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Glossary

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Molecular Biology of the Cell

Glossary 1190 terms


α helix see alpha helix

ABC transporter proteins Large superfamily of membrane transport proteins that use the energy of hydrolysis of ATP to transfer peptides and a variety of small molecules across membranes.

acetyl Chemical group derived from acetic acid. Acetyl groups are important in metabolism and are added covalently to some proteins as a posttranslational modification.

acetyl CoA Small water-soluble molecule that carries acetyl groups in cells. It consists of an acetyl group linked to coenzyme A (CoA) by an easily hydrolyzable thioester bond. (See Figure 2–62.)

acetylcholine receptor Ion channel that opens in response to binding of acetylcholine, thereby converting a chemical signal into an electrical one. Best understood example of a transmitter-gated channel. Sometimes called the nicotinic acetylcholine receptor to distinguish it from a muscarinic acetylcholine receptor, which is a G-protein-linked cell-surface receptor.

acetylcholine Neurotransmitter that functions at a class of chemical synapses known as cholinergic synapses. Found both in the brain and in the peripheral nervous system. It is the neurotransmitter at vertebrate neuromuscular junctions. (See Figure 15–9.)

acid Substance that releases protons when dissolved in water, forming a hydronium ion (H3O+).

acid hydrolase Any of a group of diverse hydrolytic enzymes (including proteases, nucleases, glycosidases, etc.) that have their optimal activity at acid pH (around 5.0) and are found in lysosomes.

acquired immunological tolerance Unresponsiveness of the immune system to a given foreign antigen that can develop in some circumstances.

acrosomal vesicle Region at the head end of a sperm cell that contains a sac of hydrolytic enzymes used to digest the protective coating of the egg.

acrosome reaction Reaction that occurs when a sperm starts to enter an egg, in which the contents of the acrosomal vesicle are released, helping the sperm to penetrate the zona pellucida.

actin Abundant protein that forms actin filaments in all eucaryotic cells. The monomeric form is sometimes called globular or G-actin; the polymeric form is filamentous or F-actin.

actin-binding protein Protein that associates with either actin monomers or actin filaments in cells and modifies their properties. Examples include myosin, α-actinin, and profilin.

actin filament (microfilament) Helical protein filament formed by the polymerization of globular actin molecules. A major constituent of the cytoskeleton of all eucaryotic cells and part of the contractile apparatus of skeletal muscle. (See Panel 16–1, p. 909.)

action potential Rapid, transient, self-propagating electrical excitation in the plasma membrane of a cell such as a neuron or muscle cell. Action potentials, or nerve impulses, make possible long-distance signaling in the nervous system.

activated carrier Small diffusible molecule in cells that stores easily-exchangeable energy in the form of one or more energy-rich covalent bonds. Examples are ATP and NADPH. Also called a coenzyme.

activation energy Extra energy that must be possessed by atoms or molecules in addition to their ground-state energy in order to undergo a particular chemical reaction. (See Figure 9–1.)

active site Region of an enzyme surface to which a substrate molecule binds in order to undergo a catalyzed reaction.

active transport Movement of a molecule across a membrane or other barrier driven by energy other than that stored in the electrochemical gradient of the transported molecule.

acyl group Functional group derived from a carboxylic acid ( ). (R represents an alkyl group, such as methyl.)

adaptation Adjustment of sensitivity following repeated stimulation. This is the mechanism that allows a neuron, a photodetector, or a bacterium to react to small changes in stimuli even against a high background level of stimulation.

adaptin Protein that binds clathrin to the membrane surface in clathrin-coated vesicles.

adaptive immune response Response of the vertebrate immune system to a specific antigen that typically generates immunological memory.

adaptor protein General term for proteins in intracellular signaling pathways that link different proteins in the pathway directly together.

adenomatous polyposis coli (APC) Tumor suppressor protein that forms part of a protein complex that recruits free cytoplasmic β-catenin and degrades it.

adenosine triphosphate see ATP

adenylyl cyclase (adenylate cyclase) Membrane-bound enzyme that catalyzes the formation of cyclic AMP from ATP. An important component of some intracellular signaling pathways.

adherens junction Cell junction in which the cytoplasmic face of the plasma membrane is attached to actin filaments. Examples include the adhesion belts linking adjacent epithelial cells and the focal contacts on the lower surface of cultured fibroblasts.

adhesion belt Beltlike adherens junction that encircles the apical end of an epithelial cell and attaches it to the adjoining cell. Also known as the zonula adherens.

adhesion plaque see focal adhesion.

adipocyte A fat cell.

ADP (adenosine 5′-diphosphate) Nucleotide that is produced by hydrolysis of the terminal phosphate of ATP. It regenerates ATP when phosphorylated by an energy-generating process such as oxidative phosphorylation. (SeeFigure 2–57.)

adrenaline (epinephrine) Hormone released by chromaffin cells (in the adrenal gland) and by some neurons in response to stress. Produces “fight or flight” responses, including increased heart rate and blood sugar levels.

aerobic Describes a process that requires, or occurs in the presence of, gaseous oxygen (O2).

affinity chromatography Type of chromatography in which the protein mixture to be purified is passed over a matrix to which specific ligands for the required protein are attached, so that the protein is retained on the matrix.

affinity constant (association constant) (Ka) Measure of the strength of binding of the components in a complex. For components A and B and a binding equilibrium A + B AB, the association constant is given by [AB]/[A][B], and is larger the tighter the binding between A and B. (See also dissociation constant.)

affinity maturation Progressive increase in the affinity of antibodies for the immunizing antigen with the passage of time after immunization.

Akt see protein kinase B

alcohol Polar organic molecule that contains a functional hydroxyl group (–OH) bound to a carbon atom that is not in an aromatic ring. An example is ethyl alcohol (CH3CH2OH).

aldehyde Organic compound that contains the group. An example is glyceraldehyde. Can be oxidized to an acid or reduced to an alcohol.

alga (algae) Informal term used to describe a wide range of simple unicellular and multicellular eucaryotic photosynthetic organisms. Examples include Nitella, Volvox, and Fucus.

alkaloid Small but chemically complex nitrogen-containing metabolite produced by plants as a defense against herbivores. Examples include caffeine, morphine, and colchicine.

alkane (adjective aliphatic) Compound of carbon and hydrogen that has only single covalent bonds. An example is ethane (CH3CH3).

alkene Hydrocarbon with one or more carbon-carbon double bonds. An example is ethylene (CH2CH2).

alkyl group General term for a group of covalently linked carbon and hydrogen atoms such as methyl (–CH3) or ethyl (–CH2CH3) groups. These groups can be formed by removing a hydrogen atom from an alkane.

allele One of a set of alternative forms of a gene. In a diploid cell each gene will have two alleles, each occupying the same position (locus) on homologous chromosomes.

allelic exclusion The expression of an immunoglobulin chain (or T cell receptor chain) gene from only one of the two homologous loci present for that gene in the lymphocyte.

allosteric protein Protein that changes from one conformation to another when it binds another molecule or when it is covalently modified. The change in conformation alters the activity of the protein and can form the basis of directed movement.

alpha helix (α helix) Common folding pattern in proteins in which a linear sequence of amino acids folds into a right-handed helix stabilized by internal hydrogen bonding between backbone atoms.

alternative RNA splicing The production of different proteins from the same RNA transcript by splicing it in different ways.

amide Molecule containing a carbonyl group linked to an amine.

amine Chemical group containing nitrogen and hydrogen. It becomes positively charged in water.

amino acid Organic molecule containing both an amino group and a carboxyl group. Those that serve as the building blocks of proteins are alpha amino acids, having both the amino and carboxyl groups linked to the same carbon atom. (See Panel 3–1, pp. 132–133.)

aminoacyl-tRNA synthetase Enzyme that attaches the correct amino acid to a tRNA molecule to form an aminoacyl-tRNA. (See Figure 6–57.)

amino group Weakly basic functional group derived from ammonia (NH3) in which one or more hydrogen atoms are replaced by another atom. In aqueous solution it can accept a proton and carry a positive charge.

amino terminus (N terminus) The end of a polypeptide chain that carries a free α-amino group.

aminoacyl tRNA Activated form of amino acid used in protein synthesis. Consists of an amino acid linked through a labile ester bond from its carboxyl group to a hydroxyl group on tRNA. (See Figure 6–57.)

AMP (adenosine 5′-monophosphate) One of the four nucleotides in an RNA molecule. Two phosphates are added to AMP to form ATP. (See Panel 2–6, pp. 120–121.)

amphipathic Having both hydrophobic and hydrophilic regions, as in a phospholipid or a detergent molecule.

anabolism System of biosynthetic reactions in a cell by which large molecules are made from smaller ones.

anaerobic Describes a cell, organism, or metabolic process that functions in the absence of air or, more precisely, in the absence of molecular oxygen (O2).

anaphase Stage of mitosis during which the two sets of chromosomes separate and move away from each other. Composed of anaphase A (chromosomes move toward the two spindle poles) and anaphase B (spindle poles move apart).

anaphase-promoting complex (APC) Ubiquitin ligase that promotes the destruction of a set of proteins, some of which initiate the separation of sister chromatids during the metaphase-to-anaphase transition during mitosis.

anchorage dependence Dependence of cell growth on attachment to a substratum.

anchoring junction Type of cell junction that attaches cells to neighboring cells or to the extracellular matrix.

angiogenesis Growth of new blood vessels by sprouting from existing ones.

Ångstrom (Å) Unit of length used to measure atoms and molecules. Equal to 10–10 meter or 0.1 nanometer (nm).

animal pole In yolky eggs, that end free of yolk that cleaves more rapidly than the vegetal pole.

ankyrin Protein mainly responsible for attaching the spectrin cytoskeleton to the red blood cell plasma membrane.

antenna complex Part of a photosystem that captures light energy and channels it into the photochemical reaction center. It consists of protein complexes that bind large numbers of chlorophyll molecules and other pigments.

anterior Situated toward the head end of the body.

anteroposterior Describes the axis running from the head to the tail of the animal body.

antibiotic Substance such as penicillin or streptomycin that is toxic to microorganisms. Usually a product of a particular microorganism or plant.

antibody (immunoglobulin) Protein produced by B cells in response to a foreign molecule or invading microorganism. Often binds to the foreign molecule or cell extremely tightly, thereby inactivating it or marking it for destruction by phagocytosis or complement-induced lysis.

anticodon Sequence of three nucleotides in a transfer RNA molecule that is complementary to a three-nucleotide codon in a messenger RNA molecule.

antigen Molecule that is able to provoke an immune response.

antigenic determinant (epitope) Specific region of an antigenic molecule that binds to an antibody or a T cell receptor.

antigenic variation The ability to change the antigens displayed on the cell surface; a property of some pathogenic microorganisms that enables them to evade attack by the immune system.

antigen-presenting cell Cell that displays foreign antigen complexed with MHC molecules on its surface.

antiparallel Describes the relative orientation of the two strands in a DNA double helix; the polarity of one strand is oriented in the opposite direction to that of the other.

antiporter Carrier protein that transports two different ions or small molecules across a membrane in opposite directions, either simultaneously or in sequence.

antisense RNA RNA complementary to a specific RNA transcript of a gene that can hybridize to the specific RNA and block its function.

APC see adenomatous polyposis coli; anaphase-promoting complex

apical Describes the tip of a cell, a structure, or an organ. The apical surface of an epithelial cell is the exposed free surface, opposite to the basal surface. The basal surface rests on the basal lamina that separates the epithelium from other tissue.

apoptosis Form of cell death, also known as programmed cell death, in which a ‘suicide' program is activated within the cell, leading to fragmentation of the DNA, shrinkage of the cytoplasm, membrane changes and cell death without lysis or damage to neighboring cells. It is a normal phenomenon, occurring frequently in a multicellular organism.

aqueous Pertaining to water, as for example, in an aqueous solution.

archea (singular archeon) Members of one of the two major divisions of procaryotes (the Archea), the other being the Bacteria.

ARF protein Monomeric GTPase responsible for regulating both COPI coat assembly and clathrin coat assembly at Golgi membranes.

aromatic Describes a molecule that contains carbon atoms in a ring, commonly drawn as linked through alternating single and double bonds. Often a molecule related to benzene.

ARP complex (ARP2/3 complex) Complex of proteins that nucleates actin filament growth from the minus end.

asexual reproduction Any type of reproduction (such as budding in Hydra, binary fission in bacteria, or mitotic division in eucaryotic microorganisms) that does not involve gamete formation and fusion. It produces an individual genetically identical to the parent.

association constant see affinity constant

aster Star-shaped system of microtubules emanating from a centrosome or from a pole of a mitotic spindle.

astral microtubule In the mitotic spindle, any of the microtubules radiating from the aster which are not attached to a kinetochore of a chromosome.

asymmetric cell division Cell division that produces two daughter cells that differ, for example in size or in the presence or absence of some cytoplasmic constituent.

atomic weight Mass of an atom relative to the mass of a hydrogen atom. Essentially equal to the number of protons plus neutrons.

ATP (adenosine 5′-triphosphate) Nucleoside triphosphate composed of adenine, ribose, and three phosphate groups that is the principal carrier of chemical energy in cells. The terminal phosphate groups are highly reactive in the sense that their hydrolysis, or transfer to another molecule, takes place with release of a large amount of free energy. (See Figure 2–26.)

ATP synthase Enzyme complex in the inner membrane of a mitochon-drion and the thylakoid membrane of a chloroplast that catalyzes the formation of ATP from ADP and inorganic phosphate during oxidative phosphorylation and photosynthesis, respectively. Also present in the plasma membrane of bacteria.

ATPase Enzyme that catalyzes a process involving the hydrolysis of ATP. A large number of different proteins have ATPase activity.

autoantibody Antibody produced by an individual against a protein, or other potential antigen, of its own cells and tissues. Autoantibodies can cause autoimmune disease.

autocatalysis Reaction that is catalyzed by one of its products, creating a positive feedback (self-amplifying) effect on the reaction rate.

autocrine signaling Type of cell signaling in which a cell secretes signal molecules that act on itself or on other adjacent cells of the same type.

autoimmune disease A pathological state in which the body mounts an immune response against one or more of its own potential antigens.

autophagy Digestion of worn-out organelles by the cell's own lysosomes.

autoradiography Technique in which a radioactive object produces an image of itself on a photographic film. The image is called an autoradiograph or autoradiogram.

autosome Any chromosome other than a sex chromosome.

avidity Total binding strength of a polyvalent antibody with a polyvalent antigen.

Avogadro's number 6 × 1023. This is the number of atoms in 1 gram of hydrogen, and thus in the atomic or molecular weight equivalent in grams of any element or molecule.

axon Long nerve cell process that is capable of rapidly conducting nerve impulses over long distances so as to deliver signals to other cells.

axonal transport Directed transport of organelles and molecules along a nerve cell axon. It can be anterograde (outward from the cell body) or retrograde (back toward the cell body).

axoneme Bundle of microtubules and associated proteins that forms the core of a cilium or flagellum in a eucaryotic cell and is responsible for their movements.

β sheet see beta sheet

B cell (B lymphocyte) Type of lymphocyte that makes antibodies.

bacteria (singular bacterium) Members of the Bacteria, one of the two major divisions of procaryotes, the other being the Archea. Most exist as single cells and some cause disease.

bacterial artificial chromosome (BAC) Cloning vector that can accommodate large pieces of DNA up to 1 million base pairs.

bacteriophage (phage) Any virus that infects bacteria. Bacteriophages were the first entities used for the study of molecular genetics and are now widely used as cloning vectors.

bacteriorhodopsin Pigmented protein found in the plasma membrane of a salt-loving bacterium, Halobacterium halobium. It pumps protons out of the cell in response to light.

basal Situated near the base. The basal surface of a cell is opposite the apical surface.

basal body Short cylindrical array of microtubules plus their associated proteins found at the base of a eucaryotic cell cilium or flagellum. Serves as a nucleation site for the growth of the axoneme. Closely similar in structure to a centriole.

basal lamina (basal laminae) Thin mat of extracellular matrix that separates epithelial sheets, and many other types of cells such as muscle or fat cells, from connective tissue.

base A substance that can accept a proton in solution. The purines and pyrimidines in DNA and RNA are organic nitrogenous bases and are often referred to simply as bases.

base pair Two nucleotides in an RNA or DNA molecule that are held together by hydrogen bonds—for example, G pairs with C, and A with T or U.

basic Having the properties of a base.

benign Describes tumors that are self-limiting in their growth and noninvasive.

benzene Molecule composed of a six-membered ring of carbon atoms, commonly drawn containing three alternating double bonds. The benzene ring occurs as part of many biological molecules.

beta-catenin (β-catenin) Multifunctional cytoplasmic protein that is involved in cadherin-mediated cell–cell adhesion, linking cadherins to the actin cytoskeleton. Can also act independently as a gene regulatory protein. Has an important role in animal development as part of a Wnt signaling pathway.

beta sheet (β sheet) Common structural motif in proteins in which different sections of the polypeptide chain run alongside each other, joined together by hydrogen bonding between atoms of the polypeptide backbone. Also known as a β-pleated sheet.

binding site A region on the surface of one molecule (usually a protein or nucleic acid) that can interact with another molecule through noncovalent bonding.

biosphere The world of living organisms.

biotin Low-molecular-weight compound used as a coenzyme. Useful technically as a covalent label for proteins, allowing them to be detected by the egg protein avidin, which binds extremely tightly to biotin. (See Figure 2–63.)

bivalent A duplicated chromosome paired with its homologous duplicated chromosome at the beginning of meiosis.

black membrane Artificial planar lipid bilayer membrane.

blastomere One of the cells formed by the cleavage of a fertilized egg.

blastula Early stage of an animal embryo, usually consisting of a hollow ball of cells, before gastrulation begins.

blotting Biochemical technique in which macromolecules separated on an agarose or polyacrylamide gel are transferred to a nylon membrane or sheet of paper, thereby immobilizing them for further analysis. (See Northern blotting, Southern blotting, Western blotting.)

bond energy Strength of the chemical linkage between two atoms, measured by the energy in kilocalories or kilojoules needed to break it.

bright-field microscope The normal light microscope in which the image is obtained by simple transmission of light through the object being viewed.

brush border Dense covering of microvilli on the apical surface of epithelial cells in the intestine and kidney. The microvilli aid absorption by increasing the surface area of the cell.

budding yeast Common name often given to the baker's yeast Saccharomyces cerevisiae, a common experimental organism, which divides by budding off a smaller cell.

C terminus see carboxyl terminus

Ca2+/calmodulin-dependent protein kinase (CaM kinase) Protein kinase whose activity is regulated by the binding of Ca2+-activated calmodulin (Ca2+/calmodulin), and which indirectly mediates the effects of Ca2+ by phosphorylation of other proteins.

cadherin A member of a family of proteins that mediates Ca2+-dependent cell–cell adhesion in animal tissues.

caged molecule Organic molecule designed to change into an active form when irradiated with light of a specific wavelength. An example is caged ATP.

calcium pump (Ca2+ ATPase) Transport protein in the membrane of the sarcoplasmic reticulum of muscle cells (and elsewhere) that pumps Ca2+ out of the cytoplasm into the sarcoplasmic reticulum using the energy of ATP hydrolysis.

calmodulin Ubiquitous calcium-binding protein whose binding to other proteins is governed by changes in intracellular Ca2+ concentration. Its binding modifies the activity of many target enzymes and membrane transport proteins.

calorie Unit of heat. One calorie (small “c”) is the amount of heat needed to raise the temperature of 1 gram of water by 1°C. A kilocalorie (1000 calories) is the unit used to describe the energy content of foods.

Calvin cycle (Calvin-Benson cycle) Major metabolic pathway by which CO2 is incorporated into carbohydrate during the second stage of photosynthesis (carbon fixation) in plants. Also called the carbon-fixation cycle.

CAM see cell adhesion molecule

CaM kinase see Ca2+/calmodulin-dependent protein kinase

CaM-kinase II Multifunctional Ca2+/calmodulin-dependent protein kinase found in all animal cells that undergoes autophosphorylation when activated. It is especially abundant in brain and is thought to have a role in learning and memory in vertebrates.

cAMP see cyclic AMP

capacitation Poorly understood process that sperm must go through in the female reproductive tract before they are competent for fertilization.

capsid Protein coat of a virus, formed by the self-assembly of one or more protein subunits into a geometrically regular structure.

carbohydrate General term for sugars and related compounds containing carbon, hydrogen, and oxygen, usually with the empirical formula (CH2O)n.

carbon fixation Process by which green plants incorporate carbon atoms from atmospheric carbon dioxide into sugars. The second stage of photosynthesis.

carbon-fixation cycle see Calvin cycle

carbonyl group Pair of atoms consisting of a carbon atom linked to an oxygen atom by a double bond (C=O).

carboxyl group Carbon atom linked both to an oxygen atom by a double bond and to a hydroxyl group. Molecules containing a carboxyl group are weak acids—carboxylic acids ( ).

carboxyl terminus (C terminus) The end of a polypeptide chain that carries a free α-carbonyl group.

carcinogen Any agent, such as a chemical or a form of radiation, that causes cancer.

carcinogenesis Generation of a cancer.

carcinoma Cancer of epithelial cells. The most common form of human cancer.

cardiac muscle Specialized form of striated muscle found in the heart, consisting of individual heart muscle cells linked together by cell junctions.

carrier protein Membrane transport protein that binds to a solute and transports it across the membrane by undergoing a series of conformational changes.

cartilage Form of connective tissue composed of cells (chondrocytes) embedded in a matrix rich in type II collagen and chondroitin sulfate.

caspase Any of a family of intracellular proteases that are involved in initiating the cellular events of apoptosis.

catabolism General term for the enzyme-catalyzed reactions in a cell by which complex molecules are degraded to simpler ones with release of energy. Intermediates in these reactions are sometimes called catabolites.

catalyst Substance that can lower the activation energy of a reaction, thus increasing its rate.

caveola (caveolae) Invaginations at the cell surface that bud off internally to form pinocytic vesicles. Thought to form from lipid rafts, regions of membrane rich in certain lipids.

CD28 Cell-surface protein on T cells that binds the co-stimulatory B7 protein on “professional” antigen-presenting cells, providing an additional signal required for the activation of a naïve T cell by antigen.

CD4 Co-receptor protein found on helper T cells that binds to class II MHC molecules outside the antigen-binding site.

CD8 Co-receptor protein found on cytotoxic T cells that binds to class I MHC molecules outside the antigen-binding site.

cdc gene see cell-division-cycle gene

Cdk inhibitor protein (CKI) Protein that binds to and inhibits cyclin-Cdk complexes, primarily involved in the control of G1 and S phases.

Cdk-activating kinase (CAK) Protein kinase that phosphorylates Cdks in cyclin-Cdk complexes, activating the Cdk.

Cdk see cyclin-dependent kinase

cDNA DNA molecule made as a copy of messenger RNA and therefore lacking the introns that are present in genomic DNA. cDNA clones represent DNA cloned from cDNA and a collection of such clones, usually representing the genes expressed in a particular cell type or tissue, is a cDNA library.

cell adhesion molecule (CAM) Protein on the surface of an animal cell that mediates cell–cell binding or cell–matrix binding.

cell body Main part of a nerve cell that contains the nucleus. The other parts are the axons and dendrites.

cell coat see glycocalyx

cell cortex Specialized layer of cytoplasm on the inner face of the plasma membrane. In animal cells it is an actin-rich layer responsible for movements of the cell surface.

cell cycle (cell-division cycle) Reproductive cycle of a cell: the orderly sequence of events by which a cell duplicates its contents and divides into two.

cell division Separation of a cell into two daughter cells. In eucaryotic cells it entails division of the nucleus (mitosis) closely followed by division of the cytoplasm (cytokinesis).

cell fate In developmental biology, describes what a particular cell at a given stage of development will normally give rise to.

cell fusion Process in which the plasma membranes of two cells fuse down at the point of contact between them, allowing the two cytoplasms to mingle.

cell junction Specialized region of connection between two cells or between a cell and the extracellular matrix.

cell line Population of cells of plant or animal origin capable of dividing indefinitely in culture.

cell plate Flattened membrane-bounded structure that forms by fusing vesicles in the cytoplasm of a dividing plant cell and is the precursor of the new cell wall.

cell wall Mechanically strong extracellular matrix deposited by a cell outside its plasma membrane. It is prominent in most plants, bacteria, algae, and fungi. Not present in most animal cells.

cell-cycle control system Network of regulatory proteins that governs progression of a eucaryotic cell through the cell cycle.

cell-division-cycle gene (cdc gene) Gene that controls a specific step or set of steps in the eucaryotic cell cycle. Originally identified in yeasts.

cell-free system Fractionated cell homogenate that retains a particular biological function of the intact cell, and in which biochemical reactions and cell processes can be more easily studied.

cell-mediated immune response That part of an adaptive immune response in which antigen-specific T cells are activated to perform various functions such as killing infected cells and activating macrophages.

cellularization The formation of cells around each nucleus in a multinucleate cytoplasm, transforming it into a multicellular structure.

cellulose Structural polysaccharide consisting of long chains of covalently linked glucose units. It provides tensile strength in plant cell walls.

centimorgan see genetic map distance

central lymphoid organ (primary lymphoid organ) Lymphoid organ in which lymphocytes develop. In adult mammals these are the thymus and bone marrow.

central nervous system (CNS) Main information-processing organ of the nervous system. In vertebrates it consists of the brain and spinal cord.

centriole Short cylindrical array of microtubules, closely similar in structure to a basal body. A pair of centrioles is usually found at the center of a centrosome in animal cells.

centromere Constricted region of a mitotic chromosome that holds sister chromatids together. It is also the site on the DNA where the kinetochore forms that captures microtubules from the mitotic spindle.

centrosome cycle Duplication of the centrosome (during interphase) and separation of the two new centrosomes (at the beginning of mitosis), which provides two centrosomes to form the poles of the mitotic spindle.

centrosome Centrally located organelle of animal cells that is the primary microtubule-organizing center and acts as the spindle pole during mitosis. In most animal cells it contains a pair of centrioles.

CG island Region of DNA with a greater than average density of CG sequences; these regions generally remain unmethylated.

CGN see cis Golgi network

channel protein Membrane transport protein that forms an aqueous pore in the membrane through which a specific solute, usually an ion, can pass.

chaperone (molecular chaperone) Protein that helps other proteins avoid misfolding pathways that produce inactive or aggregated polypeptides.

checkpoint Point in the eucaryotic cell-division cycle where progress through the cycle can be halted until conditions are suitable for the cell to proceed to the next stage.

chelate Combine reversibly, usually with high affinity, with a metal ion such as iron, calcium, or magnesium.

chemical group Set of covalently linked atoms, such as a hydroxyl group (–OH) or an amino group (–NH2), the chemical behavior of which is well characterized.

chemiosmotic coupling Mechanism in which a gradient of hydrogen ions (a pH gradient) across a membrane is used to drive an energy-requiring process, such as ATP production or the rotation of bacterial flagella.

chemokine Small secreted protein that attracts cells, such as white blood cells, to move towards its source. Important in the functioning of the immune system.

chemotaxis Directed movement of a cell or organism towards or away from a diffusible chemical.

chiasma (chiasmata) X-shaped connection visible between paired homologous chromosomes in division I of meiosis, and which represents a site of crossing-over.

chlorophyll Light-absorbing green pigment that plays a central part in photosynthesis in bacteria, plants, and algae.

chloroplast Organelle in green algae and plants that contains chlorophyll and carries out photosynthesis. It is a specialized form of plastid.

cholesterol Lipid molecule with a characteristic four-ring steroid structure that is an important component of the plasma membranes of animal cells. (See Figure 10–10.)

chondrocyte (cartilage cell) Connective-tissue cell that secretes the matrix of cartilage.

chromaffin cell Cell that stores adrenaline in secretory vesicles and secretes it in times of stress when stimulated by the nervous system.

chromatid One copy of a chromosome formed by DNA replication that is still joined at the centromere to the other copy. The two identical chromatids are called sister chromatids.

chromatin Complex of DNA, histones, and nonhistone proteins found in the nucleus of a eucaryotic cell. The material of which chromosomes are made.

chromatography Biochemical technique in which a mixture of substances is separated by charge, size, or some other property by allowing it to partition between a moving phase and a stationary phase. (See affinity chromatography, DNA affinity chromatography, high-performance liquid chromatography.)

chromosomal crossing-over The exchange of DNA between paired homologous chromosomes in division I of meiosis. It is a sign of genetic recombination and the crossovers (chiasmata) are visible in the light microscope. (SeeFigure 20–10.)

chromosome Structure composed of a very long DNA molecule and associated proteins that carries part (or all) of the hereditary information of an organism. Especially evident in plant and animal cells undergoing mitosis or meiosis, where each chromosome becomes condensed into a compact rodlike structure visible under the light microscope.

chromosome condensation Process by which a chromosome becomes packed up into a more compact structure prior to M phase of the cell cycle.

cilium (cilia) Hairlike extension of a eucaryotic cell containing a core bundle of microtubules and capable of performing repeated beating movements. Cilia are found in large numbers on the surface of many cells, and are responsible for the swimming of many single-celled organisms.

circadian clock Internal cyclical process that produces a particular change in a cell or organism with a period of around 24 hours, for example the sleep-wakefulness cycle in humans.

cis face Face of a Golgi stack at which material enters the organelle. It is adjacent to the cis Golgi network.

cis Golgi network (CGN) Network of interconnected cisternae and tubules which receives vesicles from the endoplasmic reticulum and transfers material to the cis face of the Golgi apparatus.

cisterna (cisternae) Flattened membrane-bounded compartment, as found in the endoplasmic reticulum or Golgi apparatus.

citric acid cycle (tricarboxylic acid (TCA) cycle, Krebs cycle) Central metabolic pathway found in aerobic organisms. Oxidizes acetyl groups derived from food molecules to CO2 and H2O. In eucaryotic cells it occurs in the mitochondria.

CKI see Cdk inhibitor protein

class I MHC molecule One of the two classes of MHC molecule. It is present on almost all cell types and presents viral peptides on the surface of virus-infected cells, where they are recognized by cytotoxic T cells. (SeeFigure 24–49.)

class II MHC molecule One of the two classes of MHC molecule. It is present on “professional” antigen-presenting cells and presents foreign peptides to helper T cells. (See Figure 24–49.)

class switching The change from making one class of immunoglobulin (for example IgM) to making another class (for example IgG) that many B cells undergo during the course of an immune response.

classical pathway A pathway for activating the complement system that is initiated by IgG or IgM antibodies bound to the surface of a microbe.

clathrin Protein that assembles into a polyhedral cage on the cytosolic side of a membrane so as to form a clathrin-coated pit, which buds off by endocytosis to form an intracellular clathrin-coated vesicle.

clathrin-coated pit Region of plasma membrane of animal cells that is coated with the protein clathrin on its cytosolic face. Such regions are continually forming and budding off by endocytosis to form intracellular clathrin-coated vesicles containing extracellular fluid and the materials dissolved in it.

cleavage (1) Physical splitting of a cell into two. (2) Specialized type of cell division seen in many early embryos whereby a large cell becomes subdivided into many smaller cells without growth.

clonal selection theory Theory that explains how the adaptive immune system can respond to millions of different antigens in a highly specific way. From a population of lymphocytes with a vast repertoire of randomly generated antigen specificities, a given foreign antigen activates (selects) only those cells with the corresponding antigen specificity.

clone Population of cells or organisms formed by repeated (asexual) division from a common cell or organism. Also used as a verb: “to clone a gene” means to produce many copies of a gene by repeated cycles of replication.

cloning vector A small DNA molecule, usually derived from a bacteriophage or plasmid, which is used to carry the fragment of DNA to be cloned into the recipient cell, and which enables the DNA fragment to be replicated.

coated vesicle Small membrane-bounded organelle with a cage of proteins (the coat) on its cytosolic surface. It is formed by the pinching off of a coated region of membrane (coated pit). Some coats are made of clathrin, whereas others are made from other proteins.

codon Sequence of three nucleotides in a DNA or messenger RNA molecule that represents the instruction for incorporation of a specific amino acid into a growing polypeptide chain.

coenzyme Small molecule tightly associated with an enzyme that participates in the reaction that the enzyme catalyzes, often by forming a covalent bond to the substrate. Examples include biotin, NAD+, and coenzyme A.

coenzyme A Small molecule used in the enzymatic transfer of acyl groups in the cell. (See also acetyl CoA.)

cofactor Inorganic ion or coenzyme that is required for an enzyme's activity.

cohesin, cohesin complex Complex of proteins that holds sister chomatids together along their length before their separation.

coiled-coil Especially stable rodlike structure in proteins which is formed by two of these α helices coiled around each other.

collagen fibril Extracellular structure formed by self-assembly of secreted fibrillar collagen subunits. An abundant constituent of the extracellular matrix in many animal tissues.

collagen Fibrous protein rich in glycine and proline that is a major component of the extracellular matrix and connective tissues. Exists in many forms: type I, the most common, is found in skin, tendon, and bone; type II is found in cartilage; type IV is present in basal laminae.

colony-stimulating factor (CSF) General name for the numerous signal molecules that control the differentiation of blood cells.

colorectal tumor Common carcinoma of the epithelium lining the colon and rectum.

combinatorial control Describes the control of a step in a cellular process, such as the initiation of DNA transcription, by a combination of proteins rather than by any individual one.

communicating junction Type of cell junction that allows the passage of chemical or electrical signals from one cell to another.

compartment Regions in the embryo that are formed exclusively from the descendants of a few founder cells; there is no cell movement beween compartments once delimited.

complement system System of serum proteins activated by antibody–antigen complexes or by microorganisms. Helps eliminate pathogenic microorganisms by directly causing their lysis or by promoting their phagocytosis.

complementary DNA see cDNA

complementary Two nucleic acid sequences are said to be complementary if they can form a perfect base-paired double helix with each other.

complex oligosaccharide Chain of sugars attached to a glycoprotein that is generated by trimming of the original oligosaccharide attached in the endoplasmic reticulum and subsequent addition of further sugars. (See Figure 13–25.)

complex Assembly of molecules that are held together by noncovalent bonds. Protein complexes perform most cell functions.

condensation reaction Chemical reaction in which two molecules are covalently linked through –OH groups with the removal of a molecule of water.

condensin, condensin complex Complex of proteins involved in chromosome condensation prior to mitosis. Target for the M-Cdk.

conditional mutation A mutation that changes a protein or RNA molecule so that its function is altered only under some conditions, such as at an unusually high or an unusually low temperature.

confocal microscope Type of light microscope that produces a clear image of a given plane within a solid object. It uses a laser beam as a pinpoint source of illumination and scans across the plane to produce a two-dimensional ‘optical section.'

conformation The spatial arrangement of atoms in three dimensions in a macromolecule such as a protein or nucleic acid.

connective tissue Any supporting tissue that lies between other tissues and consists of cells embedded in a relatively large amount of extracellular matrix. Includes bone, cartilage, and loose connective tissue.

connective-tissue cell Any of the various cell types found in connective tissue, e.g. fibroblasts, cartilage cells (chondrocytes), bone cells (osteoblasts and osteocytes), fat cells (adipocytes) and smooth muscle cells.

connexon Water-filled pore in the plasma membrane formed by a ring of six protein subunits. Part of a gap junction: connexons from two adjoining cells join to form a continuous channel between the two cells.

consensus sequence Average or most typical form of a sequence that is reproduced with minor variations in a group of related DNA, RNA, or protein sequences. The consensus sequence shows the nucleotide or amino acid most often found at each position. The preservation of a consensus implies that the sequence is functionally important. (See Figure 6–12.)

constitutive secretory pathway Pathway present in all cells by which molecules such as plasma membrane proteins are continually delivered to the plasma membrane from the Golgi apparatus in vesicles that fuse with the plasma membrane. (See also default pathway.)

constitutive Produced in constant amount; opposite of regulated.

contact-dependent signaling Cell–cell communication in which the signal molecule remains bound to the signaling cell and only influences cells that physically contact it.

contractile ring Ring containing actin and myosin that forms under the surface of animal cells undergoing cell division and contracts to pinch the two daughter cells apart.

convergent extension Cellular rearrangement within a tissue that causes it to extend in one dimension (e.g. length) and shrink in another (e.g. width).

cooperativity Phenomenon in which the binding of one ligand molecule to a target molecule promotes the binding of successive ligand molecules. Seen in the assembly of large complexes, as well as in enzymes and receptors composed of multiple allosteric subunits, where it sharpens the response to a ligand. (See Figure 15–22.)

cortical granule Specialized secretory vesicle present under the plasma membrane of unfertilized eggs, including those of mammals; after fertilization it is involved in preventing the entry of further sperm.

co-translational Describes import of a protein into the endoplasmic reticulum before the polypeptide chain is completely synthesized.

co-transport (coupled transport) Membrane transport process in which the transfer of one molecule depends on the simultaneous or sequential transfer of a second molecule.

coupled reaction Linked pair of chemical reactions in which the free energy released by one of the reactions serves to drive the other.

covalent bond Stable chemical link between two atoms produced by sharing one or more pairs of electrons.

crista (cristae) (1) (2) One of the folds of the inner mitochondrial membrane. (3) A sensory structure in the inner ear.

critical concentration Concentration of a protein monomer, such as actin or tubulin, that is in equilibrium with the assembled form of the protein (i.e. assembled into actin filaments or microtubules respectively). (See Panel 16–2, pp. 912–913.)

crossing-over see chromosomal crossing-over

cryoelectron microscopy Electron microscopy technique in which the objects to be viewed, such as macromolecules and viruses, are rapidly frozen.

cryptochrome Flavoprotein responsive to blue light, found in both plants and animals. In animals it is involved in circadian rhythms.

cut-and-paste transposition Type of movement of a transposable element in which it is cut out of the DNA and inserted into a new site by a special transposase enzyme.

cyclic AMP (cAMP) Nucleotide that is generated from ATP by adenylyl cyclase in response to stimulation of many types of cell-surface receptors. cAMP acts as an intracellular signaling molecule by activating cyclic-AMP-dependent kinase (protein kinase A, PKA). It is hydrolyzed to AMP by a phosphodiesterase.

cyclic AMP-dependent protein kinase (protein kinase A, PKA) Enzyme that phosphorylates target proteins in response to a rise in intracellular cyclic AMP.

cyclic GMP Small soluble intracellular signaling molecule formed from GTP by the enzyme guanylyl cyclase in response to photoreceptor stimulation in the retina.

cyclin Protein that periodically rises and falls in concentration in step with the eucaryotic cell cycle. Cyclins activate crucial protein kinases (called a cyclin-dependent protein kinase, or Cdk) and thereby help control progression from one stage of the cell cycle to the next.

cyclin-Cdk complex Protein complexes that are formed periodically during the eucaryotic cell cycle as the level of cyclin increases, and in which the cyclin-dependent kinase (Cdk) becomes partially activated.

cyclin-dependent kinase (Cdk) Protein kinase that has to be complexed with a cyclin protein in order to act. Different Cdk-cyclin complexes trigger different steps in the cell-division cycle by phosphorylating specific target proteins.

cytochrome b-c1 complex Second of the three electron-driven proton pumps in the respiratory chain. It accepts electrons from ubiquinone.

cytochrome oxidase complex Third of the three electron-driven proton pumps in the respiratory chain. It accepts electrons from cytochrome c and generates water using molecular oxygen as an electron acceptor.

cytochrome Colored, heme-containing protein that transfers electrons during cellular respiration and photosynthesis.

cytokine Extracellular signal protein or peptide that acts as a local mediator in cell–cell communication.

cytokine receptor Type of cell-surface receptor whose ligands are cytokines such as interferons, growth hormone and prolactin, and which acts through the Jak-STAT pathway.

cytokinesis Division of the cytoplasm of a plant or animal cell into two, as distinct from the division of its nucleus (which is mitosis)

cytoplasm Contents of a cell that are contained within its plasma membrane but, in the case of eucaryotic cells, outside the nucleus.

cytoskeleton System of protein filaments in the cytoplasm of a eucaryotic cell that gives the cell shape and the capacity for directed movement. Its most abundant components are actin filaments, microtubules, and intermediate filaments.

cytosol Contents of the main compartment of the cytoplasm, excluding membrane-bounded organelles such as endoplasmic reticulum and mitochondria. Originally defined operationally as the cell fraction remaining after membranes, cytoskeletal components, and other organelles have been removed by low-speed centrifugation.

cytotoxic T cell Type of T cell responsible for killing infected cells.

Δ see standard free-energy change

ΔG see free-energy change

dalton Unit of molecular mass. Approximately equal to the mass of a hydrogen atom (1.66 × 10–24 g).

default pathway Constitutive secretory pathway that automatically delivers material from the Golgi apparatus to the plasma membrane if no other sorting signals are present.

degenerate Not a moral judgment but an adjective that describes multiple states that amount to the same thing: different triplet combinations of nucleotide bases (codons) that code for the same amino acid, for example.

deletion Type of mutation in which a single nucleotide or sequence of nucleotides has been removed from the DNA.

denaturation Dramatic change in conformation of a protein or nucleic acid caused by heating or by exposure to chemicals and usually resulting in the loss of biological function.

dendrite Extension of a nerve cell, typically branched and relatively short, that receives stimuli from other nerve cells.

dendritic cell Cell derived from bone marrow and present in lymphoid and other tissues that is specialized for the uptake of particulate material by phagocytosis and which acts as a “professional” antigen-presenting cell in immune responses.

deoxyribonucleic acid see DNA

desensitization see adaptation

desmosome Type of anchoring cell–cell junction, usually formed between two epithelial cells, characterized by dense plaques of protein into which intermediate filaments in the two adjoining cells insert.

detergent Type of small amphipathic molecule that tends to coalesce in water, with its hydrophobic tails buried and its hydrophilic heads exposed. It is widely used to solubilize membrane proteins.

determined In developmental biology, an embryonic cell is said to be determined if it has become committed to a particular specialized path of development. This determination reflects a change in the internal character of the cell, and it precedes the much more readily detected process of cell differentiation.

development Succession of changes that take place in an organism as a fertilized egg gives rise to an adult plant or animal.

diacylglycerol Lipid produced by the cleavage of inositol phospholipids in response to extracellular signals. Composed of two fatty acid chains linked to glycerol, it serves as a signaling molecule to help activate protein kinase C.

dideoxy method The standard method of DNA sequencing.

differentiation Process by which a cell undergoes a change to an overtly specialized cell type.

diffraction pattern Pattern set up by wave interference between radiation transmitted or scattered by different parts of an object.

diffusion Net drift of molecules in the direction of lower concentration due to random thermal movement.

diploid Containing two sets of homologous chromosomes and hence two copies of each gene or genetic locus.

diplotene Fourth stage of division I of meiosis, in which chiasmata are first seen.

disaccharide Carbohydrate molecule consisting of two covalently joined monosaccharide units. (See Panel 2–4, p. 116–117.)

dissociation constant (Kd) Measure of the tendency of a complex to dissociate. For components A and B and the binding equilibrium A + B AB, the dissociation constant is given by [A][B]/[AB], and it is smaller the tighter the binding between A and B. (See also association constant.)

disulfide bond (–S–S–) Covalent linkage formed between two sulfhydryl groups on cysteines. For extracellular proteins, a common way of joining two proteins together or linking different parts of the same protein. Formed in the endoplasmic reticulum of eucaryotic cells.

division I of meiosis The first cell division of meiosis, in which the members of each pair of (duplicated) homologous chromosomes are segregated to opposite poles of the dividing cell.

division II of meiosis The second cell division of meiosis, in which the chromatids of each duplicated chromosome are segregated to opposite poles of the dividing cell.

DNA (deoxyribonucleic acid) Polynucleotide formed from covalently linked deoxyribonucleotide units. It serves as the store of hereditary information within a cell and the carrier of this information from generation to generation.

DNA affinity chromatography Technique for purifying sequence-specific DNA-binding proteins by their binding to a matrix to which the appropriate DNA fragments are attached.

DNA footprinting Technique for determining the DNA sequence to which a DNA-binding protein binds.

DNA helicase Enzyme that is involved in opening the DNA helix into its single strands for DNA replication.

DNA library Collection of cloned DNA molecules, representing either an entire genome (genomic library) or DNA copies of the messenger RNA produced by a cell (cDNA library).

DNA ligase Enzyme that joins the ends of two strands of DNA together with a covalent bond to make a continuous DNA strand.

DNA methylation Addition of a methyl group to DNA. Extensive methylation of the cytosine base in CG sequences is used in vertebrates to keep genes in an inactive state.

DNA microarray Technique for analyzing the simultaneous expression of large numbers of genes in cells, in which isolated cellular RNA is hybridized to a large array of short DNA probes immobilized on glass slides.

DNA polymerase Enzyme that synthesizes DNA by joining nucleotides together using a DNA template as a guide.

DNA primase Enzyme that synthesizes a short strand of RNA on a DNA template, producing a primer for DNA synthesis.

DNA repair Collective name for those biochemical processes that correct accidental changes in the DNA.

DNA sequencing Determination of the order of nucleotides in a DNA molecule. (See Figure 8–36.)

DNA supercoiling Additional twisting of the DNA helix that occurs in response to the superhelical tension created when, for example, a circular DNA is partly unwound (See Figure 6–20.)

DNA topoisomerase Enzyme that binds to DNA and reversibly breaks a phosphodiester bond in one or both strands, allowing the DNA to rotate at that point. It prevents DNA tangling during replication.

DNA transcription see transcription

DNA tumor virus A general term for a variety of different DNA viruses that can cause tumors.

DNA-only transposon Type of transposable element that exists as DNA throughout its life cycle. Many types move by cut-and-paste transposition.

domain see protein domain

dominant negative mutation Mutation that dominantly affects the phenotype by means of a defective protein or RNA molecule that interferes with the function of the normal gene product in the same cell.

dominant In genetics, refers to the member of a pair of alleles that is expressed in the phenotype of the organism while the other allele is not, even though both alleles are present. Opposite of recessive.

dorsal Relating to the back of an animal. Also the upper surface of a leaf, wing, etc.

dorsoventral Describes the axis running from the back to the belly of an animal or from the upper side to the underside of a structure.

double helix The three-dimensional structure of DNA, in which two DNA chains held together by hydrogen bonding between the bases are wound into a helix.

Drosophila melanogaster Species of small fly, commonly called a fruit fly, much used in genetic studies of development.

dynamic instability The property of sudden conversion from growth to shrinkage, and vice versa, in a protein filament such as a microtubule or actin filament. (See Panel 16–2, pp. 912–913.)

dynamin Cytosolic GTPase that binds to the neck of a clathrin-coated vesicle in the process of budding from the membrane, and which is involved in completing vesicle formation.

dynein Member of a family of large motor proteins that undergo ATP-dependent movement along microtubules. In cilia, dynein forms the side arms in the axoneme that cause adjacent microtubule doublets to slide past one another.

dysplasia A change in cell growth and behavior in a tissue in which the structure becomes disordered.

ectoderm Embryonic tissue that is the precursor of the epidermis and nervous system.

effector cell A cell that carries out the final response or function of a particular process. The main effector cells of the immune system, for example, are activated lymphocytes and phagocytes—the cells involved in destroying pathogens and removing them from the body.

egg The mature female gamete in sexually reproducing organisms. It is usually a large and immobile cell.

elastin Hydrophobic protein that forms extracellular extensible fibres (elastic fibres) that give tissues their stretchability and resilience.

electrochemical gradient The combined influence of a difference in the concentration of an ion on the two sides of the membrane and the electrical charge difference across the membrane (membrane potential). It produces a driving force that causes the ion to move across the membrane.

electrochemical proton gradient The result of a combined pH gradient (proton gradient) and the membrane potential.

electron Negatively charged subatomic particle that generally occupies orbitals surrounding the nucleus in an atom.

electron acceptor Atom or molecule that takes up electrons readily, thereby gaining an electron and becoming reduced.

electron carrier Molecule such as cytochrome c, which transfers an electron from a donor molecule to an acceptor molecule.

electron donor Molecule that easily gives up an electron, becoming oxidized in the process.

electron microscope Type of microscope that uses a beam of electrons to create the image.

electron-transport chain Series of electron carrier molecules along which electrons move from a higher to a lower energy level to a final acceptor molecule. The energy released during electron movement can be used to power various processes. Electron-transport chains present in the inner mitochondrial membrane and in the thylakoid membrane of chloroplasts generate a proton gradient across the membrane that is used to drive ATP synthesis.

elongation factor Protein required for the addition of amino acids to growing polypeptide chains on ribosomes.

embryogenesis Development of an embryo from a fertilized egg, or zygote.

embryonic stem cell (ES cell) Cell derived from the inner cell mass of the early mammalian embryo that can give rise to all the cells in the body. It can be grown in culture, genetically modified and inserted into a blastocyst to develop a transgenic animal.

endocrine cell Specialized animal cell that secretes a hormone into the blood. Usually part of a gland, such as the thyroid or pituitary gland.

endocytic-exocytic cycle The processes of endocytosis and exocytosis that, respectively, add and remove plasma membrane from the cell, resulting in no overall change in the cell's surface area and volume.

endocytosis Uptake of material into a cell by an invagination of the plasma membrane and its internalization in a membrane-bounded vesicle. (See also pinocytosis and phagocytosis.)

endoderm Embryonic tissue that is the precursor of the gut and associated organs.

endoplasmic reticulum (ER) Labyrinthine membrane-bounded compartment in the cytoplasm of eucaryotic cells, where lipids are synthesized and membrane-bound proteins and secretory proteins are made.

endosome Membrane-bounded organelle in animal cells that carries materials newly ingested by endocytosis and passes many of them on to lysosomes for degradation.

endothelial cell Flattened cell type that forms a sheet (the endothelium) lining all blood vessels.

enhancer Regulatory DNA sequence to which gene regulatory proteins bind, influencing the rate of transcription of a structural gene that can be many thousands of base pairs away.

entropy Thermodynamic quantity that measures the degree of disorder in a system; the higher the entropy, the greater the disorder.

enveloped virus Virus with a capsid surrounded by a lipid membrane (the envelope), which is derived from the host cell plasma membrane when the virus buds from the cell.

enzyme Protein that catalyzes a specific chemical reaction.

enzyme-linked receptor Major type of cell-surface receptor in which the cytoplasmic domain either has enzymatic activity itself or is associated with an intracellular enzyme. In both cases enzymatic activity is stimulated by ligand binding to the receptor.

epidermis Epithelial layer covering the outer surface of the body. It has different structures in different animal groups. The outer layer of plant tissue is also called the epidermis.

epimerization Reaction that alters the steric arrangement around one atom, as in a sugar molecule.

epinephrine see adrenaline

epithelial tissue see epithelium

epithelium (epithelia) Coherent cell sheet formed from one or more layers of cells covering an external surface or lining a cavity.

epitope see antigenic determinant

equilibrium constant (K) Ratio of forward and reverse rate constants for a reaction and equal to the association constant. (See Figure 3–44.)

equilibrium State where there is no net change in a system. For example, equilibrium is reached in a chemical reaction when the forward and reverse rates are equal.

ER lumen The space enclosed by the membrane of the endoplasmic reticulum (ER).

ER resident protein Protein that remains in the endoplasmic reticulum (ER) or its membranes and carries out its function there, as opposed to proteins that are present in the ER only in transit.

ER retention signal Short amino acid sequence on a protein that prevents it moving out of the endoplasmic reticulum (ER). Found on proteins that are resident in the ER and function there.

ER signal sequence N-terminal signal sequence that directs proteins to enter the endoplasmic reticulum (ER). It is cleaved off by signal peptidase after entry.

ER see endoplasmic reticulum

erythrocyte (red blood cell) Small, hemoglobin-containing blood cell of vertebrates that transports oxygen and carbon dioxide to and from tissues.

erythropoietin Growth factor that stimulates the production of red blood cells. It is produced by the kidney and acts on precursor cells in bone marrow.

ES cell see embryonic stem cell

Escherichia coli (E. coli) Rodlike bacterium normally found in the colon of humans and other mammals and widely used in biomedical research.

ester Molecule formed by the condensation reaction of an alcohol group with an acidic group. Phosphate groups usually form esters when linked to a second molecule. (See Panel 2–1, 110–111.)

ethyl (–CH2CH3) Hydrophobic chemical group derived from ethane (CH3CH3).

eucaryote (eukaryote) Organism composed of one or more cells with a distinct nucleus and cytoplasm. Includes all forms of life except viruses and procaryotes (bacteria and archea).

euchromatin Region of an interphase chromosome that stains diffusely; “normal” chromatin, as opposed to the more condensed heterochromatin.

exocytosis Process by which most molecules are secreted from a eucaryotic cell. These molecules are packaged in membrane-bounded vesicles that fuse with the plasma membrane, releasing their contents to the outside.

exon Segment of a eucaryotic gene that consists of a sequence of nucleotides that will be represented in messenger RNA or the final transfer RNA or ribosomal RNA. In protein-coding genes, exons encode amino acids in the protein. An exon is usually adjacent to a noncoding DNA segment called an intron.

expression vector A virus or plasmid that carries a DNA sequence into a suitable host cell and there directs the synthesis of the protein encoded by the sequence.

expression Production of an observable phenotype by a gene—usually by directing the synthesis of a protein.

extracellular matrix Complex network of polysaccharides (such as glycosaminoglycans or cellulose) and proteins (such as collagen) secreted by cells. Serves as a structural element in tissues and also influences their development and physiology.

facilitated diffusion see passive transport

FADH2 (reduced flavin adenine dinucleotide) Activated carrier molecule that is produced by the citric acid cycle.

FAK see focal adhesion kinase

Fas protein (Fas) Membrane-bound receptor that initiates apoptosis in the receptor-bearing cell after binding to its ligand (Fas ligand).

fat Energy-storage lipid in cells. It is composed of triglycerides—fatty acids esterified with glycerol.

fat cell Connective-tissue cell that produces and stores fat in animals.

fatty acid Compound such as palmitic acid that has a carboxylic acid attached to a long hydrocarbon chain. Used as a major source of energy during metabolism and as a starting point for the synthesis of phospholipids. (See Panel 2–5, pp. 118–119.)

Fc receptor One of a family of receptors specific for the invariant constant region (Fc region) of immunoglobulins (other than IgM and IgD); different Fc receptors are specific for IgG, IgA, IgE and their subclasses.

feedback inhibition Type of regulation of metabolism in which an enzyme acting early in a reaction pathway is inhibited by a late product of that pathway.

fermentation Anaerobic energy-yielding metabolic pathway in which pyruvate produced by glycolysis is converted, for example, into lactate or ethanol, with the conversion of NADH to NAD+.

fertilization Fusion of a male and a female gamete (both haploid) to form a diploid zygote, which develops into a new individual.

fibrillar collagen Type of collagen molecule which assembles into rope-like structures. Collagens type I (common in skin), II, III, V and XI are of this type.

fibroblast Common cell type found in connective tissue. Secretes an extracellular matrix rich in collagen and other extracellular matrix macromolecules. Migrates and proliferates readily in wounded tissue and in tissue culture.

fibronectin Extracellular matrix protein that is involved in adhesion of cells to the matrix and the guidance of migrating cells during embryogenesis. Integrins on the cell surface are receptors for fibronectin.

filopodium (filopodia) Thin, spike-like protrusion with an actin filament core, generated on the leading edge of a crawling animal cell.

fission yeast Common name often given to the yeast Schizosaccharomyces pombe, a common experimental organism. It divides to give two equal-sized cells.

fixative Chemical reagent such as formaldehyde or osmium tetroxide used to preserve cells for microscropy. Samples treated with these reagents are said to be “fixed,” and the process is called fixation.

flagellum (flagella) Long, whiplike protrusion whose undulations drive a cell through a fluid medium. Eucaryotic flagella are longer versions of cilia. Bacterial flagella are smaller and completely different in construction and mechanism of action.

fluid-phase endocytosis Type of endocytosis in which small vesicles bud off internally from the plasma membrane, carrying extracellular fluid and dissolved material into the cell. (See also pinocytosis.)

fluorescein Fluorescent dye that fluoresces green when illuminated with blue light or ultraviolet light.

fluorescence microscope Microscope designed to view material stained with fluorescent dyes. Similar to a light microscope but the illuminating light is passed through one set of filters before the specimen, to select those wavelengths that excite the dye, and through another set of filters before it reaches the eye, to select only those wavelengths emitted when the dye fluoresces.

fluorescent dye Molecule that absorbs light at one wavelength and responds by emitting light at another wavelength. The emitted light is of longer wavelength (and hence of lower energy) than the light absorbed.

fluorescent resonance energy transfer (FRET) Technique for monitoring the closeness of two fluorescently labeled molecules (and thus their interaction) in cells.

focal adhesion kinase (FAK) Cytoplasmic tyrosine kinase present at cell-matrix junctions (focal adhesions) in association with the cytoplasmic tails of integrins.

focal adhesion, focal contact (adhesion plaque) A type of anchoring cell junction, forming a small region on the surface of a fibroblast or other cell that is anchored to the extracellular matrix. Attachment is mediated by transmembrane proteins such as integrins, which are linked, through other proteins, to actin filaments in the cytoplasm.

follicle cell One of the cell types that surround a developing oocyte or egg.

free energy (G) The energy that can be extracted from a system to drive reactions. Takes into account changes in both energy and entropy.

free ribosome Ribosome that is free in the cytosol, unattached to any membrane. It is the site of synthesis of all proteins encoded by the nuclear genome other than those destined to enter the endoplasmic reticulum.

free-energy change (ΔG) Change in the free energy during a reaction: the free energy of the product molecules minus the free energy of the starting molecules. A large negative value of ΔG indicates that the reaction has a strong tendency to occur. (See Panel 2–7, pp. 122–123.)

freeze-fracture electron microscopy Technique for studying membrane structure, in which the membrane of a frozen cell is fractured along the interior of the bilayer, separating it into the two monolayers with the interior faces exposed.

FRET see fluorescent resonance energy transfer

fungus (fungi) Kingdom of eucaryotic organisms that includes the yeasts, molds, and mushrooms. Many plant diseases and a relatively small number of animal diseases are caused by fungi.

γ-tubulin ring complex (γTuRC) Protein complex containing γ-tubulin and other proteins that is an efficient nucleator of microtubules.

G see free energy

G0 G-“zero” phase. State of withdrawal from the eucaroytic cell-division cycle by entry into a quiescent G1 phase. A common state for differentiated cells.

G1 phase Gap 1 phase of the eucaryotic cell-division cycle, between the end of cytokinesis and the start of DNA synthesis.

G1/S-Cdk Complex formed in vertebrate cells by a G1/S-cyclin and the corresponding cyclin-dependent kinase (Cdk).

G1-Cdk Complex formed in vertebrate cells by a G1-cyclin and the corresponding cyclin-dependent kinase (Cdk).

G2 phase Gap 2 phase of the eucaryotic cell-division cycle, between the end of DNA synthesis and the beginning of mitosis.

GAG see glycosaminoglycan

gamete Specialized haploid cell, either a sperm or an egg, serving for sexual reproduction.

ganglion (ganglia) Cluster of nerve cells and associated glial cells located outside the central nervous system.

ganglioside Any glycolipid having one or more sialic acid residues in its structure. Found in the plasma membrane of eucaryotic cells and especially abundant in nerve cells.

gap junction Communicating cell–cell junction that allows ions and small molecules to pass from the cytoplasm of one cell to the cytoplasm of the next.

gastrulation The stage in animal embryogenesis during which the embryo is transformed from a ball of cells to a structure with a gut (a gastrula).

gene activator protein A gene regulatory protein that when bound to its regulatory sequence in DNA activates transcription.

gene control region DNA sequences required to initiate transcription of a given gene and control the rate of initiation.

gene conversion Process by which DNA sequence information can be transferred from one DNA helix (which remains unchanged) to another DNA helix whose sequence is altered. It occurs occasionally during general recombination.

gene regulatory protein General name for any protein that binds to a specific DNA sequence to alter the expression of a gene.

gene repressor protein A gene regulatory protein that prevents the initiation of transcription.

gene Region of DNA that controls a discrete hereditary characteristic, usually corresponding to a single protein or RNA. This definition includes the entire functional unit, encompassing coding DNA sequences, noncoding regulatory DNA sequences, and introns.

general recombination, general genetic recombination Recombination that takes place between two homologous chromosomes (as in meiosis).

general transcription factor Any of the proteins whose assembly around the TATA box is required for the initiation of transcription of most eucaryotic genes.

genetic code Set of rules specifying the correspondence between nucleotide triplets (codons) in DNA or RNA and amino acids in proteins.

genetic map Map of the chromosomes in which the distance of genes relative to each other is determined by the amount of genetic recombination that occurs between them.

genetic recombination see recombination

genetic screen A search through a large collection of mutants for a mutant with a particular phenotype.

genome The totality of genetic information belonging to a cell or an organism; in particular, the DNA that carries this information.

genomic DNA DNA constituting the genome of a cell or an organism. Often used in contrast to cDNA (DNA prepared by reverse transcription from messenger RNA). Genomic DNA clones represent DNA cloned directly from chromosomal DNA, and a collection of such clones from a given genome is a genomic DNA library.

genomic imprinting Situation where a gene is either expressed or not expressed in the embryo depending on which parent it is inherited from.

genomics The science of studying the DNA sequences and properties of entire genomes.

genotype Genetic constitution of an individual cell or organism.

germ cell Precursor cell that will give rise to gametes.

germ line The lineage of germ cells (which contribute to the formation of a new generation of organisms), as distinct from somatic cells (which form the body and leave no descendants).

GFP see green fluorescent protein

giga- Prefix denoting 109. (From Greek gigas, giant.)

Gi see inhibitory G protein

glial cell Supporting cell of the nervous system, including oligodendrocytes and astrocytes in the vertebrate central nervous system and Schwann cells in the peripheral nervous system.

globular protein Any protein with an approximately rounded shape. Such proteins are contrasted with highly elongated, fibrous proteins such as collagen.

glucose Six-carbon sugar that plays a major role in the metabolism of living cells. Stored in polymeric form as glycogen in animal cells and as starch in plant cells. (See Panel 2–4, pp. 116–117.)

glutaraldehyde Small reactive molecule with two aldehyde groups that is often used as a cross-linking fixative.

glycerol Small organic molecule that is the parent compound of many small molecules in the cell, including phospholipids.

glycocalyx (cell coat) Carbohydrate-rich layer that forms the outer coat of a eucaryotic cell. Composed of the oligosaccharides linked to intrinsic plasma membrane glycoproteins and glycolipids, as well as glycoproteins and proteoglycans that have been secreted and reabsorbed onto the cell surface.

glycogen Polysaccharide composed exclusively of glucose units used to store energy in animal cells. Large granules of glycogen are especially abundant in liver and muscle cells.

glycolipid Membrane lipid molecule with a sugar residue or oligosaccharide attached to the polar headgroup. (SeePanel 2–5, pp. 118–119.)

glycolysis Ubiquitous metabolic pathway in the cytosol in which sugars are incompletely degraded with production of ATP. (Literally, “sugar splitting.”)

glycoprotein Any protein with one or more oligosaccharide chains covalently linked to amino-acid side chains. Most secreted proteins and most proteins exposed on the outer surface of the plasma membrane are glycoproteins.

glycosaminoglycan (GAG) Long, linear, highly charged polysaccharide composed of a repeating pair of sugars, one of which is always an amino sugar. Mainly found covalently linked to a protein core in extracellular matrix proteoglycans. Examples include chondroitin sulfate, hyaluronic acid, and heparin.

glycosylation The process of adding one or more sugars to a protein or lipid molecule. (See also O-linked glycosylation, N-linked glycosylation.)

glycosylphosphatidylinositol anchor (GPI anchor) Type of lipid linkage by which some membrane proteins are bound to the membrane. It is formed as the proteins travel through the endoplasmic reticulum.

Golgi apparatus (Golgi complex) Membrane-bounded organelle in eucaryotic cells in which proteins and lipids transferred from the endoplasmic reticulum are modified and sorted. It is the site of synthesis of many cell wall polysaccharides in plants and extracellular matrix glycosaminoglycans in animal cells.

GPI anchor see glycosylphosphatidylinositol anchor

G-protein see GTP-binding protein

G-protein-linked receptor Cell-surface receptor that associates with an intracellular trimeric GTP-binding protein (G protein) after receptor activation by an extracellular ligand. These receptors are seven-pass transmembrane proteins.

Gq Class of receptor-coupled G protein that activates phospholipase C-β and originates the inositol phospholipid signaling pathway.

grana (singular granum) Stacked membrane discs (thylakoids) in chloroplasts that contain chlorophyll and are the site of the light-trapping reactions of photosynthesis.

granulocyte Category of white blood cell distinguished by conspicuous cytoplasmic granules. Includes neutrophils, basophils, and eosinophils.

gray crescent Band of pale pigmentation that appears in the egg of some species of amphibian opposite the site of sperm entry following fertilization. Caused by rotation of the egg cortex and associated pigment granules. Marks the future dorsal side.

green fluorescent protein (GFP) Fluorescent protein isolated from a jellyfish. Widely used as a marker in cell biology.

growth cone Migrating motile tip of a growing nerve cell axon or dendrite.

growth factor Extracellular polypeptide signal molecule that can stimulate a cell to grow or proliferate. Examples are epidermal growth factor (EGF) and platelet-derived growth factor (PDGF). Most growth factors also have other actions.

growth regulator see plant growth regulator

Gs see stimulatory G protein

GTP (guanosine 5′-triphosphate) Nucleoside triphosphate produced by phosphorylating GDP (guanosine diphosphate). Like ATP it releases a large amount of free energy on hydrolysis of its terminal phosphate group. It has a special role in microtubule assembly, protein synthesis, and cell signaling.

GTPase Enzyme activity that converts GTP to GDP. Also the common name used for monomeric GTP-binding proteins. (See GTP-binding protein.)

GTPase-activating protein (GAP) Protein that binds to a GTP-binding protein and inactivates it by stimulating its GTPase activity so that it hydrolyzes its bound GTP to GDP.

GTP-binding protein, G protein Protein with GTPase activity that binds GTP, which activates the protein. The intrinsic GTPase activity eventually converts the GTP to GDP which inactivates the protein. These GTPases act as molecular switches in, for example, intracellular signaling pathways. One family is composed of three different subunits (heterotrimeric GTP-binding proteins). The members of the other, very large family are monomeric GTP-binding proteins; these are commonly referred to as monomeric GTPases.

guanine nucleotide exchange factor (GEF) Protein that binds to a GTP-binding protein and activates it by stimulating it to release its tightly bound GDP, thereby allowing it to bind GTP in its place.

H+ see proton

haploid Having only one set of chromosomes, as in a sperm cell or a bacterium, as distinct from diploid (having two sets of chromosomes).

heat shock protein (stress-response protein) Protein synthesized in increased amounts in response to an elevated temperature or other stressful treatment, and which usually helps the cell to survive the stress. Prominent examples are hsp60 and hsp70.

heavy chain (H chain) The larger of the two types of polypeptide in an immunoglobulin molecule.

HeLa cell Line of human epithelial cells that grows vigorously in culture. Derived from a human cervical carcinoma.

helix-loop-helix (HLH) DNA-binding structural motif present in many gene regulatory proteins. Should not be confused with the helix-turn-helix.

helper T cell Type of T cell that helps stimulate B cells to make antibodies and activates macrophages to kill ingested microorganisms.

heme Cyclic organic molecule containing an iron atom that carries oxygen in hemoglobin and carries an electron in cytochromes. (See Figure 14–22.)

hemidesmosome Specialized anchoring cell junction between an epithelial cell and the underlying basal lamina.

hemoglobin The major protein in red blood cells that associates with O2 in the lungs by means of a bound heme group.

hemopoiesis Generation of blood cells, mainly in the bone marrow.

hepatocyte Liver cell.

heterocaryon Cell with two or more genetically different nuclei; produced by the fusion of two or more different cells.

heterochromatin Region of a chromosome that remains unusually condensed chromatin; transcriptionally inactive during interphase.

heterodimer Protein complex composed of two different polypeptide chains.

heterozygote Diploid cell or individual having two different alleles of one or more specified genes.

high-energy bond Covalent bond whose hydrolysis releases an unusually large amount of free energy under the conditions existing in a cell. A group linked to a molecule by such a bond is readily transferred from one molecule to another. Examples include the phosphodiester bonds in ATP and the thioester linkage in acetyl CoA.

high-mannose oligosaccharide Chain of sugars attached to a glycoprotein which contains many mannose residues. It is generated by a trimming of the original mannose-rich oligosaccharide that leaves most of the mannose residues with no subsequent addition of further sugars. (See Figure 13–26.)

high-performance liquid chromatography (HPLC) Type of chromatography that uses columns packed with tiny beads of matrix; the solution to be separated is pushed through under high pressure.

histidine-kinase-associated receptor Type of transmembrane receptor found in the plasma membrane of bacteria, yeast and plant cells, and involved, for example, in sensing stimuli that cause bacterial chemotaxis. Associated with a histidine protein kinase on its cytoplasmic side.

histone One of a group of small abundant proteins, rich in arginine and lysine, four of which form the nucleosome on the DNA in eucaryotic chromosomes.

HIV Human immunodeficiency virus, the retrovirus that is the cause of AIDS.

HLH see helix-loop-helix

hnRNP protein (heterogeneous nuclear ribonuclear protein) Any of a group of proteins that assemble on newly synthesized RNA, organizing it into a more compact form.

Holliday junction X-shaped structure observed in DNA undergoing recombination, in which the two DNA molecules are held together at the site of crossing-over, also called a cross-strand exchange.

homeobox Short (180 base pairs long) conserved DNA sequence that encodes a DNA-binding protein motif (homeodomain) famous for its presence in genes that are involved in orchestrating development in a wide range of organisms.

homeodomain DNA-binding domain that defines a class of gene regulatory proteins important in animal development.

homeotic mutation Mutation that causes cells in one region of the body to behave as though they were located in another, causing a bizarre disturbance of the body plan.

homolog (1) One of two or more genes that are similar in sequence as a result of derivation from the same ancestral gene. The term covers both orthologs and paralogs. (2) See homologous chromosome.

homologous Describes organs or molecules that are similar because of their common evolutionary origin. Specifically it describes similarities in protein or nucleic acid sequence.

homologous chromosome (homolog) One of the two copies of a particular chromosome in a diploid cell, each copy being derived from a different parent.

homozygote Diploid cell or organism having two identical alleles of a specified gene or set of genes.

hormone Signal molecule secreted by an endocrine cell into the bloodstream, which can then carry it to distant target cells.

housekeeping gene Gene serving a function required in all the cell types of an organism, regardless of their specialized role.

Hox complex Two tightly linked clusters of genes in Drosophila (the bithorax and Antennapedia complexes) that control the differences between the different segments of the body. Homologous Hox complexes are found in other animals, where they also determine pattern along the anteroposterior axis.

HPLC see high-performance liquid chromatography

hybridization In molecular biology, the process whereby two complementary nucleic acid strands form a double helix. Forms the basis of a powerful technique for detecting specific nucleotide sequences.

hybridoma Cell line used in the production of monoclonal antibodies. Obtained by fusing antibody-secreting B cells with cells of a lymphocyte tumor.

hydrocarbon Compound that has only carbon and hydrogen atoms. (See Panel 2–1, p 110–111.)

hydrogen bond Noncovalent bond in which an electropositive hydrogen atom is partially shared by two electronegative atoms.

hydrolysis (adjective hydrolytic) Cleavage of a covalent bond with accompanying addition of water, –H being added to one product of the cleavage and –OH to the other.

hydronium ion (H3O) Water molecule associated with an additional proton.

hydrophilic Describes a polar molecule or part of a molecule that forms enough energetically favorable interactions with water molecules to dissolve readily in water. (Literally, “water loving.”)

hydrophobic (lipophilic) Describes a nonpolar molecule or part of a molecule that cannot form energetically favorable interactions with water molecules and therefore does not dissolve in water. (Literally, “water hating.”)

hydrophobic force Force exerted by the hydrogen-bonded network of water molecules that brings two nonpolar surfaces together by excluding water between them.

hydroxyl (–OH) Chemical group consisting of a hydrogen atom linked to an oxygen, as in an alcohol.

hypertonic Describes any medium with a sufficiently high concentration of solutes to cause water to move out of a cell due to osmosis.

hypervariable region Any of three small regions within the variable region of an immunoglobulin light or heavy chain that show the highest variability from molecule to molecule. These regions determine the specificity of the antigen-binding site.

hypotonic Describes any medium with a sufficiently low concentration of solutes to cause water to move into a cell due to osmosis.

IAP family Intracellular protein inhibitors of apoptosis.

Ig see immunoglobulin

Ig superfamily Large family of proteins that contain immunoglobulin domains or immunoglobulin-like domains. Most are involved in cell-cell interactions or antigen recognition.

image processing Computer treatment of images gained from microscopy that reveal information not immediately visible to the eye.

imaginal disc Group of cells that are set aside in the Drosophila embryo and which will develop into an adult structure, e.g. eye, leg, wing.

immature secretory vesicle Secretory vesicle that appears to have just pinched off the Golgi stack. Its structure resembles that of a cisterna of the trans Golgi network.

immortalization Production of a cell line capable of an unlimited number of cell divisions. Can be the result of a chemical or viral transformation or of fusion of the original cells with cells of a tumor line.

immune response Response made by the immune system when a foreign substance or microorganism enters its body. (See also innate immune response, adaptive immune response, primary immune response, secondary immune response.)

immune system Population of lymphocytes and other white blood cells in the vertebrate body that defends it against infection.

immunoglobulin (Ig) An antibody molecule. Higher vertebrates have five classes of immunoglobulin—IgA, IgD, IgE, IgG, and IgM—each with a different role in the immune response.

immunoglobulin domain (Ig domain) Characteristic protein domain of about 100 amino acids that is found in immunoglobulin light and heavy chains. Similar domains, known as immunoglobulin-like (Ig-like) domains, are present in many other proteins involved in cell–cell interactions and antigen recognition and define the Ig superfamily.

immunogold electron microscopy Electron microscopy technique in which cellular structures or molecules of interest are labeled with antibodies tagged with electron-dense gold particles. These show up as black spots on the image.

immunological memory Long-lived state that follows a primary immune response to many antigens, in which subsequent encounter with that antigen will provoke a rapid secondary immune response.

immunoprecipitation Use of a specific antibody to draw the corresponding protein antigen out of solution. The technique can identify complexes of interacting proteins in cell extracts by using an antibody specific for one of the proteins to precipitate the complex.

in situ hybridization Technique in which a single-stranded RNA or DNA probe is used to locate a gene or a messenger RNA molecule in a cell or tissue by hybridization.

in vitro Term used by biochemists to describe a process taking place in an isolated cell-free extract. Also used by cell biologists to refer to cells growing in culture (in vitro), as opposed to in an organism (in vivo). (Latin for “in glass.”)

in vivo In an intact cell or organism. (Latin for “in life.”)

induction In developmental biology, a change in the developmental fate of one tissue caused by an interaction with another tissue. Such an interaction is called an inductive interaction.

inflammatory response Local response of a tissue to injury or infection—characterized by tissue redness, swelling, heat, and pain. Caused by invasion of white blood cells, which release various local mediators such as histamine.

inhibitor of apoptosis family see IAP family

inhibitory G protein (Gi) G protein that can regulate ion channels and inhibit the enzyme adenylyl cyclase.

inhibitory neurotransmitter Neurotransmitter that opens transmitter-gated Cl or K+ channels in the postsynaptic membrane of a nerve or muscle cell and thus tends to inhibit the generation of an action potential.

initiation factor Protein that promotes the proper association of ribosomes with messenger RNA and is required for the initiation of protein synthesis.

initiator tRNA Special tRNA that intiates translation. It always carries the amino acid methionine.

innate immune response Immune response (of both vertebrates and invertebrates) to a pathogen that involves the pre-existing defenses of the body—the innate immune system—such as barriers formed by skin and mucosa, antimicrobial molecules and phagocytes. Such a response is not specific for the pathogen.

inner membrane The innermost of two membranes surrounding an organelle. In the mitochondrion, it encloses the matrix and contains the respiratory electron transport chains.

inner nuclear membrane The innermost of the two nuclear membranes. It contains binding sites for chromatin and the nuclear lamina on its internal face.

inositol phospholipids (phosphoinositides) One of a family of lipids containing phosphorylated inositol derivatives. Although minor components of the plasma membrane, they are important in signal transduction in eucaryotic cells. (See Figure 15–34.)

insulator element DNA sequence that prevents a gene regulatory protein bound to DNA in the control region of one gene from influencing the transcription of adjacent genes.

insulin Polypeptide hormone that is secreted by β cells in the pancreas and helps regulate glucose metabolism in animals.

integral membrane protein Protein that is held tightly in a membrane and can only be removed by treatments that disrupt the lipid bilayer.

integrin Member of a large family of transmembrane proteins involved in the adhesion of cells to the extracellular matrix and to each other.

intercalary regeneration Type of regeneration that fills in the missing tissues when two mismatched parts of a structure are grafted together.

interferon-γ (IFN-γ) Cytokine secreted by certain types of T cells after activation, and which enhances the anti-viral response and macrophage activation.

interleukin Secreted peptide or protein that mainly mediates local interactions between white blood cells (leucocytes) during inflammation and immune responses.

intermediate filament Fibrous protein filament (about 10 nm in diameter) that forms ropelike networks in animal cells. One of the three most prominent types of cytoskeletal filaments. (See Panel 16–1, p. 909.)

intermembrane space (1) The subcompartment formed between the inner and outer mitochondrial membranes. (2) The corresponding compartment in a chloroplast.

internal membrane Eucaryotic cell membrane other than the plasma membrane. The membranes of the endoplasmic reticulum and the Golgi apparatus are examples.

interphase Long period of the cell cycle between one mitosis and the next. Includes G1 phase, S phase, and G2phase.

intracellular signaling protein Protein that relays a signal as part of an intracellular signaling pathway. It may either activate the next protein in the pathway or generate a small intracellular mediator.

intron Noncoding region of a eucaryotic gene that is transcribed into an RNA molecule but is then excised by RNA splicing during production of the messenger RNA or other functional structural RNA.

inversion Type of mutation in which a segment of chromosome is inverted.

ion channel Transmembrane protein complex that forms a water-filled channel across the lipid bilayer through which specific inorganic ions can diffuse down their electrochemical gradients.

ion An atom that has either gained or lost electrons to acquire a charge; for example Na+ and Cl.

ionic bond Cohesion between two atoms, one with a positive charge, the other with a negative charge. One type of noncovalent bond.

ionophore Small hydrophobic molecule that dissolves in lipid bilayers and increases their permeability to specific inorganic ions.

iron-sulfur center Electron-transporting group consisting of either two or four iron atoms bound to an equal number of sulfur atoms, found in a class of electron-transport proteins.

isoelectric point The pH at which a charged molecule in solution has no net electric charge and therefore does not move in an electric field.

isomers Molecules that are formed from the same atoms in the same chemical linkages but have different three-dimensional conformations. (See Panel 2–4, pp. 116–117.)

isoprenoid (polyisoprenoid) Member of a large family of lipid molecules with a carbon skeleton based on multiple five-carbon isoprene units. Examples include retinoic acid and dolichol.

isotope One of a number of forms of an atom that differ in atomic weight but have the same number of protons and electrons, and therefore the same chemistry. May be either stable or radioactive.

Jak-STAT signaling pathway Rapid signaling pathway by which some extracellular signals (for example interferon) activate gene expression. Involves cell-surface receptors and cytoplasmic Janus kinases (Jaks) plus signal transducers and activators of transcription (STATs).

joule Standard unit of energy in the meter-kilogram system. One joule is the energy delivered in one second by a one-watt power source. Approximately equal to 0.24 calories.

K see equilibrium constant

K+ leak channel A K+-transporting ion channel in the plasma membrane of animals cells that remains open even in a “resting” cell.

karyotype Full set of chromosomes of a cell arranged with respect to size, shape, and number.

Ka see affinity constant

keratin Member of the family of proteins that form keratin intermediate filaments, mainly in epithelial cells. Specialized keratins are found in hair, nails, and feathers.

ketone Organic molecule containing a carbonyl group linked to two alkyl groups.

kilo-Prefix denoting 103 .

kilocalorie (kcal) Unit of heat energy equal to 1000 calories. Often used to express the energy content of food or molecules: bond strengths, for example, are measured in kcal/mole. An alternative unit in wide use is the kilojoule, equal to 0.24 kcal.

kilojoule Standard unit of energy equal to 1000 joules, or 0.24 kilocalories.

kinesin One type of motor protein that uses the energy of ATP hydrolysis to move along a microtubule.

kinetochore Complex structure formed from proteins on a mitotic chromosome to which microtubules attach and which plays an active part in the movement of chromosomes to the poles. The kinetochore forms on the part of the chromosome known as the centromere.

kinetochore microtubule In a mitotic or meiotic spindle, a microtubule with one end attached to the kinetochore on a chromosome.

Krebs cycle see citric acid cycle

label Chemical group, radioactive atom or fluorescent dye added to a molecule in order to follow its progress through a biochemical reaction or to locate it spatially. Also, as a verb, to add such a group or atom to a cell or molecule.

lagging strand One of the two newly synthesized strands of DNA found at a replication fork. The lagging strand is made in discontinuous lengths that are later joined covalently.

lambda bacteriophage (γ bacteriophage) Virus that infects E. coli. Widely used as a DNA cloning vector

lamellipodium (lamellipodia) Flattened, sheetlike protrusion supported by a meshwork of actin filaments, which is extended at the leading edge of a crawling animal cell.

laminin Extracellular matrix protein found in basal laminae, where it forms a sheetlike network.

lamin see nuclear lamin

lampbrush chromosome Paired chromosome in meiosis in immature amphibian eggs, in which the chromatin forms large stiff loops extending out from the linear axis of the chromosome.

leading strand One of the two newly synthesized strands of DNA found at a replication fork. The leading strand is made by continuous synthesis in the 5′-to-3′ direction.

lectin Protein that binds tightly to a specific sugar. Abundant lectins from plant seeds are often used as affinity reagents to purify glycoproteins or to detect them on the surface of cells.

leptotene The first phase of division I of meiosis, in which the paired duplicated homologous chromosomes condense and become visible in the light microscope.

lethal mutation A mutation that causes the death of the cell or the organism that contains it.

leucine zipper Structural motif seen in many DNA-binding proteins in which two α helices from separate proteins are joined together in a coiled-coil (rather like a zipper), forming a protein dimer.

leucine-rich repeat protein (LRR protein) Common type of receptor serine/threonine kinase in plants. Characterized by a tandem array of leucine-rich repeat sequences in the extracellular portion.

leucocyte see white blood cell

leukemia Cancer of white blood cells.

ligand Any molecule that binds to a specific site on a protein or other molecule. (From Latin ligare, to bind.)

ligase Enzyme that joins together (ligates) two molecules in an energy-dependent process. DNA ligase, for example, joins two DNA molecules together end to end through phosphodiester bonds.

light chain One of the smaller polypeptides of a multisubunit protein such as myosin or immunoglobulin. Abbreviated as L chain in immunoglobulins.

lineage analysis Tracing the ancestry of individual cells in a developing embryo.

linkage (1) Mutual effect of the binding of one ligand on the binding of another that is a central feature of the behavior of all allosteric proteins. (2) Co-inheritance of two genetic loci that lie near each other on the same chromosome. The closer together the two loci, that is, the greater the linkage, the lower the frequency of recombination between them.

lipase Enzyme that catalyzes the cleavage of fatty acids from the glycerol moiety of a triglyceride.

lipid Organic molecule that is insoluble in water but tends to dissolve in nonpolar organic solvents. A special class, the phospholipids, forms the structural basis of biological membranes.

lipid bilayer Thin bimolecular sheet of mainly phospholipid molecules that forms the core structure of all cell membranes. The two layers of lipid molecules are packed with their hydrophobic tails pointing inward and their hydrophilic heads outward, exposed to water.

lipid raft Small region of the plasma membrane enriched in sphingolipids and cholesterol.

lipophilic see hydrophobic

liposome Artificial phospholipid bilayer vesicle formed from an aqueous suspension of phospholipid molecules.

local mediator Secreted signal molecule that acts at short range on adjacent cells.

locus In genetics, the position of a gene on a chromosome. Different alleles of the same gene all occupy the same locus.

long-term potentiation Long-lasting increase (days to weeks) in the sensitivity of certain synapses in the hippocampus. Induced by a short burst of repetitive firing in the presynaptic neurons.

low-density lipoprotein (LDL) Large complex composed of a single protein molecule and many esterified cholesterol molecules, together with other lipids. The form in which cholesterol is transported in the blood and taken up into cells.

LTP see long-term potentiation

lumen Cavity enclosed by an epithelial sheet (in a tissue) or by a membrane (in a cell).

lymph Colorless fluid derived from blood by filtration through capillary walls. Carries lymphocytes in a special system of ducts and vessels—the lymphatic vessels.

lymphocyte Type of white blood cell responsible for the specificity of adaptive immune responses. There are two main types: B cells, which produce antibody, and T cells, which interact directly with other effector cells of the immune system and with infected cells. T cells develop in the thymus and are responsible for cell-mediated immunity. B cells develop in the bone marrow in mammals and are responsible for the production of circulating antibodies.

lymphoid organ Organs involved in the production or function of lymphocytes, such as thymus, spleen, lymph nodes, and tonsils.

lysis Rupture of a cell's plasma membrane, leading to the release of cytoplasm and the death of the cell.

lysogeny State of a bacterium in which it carries the DNA of an inactive virus integrated into its genome. The virus can subsequently be activated to replicate and lyse the cell.

lysosome Membrane-bounded organelle in eucaryotic cells containing digestive enzymes, which are typically most active at the acid pH found in the lumen of lysosomes.

lysozyme Enzyme that catalyzes the cutting of polysaccharide chains in the cell walls of bacteria.

M phase Period of the eucaryotic cell cycle during which the nucleus and cytoplasm divide.

M6P see mannose 6-phosphate

macromolecule Molecule such as a protein, nucleic acid, or polysaccharide with a molecular mass greater than a few thousand daltons.

macrophage Phagocytic cell derived from blood monocytes, typically resident in most tissues. It has both scavenger and antigen-presenting functions in immune responses.

major histocompatibility complex (MHC) Complex of highly polymorphic genes in vertebrates. They code for a large family of cell-surface glycoproteins (MHC molecules) that bind peptide fragments of foreign proteins and present them to T cells to induce an immune response. (See Figure 24–50.)

malaria Potentially fatal human disease caused by the protozoan parasite Plasmodium, which is transmitted by the bite of an infected mosquito.

malignant Describes tumors and tumor cells that are invasive and/or able to undergo metasis. A malignant tumor is a cancer.

mannose 6-phosphate (M6P) Unique marker attached to the oligosaccharides on some glycoproteins destined for lysosomes.

map unit see genetic map distance

MAP see microtubule-associated protein

MAP-kinase (mitogen-activated protein kinase) Protein kinase that performs a crucial step in relaying signals from the plasma membrane to the nucleus. Turned on by a wide range of proliferation- or differentiation-inducing signals.

mating-type locus (MAT locus) In budding yeast, the locus that determines the mating type (α or a) of the haploid yeast cell.

matrix space (1) Central subcompartment of a mitochondrion, bounded by the inner mitochondrial membrane. (2) The corresponding compartment in a chloroplast, which is more commonly known as the stroma.

M-Cdk see M-phase Cdk

Mcm proteins Proteins in the eucaryotic cell that bind to origin recognition complexes in DNA in early G1 and are involved in forming the pre-replicative complex.

M-cyclin Type of cyclin found in all eucaryotic cells that promotes the events of mitosis.

MDR protein see multidrug resistance protein

mega- Prefix denoting 106. (From Greek megas, huge, powerful.)

megakaryocyte Large myeloid cell with a multilobed nucleus that remains in the bone marrow when mature. It buds off platelets from long cytoplasmic processes.

meiosis Special type of cell division by which eggs and sperm cells are produced. It comprises two successive nuclear divisions with only one round of DNA replication, which produces four haploid daughter cells from an initial diploid cell.

melanocyte Cell that produces the dark pigment melanin. Responsible for the pigmentation of skin and hair.

membrane The lipid bilayer plus associated proteins that encloses all cells and, in eucaryotic cells, many organelles as well.

membrane-bound ribosome Ribosome attached to the cytosolic face of the endoplasmic reticulum. The site of synthesis of proteins that enter the endoplasmic reticulum.

membrane channel Transmembrane protein complex that allows inorganic ions or other small molecules to diffuse passively across the lipid bilayer.

membrane potential Voltage difference across a membrane due to a slight excess of positive ions on one side and of negative ions on the other. A typical membrane potential for an animal cell plasma membrane is –60 mV (inside negative relative to the surrounding fluid).

membrane protein Protein that is normally closely associated with a cell membrane. (See Figure 10–17.)

membrane transport Movement of molecules across a membrane mediated by a membrane transport protein.

membrane transport protein Membrane protein that mediates the passage of ions or molecules across a membrane. Examples are ion channels and carrier proteins.

meristem An organized group of dividing cells whose derivatives give rise to the tissues and organs of a flowering plant. Key examples are the apical meristems at the tips of shoots and roots.

mesenchyme Immature, unspecialized form of connective tissue in animals, consisting of cells embedded in a thin extracellular matrix.

mesoderm Embryonic tissue that is the precursor to muscle, connective tissue, skeleton and many of the internal organs.

messenger RNA (mRNA) RNA molecule that specifies the amino acid sequence of a protein. Produced by RNA splicing (in eucaryotes) from a larger RNA molecule made by RNA polymerase as a complementary copy of DNA. It is translated into protein in a process catalyzed by ribosomes.

metabolism The sum total of the chemical processes that take place in living cells.

metaphase Stage of mitosis at which chromosomes are firmly attached to the mitotic spindle at its equator but have not yet segregated toward opposite poles.

metaphase plate Imaginary plane at right angles to the mitotic spindle and midway between the spindle poles; the plane in which chromosomes are positioned at metaphase.

metaplasia A change in the pattern of cell differentiation in a tissue.

metastasis Spread of cancer cells from their site of origin to other sites in the body.

methyl (–CH3) Hydrophobic chemical group derived from methane (CH4).

MHC molecule One of a large family of ubiquitous cell-surface glycoproteins encoded by genes of the major histocompatibility complex (MHC). They bind peptide fragments of foreign antigens and present them to T cells to induce an immune response. (See also class I MHC molecule, class II MHC molecule.)

MHC see major histocompatibility complex

micro- Prefix denoting 10–6.

microelectrode, micropipette Piece of fine glass tubing pulled to an even finer tip. Used to penetrate a cell to study its physiology or to inject electric current or molecules.

microfilament see actin filament

micrograph Photograph of an image seen through a microscope. May be either a light micrograph or an electron micrograph depending on the type of microscope employed.

microinjection Injection of molecules into a cell using a micropipette.

micron (μm or micrometer) Unit of measurement often applied to cells and organelles. Equal to 10–6 meter or 10–4 centimeter.

micropipette see microelectrode

microsome Small vesicle that is derived from fragmented endoplasmic reticulum produced when cells are homogenized.

microtubule Long hollow cylindrical structure composed of the protein tubulin. It is one of the three major classes of filaments of the cytoskeleton. (See Panel 16–1, p. 909.)

microtubule-associated protein (MAP) Any protein that binds to microtubules and modifies their properties. Many different kinds have been found, including structural proteins, such as MAP-2, and motor proteins, such as dynein.

microtubule-organizing center (MTOC) Region in a cell, such as a centrosome or a basal body, from which microtubules grow.

microvillus (microvilli) Thin cylindrical membrane-covered projection on the surface of an animal cell containing a core bundle of actin filaments. Present in especially large numbers on the absorptive surface of intestinal epithelial cells.

midbody Structure formed at the end of cleavage that can persist for some time as a tether between the two daughter cells in animals.

milli- Prefix denoting 10–3.

minus end The end of a microtubule or actin filament at which the addition of monomers occurs least readily; the “slow-growing” end of the microtubule or actin filament. The minus end of an actin filament is also known as the pointed end. (See Panel 16–2, pp. 912–913.)

mismatch repair DNA repair process that corrects mismatched nucleotides inserted during DNA replication. A short stretch of newly synthesized DNA including the mismatched nucleotide is removed and replaced with the correct sequence with reference to the template strand.

mitochondrial precursor protein Mitochondrial protein encoded by a nuclear gene, synthesized in the cytosol, and subsequently transported into mitochondria.

mitochondrion (mitochondria) Membrane-bounded organelle, about the size of a bacterium, that carries out oxidative phosphorylation and produces most of the ATP in eucaryotic cells.

mitogen An extracellular substance, such as a growth factor, that stimulates cell proliferation.

mitogen-activated protein kinase see MAP-kinase

mitosis Division of the nucleus of a eucaryotic cell, involving condensation of the DNA into visible chromosomes, and separation of the duplicated chromosomes to form two identical sets. (From Greek mitos, a thread, referring to the threadlike appearance of the condensed chromosomes.)

mitotic chromosome Highly condensed duplicated chromosome with the two new chromosomes still held together at the centromere as sister chromatids.

mitotic spindle Array of microtubules and associated molecules that forms between the opposite poles of a eucaryotic cell during mitosis and serves to move the duplicated chromosomes apart.

model organism A species, such as Drosophila melanogaster or Escherichia coli, that has been studied intensively over a long period and thus serves as a “model” of the biology of a particular type of organism.

module In proteins or nucleic acids, a unit of structure or function that is found in a variety of different contexts in different molecules.

molar Describes a solution with a concentration of 1 mole of a substance dissolved in 1 liter of solution (abbreviated as 1 M).

mole X grams of a substance, where X is its relative molecular mass (molecular weight). A mole consists of 6 × 1023 molecules of the substance.

molecular chaperone see chaperone

molecular weight Numerically, the same as the relative molecular mass of a molecule expressed in daltons. For example, a protein of relative molecular mass 20,000 has a molecular weight of 20,000.

molecule Group of atoms joined together by covalent bonds.

monoclonal antibody Antibody secreted by a hybridoma clone. Because each such clone is derived from a single B cell, all of the antibody molecules produced are identical.

monocyte Type of white blood cell that leaves the bloodstream and matures into a macrophage in tissues.

monomer Small molecular building block that can serve as a subunit, being linked to others of the same type to form a larger molecule (a polymer).

monosaccharide Simple sugar with the general formula (CH2O)n, where n = 3 to 8.

morphogen Signal molecule that can impose a pattern on a field of cells by causing cells in different places to adopt different fates.

mosaic In developmental biology, an organism made of a mixture of cells with different genotypes.

motif Element of structure or pattern that recurs in many contexts. Specifically, a small structural domain that can be recognized in a variety of proteins.

motor protein Protein that uses energy derived from nucleoside triphosphate hydrolysis to propel itself along a protein filament or another polymeric molecule.

M-phase Cdk (M-Cdk) Complex formed in vertebrate cells by an M-cyclin and the corresponding cyclin-dependent kinase (Cdk).

mRNA see messenger RNA

MTOC see microtubule-organizing center

multidrug resistance protein (MDR protein) Type of ABC transporter protein that can pump hydrophobic drugs (such as some anti-cancer drugs) out of the cytoplasm of eucaryotic cells.

multipass transmembrane protein Membrane protein in which the polypeptide chain crosses the lipid bilayer more than once.

mutant Organism in which a mutation has occurred that makes it different from wild-type or from the ‘normal' extent of variation in the population.

mutation rate The rate at which observable changes occur in a DNA sequence.

mutation Heritable change in the nucleotide sequence of a chromosome.

myelin sheath Insulating layer of specialized cell membrane wrapped around vertebrate axons. Produced by oligodendrocytes in the central nervous system and by Schwann cells in the peripheral nervous system.

myeloid cell Any white blood cell other than lymphocytes.

myoblast Mononucleated, undifferentiated muscle precursor cell. A skeletal muscle cell is formed by the fusion of multiple myoblasts.

myoepithelial cell Type of unstriated muscle cell found in epithelia, e.g. in the iris of the eye and in glandular tissue.

myofibril Long, highly organized bundle of actin, myosin, and other proteins in the cytoplasm of muscle cells that contracts by a sliding filament mechanism.

N terminus see amino terminus

Na+-K+ pump (Na+-K+ ATPase) Transmembrane carrier protein found in the plasma membrane of most animal cells that pumps Na+ out of and K+ into the cell, using energy derived from ATP hydrolysis.

NAD+ (nicotine adenine dinucleotide) Activated carrier that participates in an oxidation reaction by accepting a hydride ion (H) from a donor molecule. The NADH formed is an important carrier of electrons for oxidative phosphorylation.

NADH dehydrogenase complex First of the three electron-driven proton pumps in the mitochondrial respiratory chain. It accepts electrons from NADH.

NADP+ (nicotine adenine dinucleotide phosphate) Activated carrier closely related to NAD+ that is used extensively in biosynthetic, rather than catabolic, pathways. The reduced form is NADPH.

nano- Prefix denoting 10–9.

nanometer (nm) Unit of length commonly used to measure molecules and cell organelles. 1 nm = 10–3 micrometer (μm) = 10–9 meter.

natural killer cell (NK cell) Cytotoxic cell of the innate immune system that can kill virus-infected cells.

N-CAM see neural cell adhesion molecule

negative control Type of control of gene expression in which the active DNA-binding form of the regulatory protein turns the gene off.

negative staining Staining technique for use in the electron electron microscope in which a reverse, or negative, image of the object is created.

Nernst equation Quantitative expression that relates the equilibrium ratio of concentrations of an ion on either side of a permeable membrane to the voltage difference across the membrane. (See Panel 11–2, p. 634.)

nerve cell see neuron

neural cell adhesion molecule (N-CAM) Cell adhesion molecule of the immunoglobulin superfamily, expressed by many cell types including most nerve cells. It mediates Ca2+-independent cell-cell attachment in vertebrates.

neural tube Tube of ectoderm that will form the brain and spinal cord in a vertebrate embryo.

neurite Long process growing from a nerve cell in culture. A generic term that does not specify whether the process is an axon or a dendrite.

neurofilament Type of intermediate filament found in nerve cells.

neuromuscular junction Specialized chemical synapse between an axon terminal of a motor neuron and a skeletal muscle cell.

neuron (nerve cell) Cell with long processes specialized to receive, conduct, and transmit signals in the nervous system.

neuropeptide Peptide secreted by neurons as a signaling molecule either at synapses or elsewhere.

neurotransmitter Small signal molecule secreted by the presynaptic nerve cell at a chemical synapse to relay the signal to the postsynaptic cell. Examples include acetylcholine, glutamate, GABA, glycine, and many neuropeptides.

neutron Uncharged subatomic particle that forms part of an atomic nucleus.

neutrophil White blood cell that is specialized for the uptake of particulate material by phagocytosis and which enters tissues that become infected or inflamed.

nicotine adenine dinucleotide phosphate see NADP+

nicotine adenine dinucleotide see NAD+

nitric oxide (NO) Gaseous signal molecule in both animals and plants. In animals it regulates smooth muscle contraction, for example; in plants it is involved in responses to injury or infection.

nitrogen cycle The natural circulation of nitrogen between molecular nitrogen in the atmosphere, inorganic molecules in the soil, and organic molecules in living organisms.

nitrogen fixation Biochemical process carried out by certain bacteria that reduces atmospheric nitrogen (N2) to ammonia, leading eventually to various nitrogen-containing metabolites.

nitrogenase complex Complex of enzymes in nitrogen-fixing bacteria that catalyzes the reduction of atmospheric N2 to ammonia.

NK cell see natural killer cell

N-linked oligosaccharide Chain of sugars attached to a protein through the NH2 group of the side chain of an asparagine residue.

NMR (nuclear magnetic resonance) Resonant absorption of electromagnetic radiation at a specific frequency by atomic nuclei in a magnetic field, due to flipping of the orientation of their magnetic dipole moments. The NMR spectrum provides information about the chemical environment of the nuclei. Two-dimensional NMR is used widely to determine the three-dimensional structure of small proteins.

nm see nanometer

noncovalent attraction Chemical bond in which, in contrast to a covalent bond, no electrons are shared. Noncovalent bonds are relatively weak, but they can sum together to produce strong, highly specific interactions between molecules.

noncyclic photophosphorylation Photosynthetic process that produces both ATP and NADPH in plants and cyanobacteria.

nonenveloped virus Virus consisting of a nucleic acid core and protein capsid only.

nonpolar (apolar) Lacking any asymmetric accumulation of positive and negative charge. Nonpolar molecules are generally insoluble in water.

nonsense-mediated mRNA decay Mechanism for removing aberrant mRNAs containing in-frame internal stop codons before they can be translated.

Northern blotting Technique in which RNA fragments separated by electrophoresis are immobilized on a paper sheet. A specific RNA is then detected by hybridization with a labeled nucleic acid probe.

NO see nitric oxide

Notch Receptor protein involved in many instances of choice of cell fate in animal development, for example in the specification of nerve cells from ectodermal epithelium. Its ligands are cell-surface proteins such as Delta and Serrate.

notochord Stiff rod of mesoderm that runs along the back of all chordate embryos. In vertebrates it does not persist and becomes incorporated into the vertebral column.

NSF Protein with ATPase activity that disassembles a complex of a v-SNARE and a t-SNARE.

nuclear envelope Double membrane surrounding the nucleus. Consists of an outer and inner membrane and is perforated by nuclear pores.

nuclear export signal Sorting signal contained in the structure of molecules and complexes, such as RNA and new ribosomal subunits, that are transported from the nucleus to the cytosol through nuclear pore complexes.

nuclear lamin Protein subunit of the intermediate filaments of the nuclear lamina.

nuclear lamina Fibrous meshwork of proteins on the inner surface of the inner nuclear membrane. It is made up of a network of intermediate filaments formed from nuclear lamins.

nuclear localization signal (NLS) Signal sequences or signal patches found in proteins destined for the nucleus and which enable their selective transport into the nucleus from the cytosol through the nuclear pore complexes.

nuclear magnetic resonance see NMR

nuclear pore complex Large multiprotein structure forming a channel (the nuclear pore) through the nuclear envelope that allows selected molecules to move between nucleus and cytoplasm.

nuclear receptor superfamily Intracellular receptors for hydrophobic signal molecules such as steroids and retinoic acid. The receptor-ligand complex acts as a transcription factor in the nucleus.

nuclear transport Movement of macromolecules into or out of the nucleus mediated by nuclear transport receptors.

nucleation Critical stage in the assembly of a polymeric structure, such as a microtubule, at which a small cluster of monomers aggregates in the correct arrangement to initiate rapid polymerization. (See Panel 16–2, pp. 912–913.) More generally, the rate-limiting step in an assembly process.

nucleic acid RNA or DNA, a macromolecule consisting of a chain of nucleotides joined together by phosphodiester bonds.

nucleolar organizer Region of a chromosome containing a cluster of ribosomal RNA genes that gives rise to a nucleolus.

nucleolus Structure in the nucleus where ribosomal RNA is transcribed and ribosomal subunits are assembled.

nucleoporin Any of a number of different proteins that make up nuclear pore complexes.

nucleoside Molecule composed of a purine or pyrimidine base covalently linked to a ribose or deoxyribose sugar. (See Panel 2–6, pp. 120–121.)

nucleosome Beadlike structure in eucaryotic chromatin. It is composed of a short length of DNA wrapped around a core of histone proteins, and is the fundamental structural unit of chromatin.

nucleotide Nucleoside with one or more phosphate groups joined in ester linkages to the sugar moiety. DNA and RNA are polymers of nucleotides. (See Panel 2–6, pp. 120–121.)

nucleus Prominent membrane-bounded organelle in a eucaryotic cell, containing DNA organized into chromosomes.

nurse cell Cell connected by cytoplasmic bridges to a developing oocyte and which thereby supplies it with ribosomes, mRNAs, and proteins needed for the development of the early embryo.

occluding junction Type of cell junction that seals cells together in an epithelium, forming a barrier through which even small molecules cannot pass.

Okazaki fragments Short lengths of DNA produced on the lagging strand during DNA replication. They are rapidly joined by DNA ligase to form a continuous DNA strand.

oligodendrocyte Type of glial cell in the vertebrate central nervous system that forms a myelin sheath around axons.

oligomer Short polymer, usually consisting (in a cell) of amino acids (oligopeptides), sugars (oligosaccharides), or nucleotides (oligonucleotides). (From Greek oligos, few, little.)

oligosaccharide Short linear or branched chain of covalently linked sugars (see Panel 2–4, pp. 116–117.) (See also complex oligosaccharide, high-mannose oligosaccharide, N-linked oligosacharide, O-linked glycosylation.)

O-linked glycosylation Addition of an oligosaccharide chain to a protein through the OH group of a serine or threonine side chain.

oncogene An altered gene whose product can act in a dominant fashion to help make a cell cancerous. Typically, an oncogene is a mutant form of a normal gene (proto-oncogene) involved in the control of cell growth or division.

oocyte The developing egg. It is usually a large and immobile cell.

oogenesis Formation and maturation of oocytes in the ovary.

operator Short region of DNA in a bacterial chromosome that controls the transcription of an adjacent gene.

operon In a bacterial chromosome, a group of contiguous genes that are transcribed into a single mRNA molecule.

ORC see origin recognition complex

organelle Membrane-enclosed compartment in a eucaryotic cell that has a distinct structure, macromolecular composition, and function. Examples are nucleus, mitochondrion, chloroplast, Golgi apparatus.

Organizer see Spemann's Organizer

origin recognition complex (ORC) Large protein complex that is bound to the DNA at origins of replication in eucaryotic chromosomes throughout the cell cycle.

osmolarity A term used to describe the concentration of a solute in terms of the osmotic pressure it can exert.

osmosis Net movement of water molecules across a semipermeable membrane driven by a difference in concentration of solute on either side. The membrane must be permeable to water but not to the solute molecules.

osteoblast Cell that secretes matrix of bone.

osteoclast Macrophage-like cell that erodes bone, enabling it to be remodeled during growth and in response to stresses throughout life.

outer membrane Outermost of the two membranes surrounding an organelle; the membrane adjacent to the cytosol.

outer nuclear membrane The outermost of the two nuclear membranes. It is continuous with the endoplasmic reticulum and is studded with ribosomes on its cytosolic face.

overlap microtubule In the mitotic or meiotic spindle, a microtubule interdigitating at the equator with the microtubules emanating from the other pole.

ovulation Release of an egg from the ovary.

ovum see egg

oxidation (verb oxidize) Loss of electrons from an atom, as occurs during the addition of oxygen to a molecule or when a hydrogen is removed. Opposite of reduction. (See Figure 2–43.)

oxidative phosphorylation Process in bacteria and mitochondria in which ATP formation is driven by the transfer of electrons from food molecules to molecular oxygen. Involves the intermediate generation of a proton gradient (pH gradient) across a membrane and chemiosmotic coupling.

p53 Tumor suppressor gene found mutated in about half of human cancers. It encodes a gene regulatory protein that is activated by damage to DNA and is involved in blocking further progression through the cell cycle.

pachytene Third stage of division I of meiosis, in which synapsis is complete.

palindromic sequence Nucleotide sequence that is identical to its complementary strand when each is read in the same chemical direction—for example, GATC.

paracrine signaling Short-range cell-cell communication via secreted signal molecules that act on adjacent cells.

parthenogenesis Production of a new individual from an egg cell in the absence of fertilization by sperm.

passive transport Transport of a solute across a membrane down its concentration gradient or its electrochemical gradient, using only the energy stored in the gradient.

patch-clamp recording Electrophysiological technique in which a tiny electrode tip is sealed onto a patch of cell membrane, thereby making it possible to record the flow of current through individual ion channels in the patch.

pathogen (adjective pathogenic) An organism or other agent that causes disease.

PCR (polymerase chain reaction) Technique for amplifying specific regions of DNA by the use of sequence-specific primers and multiple cycles of DNA synthesis, each cycle being followed by a brief heat treatment to separate complementary strands.

peptide bond Chemical bond between the carbonyl group of one amino acid and the amino group of a second amino acid—a special form of amide linkage. Peptide bonds link amino acids together in proteins. (See Panel 3–1, pp. 132–133.)

peptide map Characteristic two-dimensional pattern (on paper or gel) formed by the separation of the mixture of peptides produced by the partial digestion of a protein.

peripheral lymphoid organ (secondary lymphoid organ) Lymphoid organ in which T cells and B cells interact with foreign antigens. Examples are spleen, lymph nodes, and mucosal-associated lymphoid tissue.

peripheral membrane protein Protein that is attached to one face of a membrane by noncovalent interactions with other membrane proteins, and which can be removed by relatively gentle treatments that leave the lipid bilayer intact.

peroxisome Small membrane-bounded organelle that uses molecular oxygen to oxidize organic molecules. Contains some enzymes that produce and others that degrade hydrogen peroxide (H2O2).

pH Common measure of the acidity of a solution: “p” refers to power of 10, “H” to hydrogen. Defined as the negative logarithm of the hydrogen ion concentration in moles per liter (M). Thus on the pH scale, pH 3 (10–3 M H+) is acidic and pH 9 (10–9 M H+) is alkaline.

PH domain see pleckstrin homology domain

phage display Technique for detecting proteins that interact with each other by screening a protein against a library of genetically modified phage, each displaying a potential binding protein on their surface.

phage see bacteriophage

phagocyte General term for a professional phagocytic cell—that is, a cell such as a macrophage or neutrophil that is specialized to take up particles and microorganisms by phagocytosis.

phagocytosis Process by which particulate material is endocytosed (“eaten”) by a cell. Prominent in carnivorous cells, such as Amoeba proteus, and in vertebrate macrophages and neutrophils. (From Greek phagein, to eat.)

phagosome Large intracellular membrane-bounded vesicle that is formed as a result of phagocytosis. Contains ingested extracellular material.

phase-contrast microscope Type of light microscope that exploits the interference effects that occur when light passes through material of different refractive indexes. Used to view living cells.

phenotype The observable character of a cell or an organism.

phosphatase Enzyme that removes phosphate groups from a molecule.

phosphatidylinositol 3-kinase (PI 3-kinase) A kinase involved in intracellular signaling pathways activated by a variety of cell-surface receptors. It phosphorylates inositol phospholipids at the 3 position of the inositol ring. (SeeFigure 15–58.)

phosphatidylinositol An inositol phospholipid. (See Figure 15–34.)

phosphodiester linkage Set of covalent chemical bonds formed when two hydroxyl groups are linked in ester linkage to the same phosphate group. This linkage joins adjacent nucleotides in RNA or DNA. (See Figure 2–28.)

phosphoinositide see inositol phospholipid

phospholipase C-β (PLC-β) Enzyme bound to the cytoplasmic face of the plasma membrane that converts membrane phosphatidylinositol 4,5-bisphosphate to diacylglycerol (which remains in the plasma membrane) and inositol 1,4,5-trisphosphate (IP3). It is activated by certain G proteins to trigger the inositol phospholipid signaling pathway.

phospholipase C-γ (PLC-γ) Like phospholipase C-β, an enzyme that cleaves inositol phospholipids to diacylglycerol and IP3 to trigger the inositol phospholipid signaling pathway. Activated by certain receptor tyrosine kinases.

phospholipid exchange protein Water-soluble carrier protein that transfers a phospholipid molecule from one membrane to another.

phospholipid The main category of lipid molecules used to construct biological membranes. Generally composed of two fatty acids linked through glycerol phosphate to one of a variety of polar groups.

phosphoprotein phosphatase Enzyme that removes a phosphate group from a protein by hydrolysis.

phosphorylation Reaction in which a phosphate group becomes covalently coupled to another molecule.

photochemical reaction center The part of a photosystem that converts light energy into chemical energy.

photon Elementary particle of light and other electromagnetic radiation.

photoreceptor Cell or molecule that is sensitive to light.

photosynthesis Process by which plants, algae and some bacteria use the energy of sunlight to drive the synthesis of organic molecules from carbon dioxide and water.

photosynthetic electron-transfer Light-driven reactions in photosynthesis in which electrons move along the electron-transport chain in the thylakoid membrane, generating ATP and NADPH.

photosystem Multiprotein complex involved in photosynthesis that captures the energy of sunlight and converts it to useful forms of energy.

phragmoplast Structure made of microtubules and actin filaments that forms in the prospective plane of division of a plant cell and guides formation of the cell plate.

phylogeny Evolutionary history of an organism or group of organisms, often presented in chart form as a phylogenetic tree.

pinocytosis Type of endocytosis in which soluble materials are taken up from the environment and incorporated into vesicles for digestion. Literally, “cell drinking.” (See also fluid-phase endocytosis.)

PKA see cyclic AMP-dependent protein kinase

PKC see protein kinase C

plant growth regulator Signal molecule (also known as a plant hormone) that helps coordinate growth and development. Examples are ethylene, auxins, gibberellins, cytokines, abscisic acid, and the brassinosteroids.

plasma membrane Membrane that surrounds a living cell.

plasmid Small circular DNA molecule that replicates independently of the genome. Modified plasmids are used extensively as plasmid vectors for DNA cloning.

plasmodesma (plasmodesmata) Communicating cell–cell junction in plants in which a channel of cytoplasm lined by plasma membrane connects two adjacent cells through a small pore in their cell walls.

plastid Cytoplasmic organelle in plants, bounded by a double membrane, that carries its own DNA and is often pigmented. Chloroplasts are plastids.

platelet Cell fragment, lacking a nucleus, that breaks off from a megakaryocyte in the bone marrow and is found in large numbers in the bloodstream. It helps initiate blood clotting when blood vessels are injured.

PLC-β see phospholipase C-β

PLC-γ see phospholipase C-γ

pleckstrin homology domain (PH domain) Protein domain found in intracellular signaling proteins by which they bind to inositol phospholipids phosphorylated by PI 3-kinase.

ploidy The number of complete sets of chromosomes in a genome. Diploid organisms have two sets in their somatic cells, polyploid organisms more than two. Natural polyploidy is the result of previous duplications of the whole genome or the introduction of complete genomes from another species during evolution.

plus end The end of a microtubule or actin filament at which addition of monomers occurs most readily; the “fast-growing” end of a microtubule or actin filament. The plus end of an actin filament is also known as the barbed end. (See Panel 16–2, pp. 912–913.)

point mutation Change of a single nucleotide in DNA, especially in a region of DNA coding for protein.

polar In the electrical sense, describes a structure (for example, a chemical bond, chemical group, or molecule) with positive charge concentrated toward one end and negative charge toward the other as a result of an uneven distribution of electrons. Polar molecules are likely to be soluble in water.

polyisoprenoid– see isoprenoid

polymer Large molecule made by covalently linking multiple identical or similar units (monomers) together.

polymerase chain reaction see PCR

polymorphic Describes a gene with many different alleles, none of which is predominant in the population.

polypeptide Linear polymer composed of multiple amino acids. Proteins are large polypeptides, and the two terms can be used interchangeably.

polypeptide backbone The chain of repeating carbon and nitrogen atoms, linked by peptide bonds, in a polypeptide or protein. The side chains of the amino acids project from this backbone.

polyploid Describes a cell or an organism that contains more than two sets of homologous chromosomes.

polyribosome (polysome) Messenger RNA molecule to which are attached a number of ribosomes engaged in protein synthesis.

polysaccharide Linear or branched polymer of monosaccharides. They include glycogen, starch, hyaluronic acid, and cellulose.

polytene chromosome Giant chromosome in which the DNA has undergone repeated replication without separation into new chromosomes.

position effect Differences in gene expression that depend on the position of the gene on the chromosome and probably reflect differences in the state of the chromatin along the chromosome.

positional information Information supplied to or possessed by cells according to their position in a multicellular organism. A cell's internal record of its positional information is called its positional value.

positive control Type of control of gene expression in which the active DNA-binding form of the regulatory protein turns the gene on.

posterior Situated toward the tail end of the body.

posttranscriptional control Any control on gene expression that is exerted at a stage after transcription has begun.

posttranslational Describes any process involving a protein that occurs after protein synthesis is completed.

posttranslational modification The enzyme-catalyzed change to a protein made after it is synthesized. Examples are acetylation, cleavage, glycosylation, methylation, phosphorylation, and prenylation.

pre-B cell Iimmediate precursor of a B cell.

prenylation Covalent attachment of an isoprenoid lipid group to a protein.

preprophase band Circumferential band of microtubules and actin filaments that forms around a plant cell under the plasma membrane prior to mitosis and cell division.

primary immune response Adaptive immune response to an antigen that is made on first encounter with that antigen.

primary structure Sequence of monomer units in a linear polymer, such as the amino acid sequence of a protein.

primordial germ cell Cell set aside early in embryonic development that is a precursor to germ cells that give rise to gametes.

primosome A complex of DNA primase and DNA helicase that is formed on the lagging strand during DNA replication, improving the efficiency of replication.

prion An infectious abnormal form of a normal protein that is replicated in the host by forcing the normal proteins of the same type to adopt the aberrant structure.

prion disease Transmissible spongiform encephalopathies such as Kreutzfeld–Jacob disease in humans, scrapie in sheep and bovine spongiform encephalopathy (BSE) in cattle, that are apparently caused and transmitted by abnormal forms of a protein (prions).

probe Defined fragment of RNA or DNA, radioactively or chemically labeled, used to locate specific nucleic acid sequences by hybridization.

procaryote (prokaryote) Single-celled microorganism whose cells lack a well-defined, membrane-enclosed nucleus. The procaryotes comprise two of the major domains of living organisms—the Bacteria and the Archaea.

programmed cell death see apoptosis

prometaphase Phase of mitosis preceding metaphase in which the nuclear envelope breaks down and chromosomes first attach to the spindle.

promoter Nucleotide sequence in DNA to which RNA polymerase binds to begin transcription.

prophase First stage of mitosis, during which the chromosomes are condensed but not yet attached to a mitotic spindle.

protease (proteinase, proteolytic enzyme) Enzyme such as trypsin that degrades proteins by hydrolyzing some of their peptide bonds.

proteasome Large protein complex in the cytosol with proteolytic activity that is responsible for degrading proteins that have been marked for destruction by ubiquitylation or by some other means.

protein The major macromolecular constituent of cells. A linear polymer of amino acids linked together by peptide bonds in a specific sequence.

protein domain Portion of a protein that has a tertiary structure of its own. Larger proteins are generally composed of several domains, each connected to the next by short flexible regions of polypeptide chain.

protein glycosylation Posttranslational addition of oligosaccharide side chains to a protein.

protein kinase Enzyme that transfers the terminal phosphate group of ATP to a specific amino acid of a target protein.

protein kinase C (PKC) Ca2+-dependent protein kinase that, when activated by diacylglycerol and an increase in the concentration of Ca2+, phosphorylates target proteins on specific serine and threonine residues.

protein module see module

protein phosphatase see phosphoprotein phosphatase

protein phosphorylation The covalent addition of a phosphate group to a side chain of a protein catalyzed by a protein kinase.

protein translocator Membrane-bound protein that mediates the transport of another protein across an organelle membrane.

proteoglycan Molecule consisting of one or more glycosaminoglycan (GAG) chains attached to a core protein.

proteolysis Degradation of a protein by hydrolysis at one or more of its peptide bonds.

proteolytic enzyme see protease

protofilament A linear chain of protein subunits joined end to end, which associates laterally with other protofilaments to form cytoskeletal components such as microtubules and intermediate filaments.

proton Positively charged subatomic particle that forms part of an atomic nucleus. Hydrogen has a nucleus composed of a single proton (H+).

proton-motive force Driving force that moves protons across a membrane as a result of an electrochemical proton gradient.

proto-oncogene Normal gene, usually concerned with the regulation of cell proliferation, that can be converted into a cancer-promoting oncogene by mutation.

protozoa Free-living or parasitic, nonphotosynthetic, single-celled, motile eucaryotic organisms, such asParamecium and Amoeba. Free-living protozoa feed on bacteria or other microorganisms.

pseudogene Gene that has accumulated multiple mutations that has rendered it inactive and nonfunctional.

pseudopodium (pseudopodia) Large cell-surface protrusion formed by amoeboid cells as they crawl. More generally, any dynamic actin-rich extension of the surface of an animal cell.

pulse-chase Technique for following the movement of a substance through a biochemical or cellular pathway, by briefly adding the radioactively labeled substance (the pulse) followed by the unlabeled substance (the chase).

pump Transmembrane protein that drives the active transport of ions or small molecules across the lipid bilayer.

purine One of the two categories of nitrogen-containing ring compounds found in DNA and RNA. Examples are adenine and guanine. (See Panel 2–6, pp. 120–121.)

pyrimidine One of the two categories of nitrogen-containing ring compounds found in DNA and RNA. Cytosine, thymine and uracil are pyrimidines. (See Panel 2–6, pp. 120–121.)

quaternary structure Three-dimensional relationship of the different polypeptide chains in a multisubunit protein or protein complex.

quinone (Q) Small, lipid soluble, mobile electron carrier molecule found in the respiratory and photosynthetic electron-transport chains. (See Figure 14–24.)

Rab protein Any of a large family of monomeric GTPases present in the plasma membrane and organelle membranes that are involved in conferring specificity on vesicle docking.

radioactive isotope Form of an atom with an unstable nucleus that emits radiation as it decays.

Ran Monomeric GTPase present in both cytosol and nucleus that is required for the active transport of macromolecules into and out of the nucleus through nculear pore complexes. Hydrolysis of GTP to GDP is thought to provide the energy required for this transport.

Ras protein The most famous member of a large family of GTP-binding proteins (called monomeric GTPases) that help relay signals from cell-surface receptors to the nucleus. Named for the ras gene, first identified in viruses that cause rat sarcomas.

reaction In chemistry, any process in which one molecule is converted into another by the removal or addition of atoms, or in which the arrangement of atoms in a molecule or molecules is altered by a change in chemical bonds.

reading frame The phase in which nucleotides are read in sets of three to encode a protein. A messenger RNA molecule can be read in any one of three reading frames, only one of which will give the required protein. (SeeFigure 6–51.)

RecA protein The prototype for a class of DNA-binding proteins that catalyze synapsis of DNA strands during genetic recombination.

receptor Protein that binds a specific extracellular signal molecule (ligand) and initiates a response in the cell. Cell-surface receptors, such as the acetylcholine receptor and the insulin receptor, are located in the plasma membrane, with their ligand-binding site exposed to the external medium. Intracellular receptors, such as steroid hormone receptors, bind ligands that diffuse into the cell across the plasma membrane.

receptor-mediated endocytosis Internalization of receptor-ligand complexes from the plasma membrane by endocytosis, It is used to take up some macromolecules, such as cholesterol-containing lipoproteins, from the extracellular fluid, and is also a means of recycling receptor proteins once they have bound their ligands.

recessive In genetics, refers to the member of a pair of alleles that fails to be expressed in the phenotype of the organism when the dominant allele is present. Also refers to the phenotype of an individual that has only the recessive allele.

recombinant DNA Any DNA molecule formed by joining DNA segments from different sources. Recombinant DNAs are widely used in the cloning of genes, in the genetic modification of organisms, and in molecular biology generally.

recombination Process in which DNA molecules are broken and the fragments are rejoined in new combinations. Can occur in the living cell—for example, through crossing-over during meiosis—or in vitro using purified DNA and enzymes that break and ligate DNA strands.

recycling endosomes Large intracellular membrane-bounded vesicle formed from a fragment of an endosome that is an intermediate stage on the passage of recycled receptors back to the cell membrane.

red blood cell see erythrocyte

redox pair Pair of molecules in which one acts as an electron donor and one as an electron acceptor in an oxidation-reduction reaction; for example, NADH (electron donor) and NAD+ (electron acceptor).

redox potential The affinity of a redox pair for electrons, generally measured as the voltage difference between an equimolar mixture of the pair and a standard reference. NADH/NAD+ has a low redox potential and O2/H2 has a high redox potential (high affinity for electrons).

redox reaction A reaction in which one component becomes oxidized and the other reduced; an oxidation-reduction reaction.

reduction (verb reduce) Addition of electrons to an atom, as occurs during the addition of hydrogen to a molecule or the removal of oxygen from it. Opposite of oxidation. (See Figure 2–43.)

regulatory sequence DNA sequence to which a gene regulatory protein binds to control the rate of assembly of the transcirptional complex at the promoter.

regulatory site Site on an enzyme, other than the active site, that binds a molecule that affects enzyme activity.

replication fork Y-shaped region of a replicating DNA molecule at which the two daughter strands are formed and separate.

replication origin Location on a DNA molecule at which duplication of the DNA begins.

replicative cell senescence Phenomenon observed in primary cell cultures as they age, in which cell proliferation slows down and finally halts.

repressor Protein that binds to a specific region of DNA to prevent transcription of an adjacent gene.

residue General term for the unit of a polymer. That portion of a sugar, amino acid, or nucleotide that is retained as part of the polymer chain during the process of polymerization.

respiration General term for a process in a cell involving the oxidative breakdown of sugars or other organic molecules, and requiring the uptake of O2 while producing CO2 and H2O as waste products.

respiratory chain Electron-transport chain in the inner mitochondrial membrane that receives high-energy electrons derived from the citric acid cycle and generates the proton gradient across the membrane that is used to power ATP synthesis.

respiratory control Regulatory mechanism that controls the rate of electron transport in the respiratory chain according to need via a direct influence of the electrochemical proton gradient.

respiratory enzyme complex Any of the major protein complexes of the mitochondrial respiratory chain that act as electron-driven proton pumps to generate the proton gradient across the inner membrane.

resting membrane potential The membrane potential in equilibrium conditions in which there is no net flow of ions across the plasma membrane.

restriction map Diagrammatic representation of a DNA molecule indicating the sites of cleavage by various restriction enzymes.

restriction nuclease (restriction enzyme) One of a large number of nucleases that can cleave a DNA molecule at any site where a specific short sequence of nucleotides occurs. Extensively used in recombinant DNA technology.

restriction point Important checkpoint in the mammalian cell cycle. Passage through the restriction point commits the cell to enter S phase. It corresponds to Start in the yeast cell cycle.

retrotransposon Type of transposable element that moves by being first transcribed into an RNA copy that is then reconverted to DNA by reverse transcriptase and inserted elsewhere in the chromosomes.

retrovirus RNA-containing virus that replicates in a cell by first making a double-stranded DNA intermediate.

reverse genetics Approach to discovering gene function that starts from the DNA (gene) and protein and then creates mutants to analyze the gene's function.

reverse transcriptase Enzyme first discovered in retroviruses that makes a double-stranded DNA copy from a single-stranded RNA template molecule.

rhodopsin G-protein-linked light-sensitive receptor protein in the rod photoreceptor cells of the retina.

ribonuclease Enzyme that cuts an RNA molecule by hydrolyzing one or more of its phosphodiester bonds.

ribonucleic acid see RNA

ribosomal RNA (rRNA) Any one of a number of specific RNA molecules that form part of the structure of a ribosome and participate in the synthesis of proteins. Often distinguished by their sedimentation coefficient, such as 28S rRNA or 5S rRNA.

ribosome Particle composed of ribosomal RNAs and ribosomal proteins that associates with messenger RNA and catalyzes the synthesis of protein.

ribozyme RNA with catalytic activity.

RNA (ribonucleic acid) Polymer formed from covalently linked ribonucleotide monomers. (See also messenger RNA, ribosomal RNA, transfer RNA.)

RNA editing Production of a functional mRNA by insertion or alteration of individual nucleotides in an RNA molecule after it is synthesized.

RNA interference (RNAi) Selective intracellular degradation of RNA that is intended to remove foreign RNAs, such as those of viruses. Fragments cleaved from free double-stranded RNA direct the degradative mechanism to other similar RNA sequences. Widely exploited in a technique used to silence the expression of selected genes.

RNA polymerase II holoenzyme Large pre-assumbled complex of RNA polymerase II, most of the general transcription factors required for its function, and the mediator protein complex.

RNA polymerase Enzyme that catalyzes the synthesis of an RNA molecule on a DNA template from nucleoside triphosphate precursors. (See Figure 6–8.)

RNA primer Short stretch of RNA synthesized on a DNA template. It is required by DNA polymerases to start their DNA synthesis.

RNA processing control Control of gene expression by controlling how the RNA transcript is spliced or otherwise processed.

RNA splicing Process in which intron sequences are excised from RNA transcripts in the nucleus during formation of messenger and other RNAs.

RNAi see RNA interference

rod photoreceptor (rod) Photoreceptor cell type in the retina that is responsible for noncolor vision in dim light.

rough endoplasmic reticulum (rough ER) Endoplasmic reticulum with ribosomes on its cytosolic surface. Involved in the synthesis of secreted and membrane-bound proteins.

rRNA see ribosomal RNA

rRNA gene Gene that specifies a ribosomal RNA (rRNA).

S phase Period of a eucaryotic cell cycle in which DNA is synthesized.

Saccharomyces Genus of yeasts that reproduce asexually by budding or sexually by conjugation. Economically important in brewing and baking, they are also widely used in genetic engineering and as simple model organisms in the study of eucaryotic cell biology.

sarcoma Cancer of connective tissue.

sarcomere Repeating unit of a myofibril in a muscle cell, composed of an array of overlapping thick (myosin) and thin (actin) filaments between two adjacent Z discs.

sarcoplasmic reticulum Network of internal membranes in the cytoplasm of a muscle cell that contains high concentrations of sequestered Ca2+ which is released into the cytosol during muscle excitation.

satellite DNA Regions of highly repetitive DNA from a eucaryotic chromosome, usually identifiable by its unusual nucleotide composition. Satellite DNA is not transcribed and has no known function.

saturated Describes a molecule containing carbon–carbon bonds that has only single covalent bonds.

scaffold protein Protein that organizes groups of interacting intracellular signaling proteins into signaling complexes.

scanning electron microscope Type of electron microscope that produces an image of the surface of an object.

S-Cdk Complex formed in vertebrate cells by an S-cyclin and the corresponding cyclin-dependent kinase (Cdk).

Schwann cell Glial cell responsible for forming myelin sheaths in the peripheral nervous system.

SDS polyacrylamide gel electrophoresis (SDS-PAGE) Type of electrophoresis in which the protein mixture to be separated is run through a gel containing the detergent sodium dodecyl sulfate (SDS) which unfolds the proteins and frees them from association with other molecules.

second messenger Small molecule that is formed in or released into the cytosol in response to an extracellular signal and helps to relay the signal to the interior of the cell. Examples include cAMP, IP3, and Ca2+.

secondary immune response Adaptive immune response to an antigen that is made on a second or subsequent encounter with a given antigen. It is more rapid in onset, stronger, and more specific than the primary immune response.

secondary structure Regular local folding pattern of a polymeric molecule. In proteins, α helices and β sheets.

secretory vesicle Membrane-bounded organelle in which molecules destined for secretion are stored prior to release. Sometimes called secretory granule because darkly staining contents make the organelle visible as a small solid object.

section A very thin slice of tissue, suitable for viewing under the microscope.

selectin Member of a family of cell-surface carbohydrate-binding proteins that mediate transient, Ca2+-dependent cell-cell adhesion in the bloodstream, for example between white blood cells and the endothelium of the blood vessel wall.

selectivity filter That part of an ion channel structure that determines which ions it can transport.

septate junction Main type of occluding cell junction in invertebrates; their structure is distinct from that of vertebrate tight junctions.

serine protease Type of protease that has a reactive serine in the active site.

sex chromosome Chromosome that may be present or absent, or present in a variable number of copies, according to the sex of the individual. In mammals, the X and Y chromosomes.

sexual reproduction Type of reproduction in which the genomes of two individuals are mixed in the formation of a new organism. Individuals produced by sexual reproduction differ from either of their parents and from each other.

SH2 domain Src homology region 2, a protein domain present in many signaling proteins; it binds a short amino acid sequence containing a phosphotyrosine.

side chain The part of an amino acid that differs between different amino acids, giving the amino acid its unique physical and chemical properties.

signal molecule Extracellular or intracellular molecule that cues the response of a cell to the behavior of other cells or objects in the environment.

signal patch Protein sorting signal that consists of a specific three-dimensional arrangement of atoms on the folded protein's surface.

signal peptidase Enzyme that removes a terminal signal sequence from a protein once the sorting process is complete.

signal-recognition particle (SRP) Ribonucleoprotein particle that binds an ER signal sequence on a partially synthesized polypeptide chain and directs the polypeptide and its attached ribosome to the endoplasmic reticulum.

signal sequence Short continuous sequence of amino acids that determines the eventual location of a protein in the cell. An example is the N-terminal sequence of 20 or so amino acids that directs nascent secretory and transmembrane proteins to the endoplasmic reticulum.

signal transduction Relaying of a signal by conversion from one physical or chemical form to another. In cell biology, the process by which a cell converts an extracellular signal into a response.

single-nucleotide polymorphism (SNP) Variation between individuals at certain nucleotide positions in the genome.

single-pass transmembrane protein Membrane protein in which the polypeptide chain crosses the lipid bilayer only once.

single-strand DNA-binding protein Protein that binds to the single strands of the opened-up DNA double helix, preventing helical structures from reforming while the DNA is being replicated.

sister chromatid see chromatid

site-directed mutagenesis Technique by which a mutation can be made at a particular site in DNA.

site-specific recombination Type of recombination that does not require extensive similarity in the two DNA sequences undergoing recombination. Can occur between two different DNA molecules or within a single DNA molecule.

small intracellular mediator see second messenger

small nuclear RNA (snRNA) Small RNA molecules that are complexed with proteins to form the ribonucleoprotein particles involved in RNA splicing.

smooth endoplasmic reticulum (smooth ER) Region of the endoplasmic reticulum not associated with ribosomes. It is involved in lipid synthesis.

smooth muscle cell Type of long, spindle-shaped mononucleate muscle cell making up the muscular tissue found in the walls of arteries and of the intestine and other viscera, and in some other locations of the vertebrate body. Called “smooth” because it lacks the striated myofibrils of skeletal and cardiac muscle cells.

SNAREs Large family of transmembrane proteins present in organelle membranes and the vesicles derived from them. They are involved in guiding vesicles to their correct destinations. They exist in pairs—a v-SNARE in the vesicle membrane that binds specifically to a complementary t-SNARE in the target membrane.

SNP see single-nucleotide polymorphism

snRNA see small nuclear RNA

solute Any molecule that is dissolved in a liquid. The liquid is called a solvent.

somatic cell Any cell of a plant or animal other than a germ cell or germ-cell precursor. (From Greek soma,body.)

somite One of a series of paired blocks of mesoderm that form during early development and lie on either side of the notochord in a vertebrate embryo. They give rise to the vertebral column, muscles and associated connective tissue. Each somite produces the musculature of one vertebral segment, plus associated connective tissue.

sorting signal Amino acid sequence that directs the delivery of a protein to a specific location outside the cytosol.

Southern blotting Technique in which DNA fragments separated by electrophoresis are immobilized on a paper sheet. Specific fragments are then detected with a labeled nucleic acid probe. (Named after E.M. Southern, inventor of the technique.)

spectrin Abundant protein associated with the cytosolic side of the plasma membrane in red blood cells, forming a rigid network that supports the membrane.

Spemann's Organizer Specialized tissue at the dorsal lip of the blastopore in an amphibian embryo; a source of signals that help to orchestrate formation of the embryonic body axis. (After H. Spemann and H. Mangold, co-discoverers.)

sperm (spermatozoon,spermatozoa) The mature male gamete in animals. It is motile and usually small compared with the egg.

spermatogenesis Development of sperm.

spindle-attachment checkpoint Checkpoint that operates during mitosis to ensure that all chromosomes are properly attached to the spindle before sister-chromatid separation starts.

spliceosome Large assembly of RNA and protein molecules that performs pre-mRNA splicing in eucaryotic cells.

Src family Family of cytoplasmic tyrosine kinases (pronounced “sark”) that associate with the cytoplasmic domains of some enzyme-linked receptors (for example, the T cell antigen receptor) that lack intrinsic tyrosine kinase activity. They transmit a signal onwards by phosphorylating the receptor itself and other signaling proteins.

SRP see signal-recognition particle

standard free-energy change (ΔG°) Free-energy change of two reacting molecules at standard temperature and pressure when all components are present at a concentration of 1 mole per liter.

starch Polysaccharide composed exclusively of glucose units, used as an energy storage material in plant cells.

start-transfer signal Short amino-acid sequence that enables a polypeptide chain to start being translocated across the endoplasmic reticulum membrane through a protein translocator. Multipass membrane proteins have both N-terminal (signal sequence) and internal start-transfer signals.

stem cell Relatively undifferentiated cell that can continue dividing indefinitely, throwing off daughter cells that can undergo terminal differentiation into particular cell types.

stereocilium A large, rigid microvillus found in “organ pipe” arrays on the apical surface of hair cells in the ear. A stereocilium contains a bundle of actin filaments, rather than microtubules, and is thus not a true cilium.

steroid Hydrophobic lipid molecule with a characteristic four-ringed structure. Many important hormones such as estrogen and testosterone are steroids. (See Panel 2–5, pp. 118–119.)

stimulatory G protein (Gs) G protein that, when activated, activates the enzyme adenylyl cyclase and thus stimulates the production of cyclic AMP.

stop-transfer signal Hydrophobic amino acid sequence that halts translocation of a polypeptide chain through the endoplasmic reticulum membrane, thus anchoring the protein chain in the membrane (See Figure 12–49).

strand-directed mismatch repair see mismatch repair

striated muscle Muscle composed of transversely striped (striated) myofibrils. Skeletal and heart muscle of vertebrates are the best-known examples.

stroma (1) The connective tissue in which a glandular or other epithelium is embedded. (2) The large interior space of a chloroplast, containing enzymes that incorporate CO2 into sugars.

structural gene Region of DNA that codes for a protein or for an RNA molecule that forms part of a structure or has an enzymatic function. Distinguished from regions of DNA that regulate gene expression.

substrate Molecule on which an enzyme acts.

substratum Solid surface to which a cell adheres.

subunit Component of a multicomponent complex—for example, one protein component of a protein complex or one polypeptide chain of a multichain protein.

sucrose Disaccharide composed of one glucose unit and one fructose unit. The major form in which glucose is transported between plant cells.

sugar Small carbohydrates with a monomer unit of general formula (CH2O)n. Examples are the monosaccharides glucose, fructose and mannose, and the disacharide sucrose (composed of a molecule of glucose and one of fructose linked together).

sulfhydryl (thiol, –SH) Chemical group containing sulfur and hydrogen found in the amino acid cysteine and other molecules. Two sulfhydryls can join to produce a disulfide bond.

supercoiled DNA Region of DNA in which the double helix is further twisted on itself. (See Figure 6–20.)

survival factor Extracellular signal required for a cell to survive; in its absence the cell will undergo apoptosis and die.

symbiosis Intimate association between two organisms of different species from which both derive a long-term selective advantage.

symporter Carrier protein that transports two types of solute across the membrane in the same direction.

synapse Communicating cell–cell junction that allows signals to pass from a nerve cell to another cell. In a chemical synapse the signal is carried by a diffusible neurotransmitter; in an electrical synapse a direct connection is made between the cytoplasms of the two cells via gap junctions.

synapsis (1) In genetic recombination, the initial formation of base pairs between complementary DNA strands in different DNA molecules that occurs at sites of crossing-over between chromosomes. (2) In meiosis, the pairing of maternal and paternal copies of a chromosome as they become attached to each other along their length.

synaptic signaling Type of cell–cell communication that occurs across chemical synapses in the nervous system.

synaptic vesicle Small neurotransmitter-filled secretory vesicle formed at the axon terminals of nerve cells and whose contents are released into the synaptic cleft by exocytosis when an action potential reaches the axon terminal.

synaptonemal complex Structure that holds paired chromosomes together during prophase I of meiosis and promotes genetic recombination.

syncytium Mass of cytoplasm containing many nuclei enclosed by a single plasma membrane. Typically the result either of cell fusion or of a series of incomplete division cycles in which the nuclei divide but the cell does not.

synteny The presence in different species of regions of chromosomes with the same genes in the same order.

T cell (T lymphocyte) Type of lymphocyte responsible for cell-mediated immunity; includes both cytotoxic T cells and helper T cells.

TATA box Consensus sequence in the promoter region of many eucaryotic genes that binds a general transcription factor and hence specifies the position at which transcription is initiated.

TCA cycle see citric acid cycle

telomerase Enzyme that elongates telomere sequences in DNA.

telomere End of a chromosome, associated with a characteristic DNA sequence that is replicated in a special way. Counteracts the tendency of the chromosome otherwise to shorten with each round of replication. (From Greek telos, end.)

telophase Final stage of mitosis in which the two sets of separated chromosomes decondense and become enclosed by nuclear envelopes.

temperature-sensitive (ts) mutant Organism or cell carrying a genetically altered protein (or RNA molecule) that performs normally at one temperature but is abnormal at another (usually higher) temperature.

template A single strand of DNA or RNA whose nucleotide sequence acts as a guide for the synthesis of a complementary strand.

terminator Signal in bacterial DNA that halts transcription.

tertiary structure Complex three-dimensional form of a folded polymer chain, especially a protein or RNA molecule.

TGF-β superfamily see transforming growth factor-β superfamily

TGN see trans Golgi network (TGN)

thioester bond High-energy bond formed by a condensation reaction between an acid (acyl) group and a thiol group (–SH); seen, for example, in acetyl CoA and in many enzyme-substrate complexes.

thiol see sulfhydryl

thylakoid Flattened sac of membrane in a chloroplast that contains chlorophyll and other pigments and carries out the light-trapping reactions of photosynthesis. Stacks of thylakoids form the grana of chloroplasts.

tight junction Cell–cell junction that seals adjacent epithelial cells together, preventing the passage of most dissolved molecules from one side of the epithelial sheet to the other.

TIM complexes Protein translocators in the mitochondrial inner membrane. The TIM23 complex mediates the transport of proteins into the matrix and the insertion of some proteins into the inner membrane; the TIM22 complex mediates the insertion of a subgroup of proteins into the inner membrane.

Toll-like receptor family (TLR) Important family of mammalian pattern recognition receptors abundant on macrophages, neutrophils and the epithelial cells of the gut. They recognize pathogen-associated immunostimulants such as lipopolysacharide and peptidoglycan.

TOM complex Multisubunit protein complex that transports proteins across the mitochondrial outer membrane.

topoisomerase (DNA topoisomerase) Enzyme that makes reversible cuts in a double-helical DNA molecule for the purpose of removing knots or unwinding excessive twists.

tracer Molecule or atom that has been labeled either chemically or radioactively so that it can be followed in a biochemical process or readily located in a cell or tissue.

trans face Face of a Golgi stack at which material leaves the organelle for the cell surface or another cell compartment. It is adjacent to the trans Golgi network.

trans Golgi network (TGN) Network of interconnected cisternae and tubules at the trans face of the Golgi apparatus, through which material is transferred out of the Golgi.

transcellular transport Transport of solutes, such as nutrients, across an epithelium, by means of membrane transport proteins in the apical and basal faces of the epithelial cells.

transcript RNA product of DNA transcription.

transcription (DNA transcription) Copying of one strand of DNA into a complementary RNA sequence by the enzyme RNA polymerase.

transcription attenuation Inhibition of gene expression in bacteria by the premature termination of transcription.

transcription factor Term loosely applied to any protein required to initiate or regulate transcription in eucaryotes. Includes both gene regulatory proteins as well as the general transcription factors.

transcriptional control Control of of gene expression by controlling when and how often the gene is transcribed.

transcytosis The uptake of material at one face of a cell by endocytosis, its transfer across a cell in vesicles, and its discharge from another face by exocytosis.

transfection Introduction of a foreign DNA molecule into a eucaryotic cell. It is usually followed by expression of one or more genes in the newly introduced DNA.

transfer RNA (tRNA) Set of small RNA molecules used in protein synthesis as an interface (adaptor) between messenger RNA and amino acids. Each type of tRNA molecule is covalently linked to a particular amino acid.

transforming growth factor-β superfamily (TGF-β superfamily) Large family of structurally related, secreted proteins that act as hormones and local mediators to control a wide range of functions in animals, including during development. It includes TGF-βs, activins, and bone morphogenetic proteins (BMPs).

transgenic organism Plant or animal that has stably incorporated one or more genes from another cell or organism and can pass them on to successive generations.

transition state Structure that forms transiently in the course of a chemical reaction and has the highest free energy of any reaction intermediate. Its formation is a rate-limiting step in the reaction.

translation (RNA translation) Process by which the sequence of nucleotides in a messenger RNA molecule directs the incorporation of amino acids into protein. It occurs on a ribosome.

translational control Control of gene expression by selection of which mRNAs in the cytoplasm are translated by ribosomes.

translocation Type of mutation in which a portion of one chromosome is broken off and attached to another.

transmembrane protein Membrane protein that extends through the lipid bilayer, with part of its mass on either side of the membrane.

transmitter-gated ion channel Ion channel in the postsynaptic plasma membranes of nerve and muscle cells that opens only in response to the binding of a specific extracellular neurotransmitter. The resulting inflow of ions leads to the generation of a local electrical signal in the postsynaptic cell.

transposable element Segment of DNA that can move from one position in a genome to another. Also called a transposon.

transposition The movement of a DNA sequence from one site to another within the genome. See also cut-and-paste transposition.

trans-splicing Type of RNA splicing present in a few eucaryotic organisms in which exons from two separate RNA molecules are joined together to form an mRNA.

treadmilling The process by which a polymeric protein filament is maintained at constant length by addition of protein subunits at one end and loss of subunits at the other. (See Panel 16–2, pp. 912–913.)

triacylglycerol Molecule composed of three fatty acids esterified to glycerol. The main constituent of fat droplets in animal tissues (where the fatty acids are saturated) and of vegetable oils (where the fatty acids are mainly unsaturated). Also known as triglyceride. (See Panel 2–5, pp. 118–119.)

tricarboxylic acid (TCA) cycle see citric acid cycle

trimeric GTP-binding protein see GTP-binding protein

tRNA see transfer RNA

t-SNARE see SNAREs

tubulin The protein subunit of microtubules.

tumor progression The process by which an initial mildly disordered cell behavior gradually evolves into a full-blown cancer.

tumor suppressor gene Gene that appears to prevent formation of a cancer. Loss-of-function mutations in such genes enhance susceptibility to cancer.

two-dimensional gel electrophoresis Type of electrophoresis in which the protein mixture is run first in one direction and then in a direction at right angles to the first. It enables better separation of individual proteins.

two-hybrid system Technique for identifying interacting proteins using genetically engineered yeast cells.

type III secretion system A bacterial system for delivering toxic proteins into the cells of their host.

ubiquitin Small, highly conserved protein present in all eucaryotic cells that becomes covalently attached to lysines of other proteins. Attachment of a short chain of ubiquitins to such a lysine tags a protein for intracellular proteolytic destruction by a proteasome.

ubiquitin ligase Any one of a large number of enzymes that attach ubiquitin to a protein, thus marking it for destruction in a proteasome. The process catalyzed by a ubiquitin ligase is called ubiquitylation.

unfolded protein response Cellular response triggered by an accumulation of misfolded proteins in the endoplasmic reticulum. It involves increased transcription of ER chaperones and degradative enzymes.

uniporter Carrier protein that transports a single solute from one side of the membrane to the other.

unsaturated Describes a molecule that contains one or more double or triple carbon-carbon bonds, such as isoprene or benzene.

V gene segment Gene segment encoding most of the variable region of the polypeptide chains of immunoglobulins and T cell receptors.

V (D) J joining Recombination process by which gene segments are brought together to form a functional gene for a polypeptide chain of an immunoglobulin or T cell receptor.

vacuole Very large fluid-filled vesicle found in most plant and fungal cells, typically occupying more than a third of the cell volume.

van der Waals attraction Type of (individually weak) noncovalent bond that is formed at close range between nonpolar atoms.

variable region Region of an immunoglobulin light or heavy chain that differs from molecule to molecule; it comprises the antigen-binding site.

vector In cell biology, the DNA of an agent (virus or plasmid) used to transmit genetic material to a cell or organism. (See also cloning vector, expression vector.)

vegetal pole The end at which most of the yolk is located in an animal egg. The end opposite the animal pole.

ventral Situated toward the belly surface of an animal, or towards the underside of a wing or leaf.

vesicle Small, membrane-bounded, spherical organelle in the cytoplasm of a eucaryotic cell.

vesicular transport Transport of proteins from one cellular compartment to another by means of membrane-bounded intermediaries such as vesicles or organelle fragments.

virulence gene Gene that contributes to an organism's ability to cause disease.

virus Particle consisting of nucleic acid (RNA or DNA) enclosed in a protein coat and capable of replicating within a host cell and spreading from cell to cell. Many viruses cause disease.

voltage-gated cation channel Type of ion channel found in the membranes of excitable cells (such as nerve cells and muscle) which opens in response to a shift in membrane potential past a threshold value.

v-SNARE see SNAREs

Western blotting Technique by which proteins are separated by electrophoresis and immobilized on a paper sheet and then analyzed, usually by means of a labeled antibody.

white blood cell (leucocyte) General name for all the nucleated blood cells lacking hemoglobin. Includes lymphocytes, neutrophils, eosinophils, basophils, and monocytes.

wild-type Normal, nonmutant form of an organism; the form found in nature (in the wild).

Xenopus laevis (South African clawed toad) Species of frog (not toad) frequently used in studies of early vertebrate development.

XIC see X-inactivation center

X-inactivation Inactivation of one copy of the X chromosome in the somatic cells of female mammals.

X-inactivation center (XIC) Site in an X chromosome at which inactivation is initiated and spreads outwards.

X-ray crystallography Technique for determining the three-dimensional arrangement of atoms in a molecule based on the diffraction pattern of X-rays passing through a crystal of the molecule.

yeast Common term for several families of unicellular fungi. Includes species used for brewing beer and making bread, as well as pathogenic species (that is, species that cause disease).

yolk Nutritional reserves rich in lipids, proteins and polysaccharides, present in the eggs of many animals.

Z disc (Z line) Platelike region of a muscle sarcomere to which the plus ends of actin filaments are attached. Seen as a dark transverse line in micrographs.

zinc finger DNA-binding structural motif present in many gene regulatory proteins. Composed of a loop of polypeptide chain held in a hairpin bend bound to a zinc atom.

zona pellucida Glycoprotein layer on the surface of the unfertilized egg. It is often a barrier to fertilization across species.

zygote Diploid cell produced by fusion of a male and female gamete. A fertilized egg.

zygotene Second stage of division I of meiosis, in which the synaptonemal complex begins to form between the two sets of sister chromatids in each bivalent chromosome.

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