A

acetyl CoA carboxylase An enzyme that catalyzes the ATP-dependent synthesis of malonyl CoA from acetyl CoA and carbon dioxide， the committed step in fatty acid synthesis.

acetylcholine receptor A ligand-gated channel that promotes a large inward current of sodium ions and triggers an action potential; composed of a pentamer of four kinds of polypeptide subunits， the channel opens to allow passage of sodium and potassium ions when two acetylcholine molecules promote transient opening of the channel.

acetylcholinesterase An enzyme in the synaptic cleft that converts acetylcholine into choline and acetate; this enzyme promotes the closing of the acetylcholine-receptor membrane by rapidly reducing the concentration of acetylcholine in the region between the presynaptic and the postsynaptic membranes.

acetyllysine-binding domain A domain consisting of a four-helix bundle that binds peptides containing acetyllysine. Also called a bromodomain.

actin A highly conserved protein found in all eukaryotes; in striated muscle， it forms the thin filaments of the sarcomere and activates the ATPase of myosin.

actinomycin A polypeptide antibiotic from Streptomyces that inhibits the elongation phase of RNA synthesis by binding to double-helical DNA by intercalating with it， thereby preventing the DNA from serving as a template.

action potential The increase in membrane potential and the changes in sodium and potassium conductances that result from alterations in the permeability of the axon membrane to those ions. Also called nerve impulse.

activated carriers Small molecules carrying activated functional groups that can be donated to other molecules; for instance， ATP carries activated phosphate groups and CoA carries activated acyl groups.

activated methyl cycle A series of reactions in which S-methyl groups from methionine are converted into a biochemically reactive form through insertion into an adenosyl group; an active S-methyl group can be transferred from S-adenosylmethionine to acceptor molecules such as norepinephrine. The remaining part of the cycle includes the regeneration of methionine from homocysteine and N5- methyltetrahydrofolate.

activation domain The structural region of a transcription factor that facilitates transcription in some manner. See also DNA-binding domain.

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active site A specific region of an enzyme that binds the substrate and carries out catalysis.

active transport The transport of an ion or a molecule against a concentration gradient， where DG for the transported species is positive; the process must be coupled to an input of free energy from a source such as ATP， an electrochemical gradient of Na+ or K+， or light.

actomyosin A complex formed in vitro between myosin and actin that displays certain properties of muscles， such as contracting in the presence of ATP.

acyl adenylate A mixed anhydride in which the carboxyl group of a molecule is linked to the phosphoryl group of AMP; the formation of acyl adenylates is a means of activating carboxyl groups in biochemical reactions， such as the formation of fatty acyl CoA molecules from a free fatty acid and coenzyme A.

acyl carrier protein A bacterial polypeptide that is linked to phosphopantetheine and acts as a carrier of the growing fatty acyl chain during fatty acid biosynthesis.

adaptation The resetting of the sensitivity level of receptors due to the continued presence of ligands. Also called desensitization.

addition to or formation of double bond A reaction in which a functional group is added to a double bond or a group is removed from a molecule to form a double bond.

adenylate cyclase An enzyme that generates cAMP， a second messenger， from ATP.

adenylate cyclase cascade A signal-transduction pathway that employs cAMP and a series of enzymes to convert an extracellular signal into an intracellular signal.

adipocytes Mammalian cells that are the major storage site for triacylglycerols.

A-DNA helix A right-handed double helix made up of antiparallel strands held together by A-T and G- C base pairing; it is wider and shorter than B-DNA and is seen in dehydrated DNA as well as in doublestranded regions of RNA and in RNA-DNA helices.

aerobic In the presence of oxygen.

affinity chromatography A protein-purification technique based on the high affinity of many proteins for specific chemical groups. Such groups are attached to an inert matrix， and the protein sample is applied; only those proteins with an affinity for the groups will bind.

affinity labeling A means of mapping the active site of an enzyme by using a substrate analog that binds to the active site and forms a covalent bond with a nearby amino acid.

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agonists Molecules that bind to receptor proteins and trigger signaling pathways.

alcaptonuria A relatively harmless hereditary disorder resulting from the aberrant breakdown of tyrosine and phenylalanine.

alcoholic fermentation The anaerobic conversion of glucose into ethanol with the concomitant production of ATP.

aldol condensation The combination of two carbonyl compounds (e.g.， an aldehyde and a ketone) to form a β-hydroxycarbonyl compound， or aldol.

aldose A monosaccharide whose C-1 carbon atom contains an aldehyde group.

alleles Alternative forms of a gene at a particular site on a chromosome.

allosteric interaction An interaction between a small molecule (a ligand) and a site on a protein that may be some distance away from the active site; the interaction causes a conformational change and consequent alteration in the catalytic activity of the protein.

α helix A common structural motif in proteins， in which a polypeptide main chain forms the inner part of a right-handed helix， with the side chains extending outward; the helix is stabilized by intrachain hydrogen bonds between NH and CO groups of the main chain.

α-ketoglutarate dehydrogenase A citric acid cycle enzyme that catalyzes the oxidative decarboxylation of α-ketoglutarate to yield succinyl CoA. This enzyme， which helps to regulate the rate of the citric acid cycle， is structurally and mechanistically similar to the pyruvate dehydrogenase complex.

alternative splicing The generation of unique but related mRNA molecules by the differential splicing of the pre-mRNA transcript. By allowing the synthesis of more than one mRNA molecule from a premRNA transcript， alternative splicing increases the encoding potential of the genome.

Amanitin A cyclic octapeptide from the mushroom Amanita phalloides (the destroying angel) that is a potent inhibitor of the elongation phase of RNA synthesis catalyzed by RNA polymerase II.

Ames test A simple， rapid means of detecting carcinogens by measuring a chemicals ability to induce mutations in Salmonella bacteria.

amiloride-sensitive sodium channel A sodium channel important in the detection of the taste of salt. Such channels are inhibited by amiloride， which also mutes the taste of salt.

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amino acid An organic acid with an α-carbon atom linked to a carboxylic acid， an amino group， a hydrogen atom， and a side chain (the R group). Twenty different amino acids are the building blocks of proteins.

amino sugar A sugar that contains an amino group rather than a hydroxyl group at the C-2 position; the most common amino sugars are glucosamine and galactosamine.

aminoacyl-tRNA An amino acid ester of transfer RNA.

aminoacyl-tRNA synthetase An enzyme that activates an amino acid and then links it to transfer RNA. Also known as an activating enzyme， each aminoacyl-tRNA synthetase is specific for a particular amino acid.

aminotransferase A class of enzymes that transfer an α-amino group from an α-amino acid to an α- keto acid. Also called transaminases.

ammoniotelic Characteristic of organisms in which excess ammonia is directly secreted; many aquatic animals are ammoniotelic.

AMP-dependent protein kinase A protein kinase， conserved among eukaryotes， that is activated on binding of AMP and inhibited by ATP; consequently， it functions as a cellular fuel gauge， inhibiting certain process by phosphorylating key enzymes when the energy supply is low.

ampere A measure of electrical current: the flow of 6.24 × 1018 charges per second.

amphibolic reactions Metabolic reactions that can be anabolic or catabolic， depending on the energy conditions in the cell.

amphipathic molecule A molecule， such as a membrane lipid， that contains both a hydrophobic and a hydrophilic moiety.

amylopectin The branched form of starch， containing glucose residues in about one α-1，6 linkage per thirty α-1，4 linkages.

amylose The unbranched form of starch， containing glucose residues in α-1，4 linkage.

amytal A barbiturate that blocks the respiratory chain by inhibiting electron transfer in the NADH-Q reductase complex.

anabolic steroid A steroid， such as testosterone， that acts through the androgen receptor to stimulate genes that enhance the development of lean muscle mass.

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anabolism The set of metabolic reactions that require energy to syntheize molecules from simpler precursors.

anaerobic In the absence of oxygen.

anapleurotic reaction From the Greek for "fill up，" referring to a reaction that replenishes intermediates removed from a metabolic pathway. The most common example is the reaction catalyzed by pyruvate carboxylase， in which the carboxylation of pyruvate produces oxaloacetate， a key component of the citric acid cycle.

Andersens disease A disease due to an inability to introduce α-1，6-glycosidic bonds during the synthesis of glycogen. Glycogen is present in normal amounts but with long outer branches; clinically， the disease is characterized by progressive cirrhosis of the liver.

androgens A class of steroid hormones， exemplified by testosterone， that are responsible for the development of male secondary sexual characteristics; synthesized by the testes.

angstrom (Å) A unit of length equal to 10-10 meter.

anomers Isomers of cyclic hemiacetals or hemiketals， with different configurations only at the carbonyl carbon atom; that carbon is known as the anomeric carbon.

antagonist A molecule that binds to a receptor protein but does not trigger the signaling pathway. Such molecules are like competitive inhibitors for enzymes.

antibody A protein synthesized by an animal in response to the presence of a foreign substance， or antigen; often binds to the antigen， neutralizing it or marking it for destruction.

anticodon Three-nucleotide sequence of tRNA that base-pairs with a codon in mRNA.

antigen A foreign substance that elicits the synthesis of an antibody.

antigenic determinant Site on an antigen to which an antibody binds. Also called an epitope.

antimycin A An antibiotic from Streptomyces that inhibits the respiratory chain by blocking electron transfer in the cytochrome reductase complex.

antiporter A transport system in which a molecule is carried across a membrane in the direction opposite that of an ion， which in turn is pumped back across the membrane through active transport linked to ATP hydrolysis.

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antiserum Serum prepared from the blood of an immunized animal containing soluble antibodies specific for a particular antigen.

apoptosis A cascade of proteolytic enzymes that results in controlled cell death in response to significant cell damage or specific developmental programs. Also called programmed cell death.

arachidonate Derived from linoleate， a 20:4 fatty acid that is a major precursor to several classes of signal molecules， including prostaglandins.

arrestin A protein that plays a role in the termination of the visual signal-transduction pathway by binding to phosphorylated rhodopsin and preventing further interaction with transducin. Arrestin may function similarly in other 7TM-dependent signal-transduction pathways.

ascorbate (vitamin C) A water-soluble vitamin that functions as an antioxidant and is required for the hydroxylation of collagen; scurvy results if ascorbate is deficient.

aspartyl proteases A class of protein-degrading enzymes whose activity is dependent on an aspartate residue at the active site. An aspartyl protease is required for HIV replication.

ATP (adenosine 5-triphosphate) A nucleotide consisting of adenine， ribose， and triphosphate units that serves as the cellular energy currency.

ATP-ADP translocase An adenine nucleotide carrying a transport protein that carries ADP into the mitochondria and ATP out in a coupled fashion.

ATP-binding cassette (ABC) domain The ATP-binding domain characteristic of specific membranetransport proteins， called ABC transporters; these transporters also contain a membrane-spanning region.

ATP-grasp fold A protein domain that surrounds ATP and orients it for nucleophilic attack at the ã phosphate. Enzymes with these domains catalyze the formation of carbon-nitrogen bonds through acylphosphate intermediates.

ATP synthase Molecular assembly of the inner mitochondrial membrane responsible for the respiratory-chain-driven synthesis of ATP. Also called Complex V， mitochondrial ATPase， H+-ATPase， or F0F1-ATPase.

atractyloside A plant glycoside that inhibits ATP-ADP translocase.

attenuation In bacteria， a mechanism for transcriptional regulation in which a decrease in the rate of translation of an mRNA operon reduces the rate of transcription of that operon.

autoimmune disease A disease， such as insulin-dependent diabetes and multiple sclerosis， that result from a failure to suppress the immune response to self-antigens.

autoradiography A means of detecting radioactive molecules immobilized in a separation medium such as polyacrylamide; the radioactivity of the molecules will blacken x-ray film.

autotrophs Photosynthetic organisms that synthesize glucose from carbon dioxide and water， by using sunlight as an energy source; the glucose is then used as a fuel for cellular metabolism.

avidity The strength of an interaction comprising multiple independent binding interactions between partners， as would take place between an antigen and antibody.

axoneme The fundamental design structure of cilia and flagella; it consists of a bundle of microtubules， enclosed in a membrane， in which nine microtubule doublets surround two microtubule singlets.

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B

blymphocyte Precursors to plasma cells， which are antibody-secreting cells.

bacitracin An antibiotic that blocks the transfer of oligosaccharides from dolichol phosphate to proteins.

bacteriorhodopsin A 26-kd integral membrane protein that absorbs light and converts its energy into proton-motive force， which is then used to synthesize ATP.

base-excision repair A means of repairing DNA in which the damaged base is removed and replaced by a base complementary to the undamaged DNA strand.

B-DNA helix A right-handed double helix with the following characteristics: the two strands are antiparallel; the bases are inside the helix and the phosphates and deoxyribose sugars are on the outside; adenine forms hydrogen bonds with thymine， and guanine forms them with cytosine; the bases in each pair are coplanar; there are 10.4 residues per turn， with a pitch of 35 Å.

Beers law The relations between the absorbance of light (A) by a compound， its extinction coefficient (å)， concentration (c) and the length (l) of the light path: A = åcl

beriberi A neurologic and cardiovascular disorder caused by a dietary deficiency of thiamine (vitamin B1).

β sheet A common structural motif in proteins， in which two or more β strands are associated as stacks of chains， stabilized by interchain hydrogen bonds; a number of β strands running in the same direction form a β pleated sheet， whereas such strands running in opposite directions form an antiparallel pleated sheet.

β-galactosidase An essential enzyme in lactose metabolism that hydrolyzes lactose into galactose and glucose.

bifunctional enzyme An enzyme with two different， often opposing， catalytic activities on one polypeptide chain. For instance， phosphofructokinase 2 synthesizes fructose 2，6-bisphosphate and fructose 2，6-bisphosphatase hydrolyzes it， yet both active sites are on the same polypeptide chain.

bile salts Polar derivatives of cholesterol that are made in the liver， stored in the gall bladder， and released into the small intestine， where they act as detergents to solubilize dietary lipids， facilitating their digestion and absorption.

binding energy The free energy released in the formation of the weak interactions between enzyme and substrate.

biopterin A cofactor from which the electron carrier tetrahydrobiopterin is derived.

biotin A vitamin that plays a role in carboxylation and decarboxylation reactions.

Bohr effect The observation made by Christian Bohr that H+ and CO2 promote the release of oxygen from oxyhemoglobin.

bongkrekic acid An antibiotic that inhibits the action of ATP-ADP translocase.

branch migration The ability of a DNA strand partly paired with its complementary strand to move， displacing its homologous resident strand and extending the pairing.

bromodomain A domain consisting of a four-helix bundle that binds peptides containing acetyllysine. Also called an acetyllysine-binding domain.

C3 plant A plants that lacks the C4 pathway.

C4 pathway A means by which four-carbon compounds， such as oxaloacetate and malate， carry carbon dioxide from mesophyll cells in contact with the air to bundle-sheath cells， which are the major sites of photosynthesis. The pathway accelerates photosynthesis by concentrating carbon dioxide in photosynthetic cells.

C4 plant A plant that utilizes the C4 pathway.

CAAT box A component of many eukaryotic promoters with the consensus sequence 5-GNCAATCT- 3.

Calcium ATPase (Ca2+ ATPase) An ATP-driven calcium pump that maintains the large electrochemical gradient of calcium ion across the plasma membrane.

calmodulin In vertebrates， a ubiquitous protein in vertebrates that， when bound to calcium， stimulates many enzymes and transporters.

calmodulin-dependent kinase A protein kinase that is activated by the binding of a Ca2+-calmodulin complex.

calnexin Anchored in the endoplasmic reticulum membrane， a chaperone protein that prevents the export of immature or defective glycoproteins by binding glucose residues on the glycoproteins.

caloric homeostasis Maintenance of a constant body weight by a complex network of hormonal interactions.

calorie The amount of heat required to raise the temperature of 1 gram of water from 14.5°C to 15.5°C.

Calvin cycle In plants， a cyclic metabolic pathway in which carbon dioxide is incorporated into ribulose 1，5-bisphosphate to give compounds that can be used for the synthesis of glucose.

5 cap A structure at the 5 end of eukaryotic mRNA that stabilizes the mRNA and enhances its translation. The cap contains a 7-methyl guanylate residue attached by a triphosphate linkage to the sugar at the 5 end of the mRNA in a rare 5-5 linkage.

capsaicin receptors Ion channels expressed in nociceptors that open in response to noxious stimuli such as heat， acidity or chemicals such as capsaicin， the chemical responsible for the "hot" taste of spicy food. Also called the vanilloid receptor 1 (VR1).

capsid The protein coat surrounding viral DNA or RNA.

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carbamoyl phosphate synthetase An enzyme that begins the urea cycle by catalyzing the synthesis of carbamoyl phosphate from bicarbonate， ammonium ion， and ATP. The enzyme also catalyzes the initial reaction in pyrimidine biosynthesis.

carbohydrates Saccharides， which are aldehyde or ketone compounds with multiple hydroxyl groups. Also defined as organic compounds with the empirical formula (CH2O)n.

carbonium ion A carbon compound that contains a positively charged carbon atom; a carbonium atom is critical for catalysis by lysozyme

carboxylase An enzyme that catalyzes a carboxyl transfer reaction; biotin is usually required as a coenzyme.

carcinoembryonic antigen (CEA) A membrane glycoprotein of the fetal gastrointestinal cells that is not significantly expressed after birth. High serum levels of CEA are evident in many patients with colorectal cancer.

cardiotonic steroids Compounds derived from cholesterol that inhibit the Na+-K+ pump by blocking the dephosphorylation of the E2 conformation of the pump protein. Inhibition of the pump in cardiactissue cells leads to a higher level of sodium ion in the cells， which slows the extrusion of calcium ion and enhances cardiac muscle contractility.

carnitine A zwitterionic compound formed from lysine that acts as a carrier of long-chain fatty acids from the cytosol to the mitochondrial matrix.

carotenoids Extended polyenes that absorb light between 400 and 500 nm and serve as accessory pigments in photosynthesis by funneling the energy to the photosynthetic reaction center.

cascade， enzymatic A sequence of reactions， in which at each step a product stimulates an ensuing reaction， generating an amplification of a relatively small stimulus or signal.

caspases Cysteine proteases that participate in the proteolytic cascade in apoptosis.

catabolism The set of metabolic reactions that transform fuels into cellular energy.

catabolite activator protein (CAP) The cAMP response protein; when bound to cAMP， CAP binds to an inverted repeated of the lac operon， near position -61 relative to the start site of transcription， to stimulate transcription.

catabolite repression The repression by glucose of catabolic enzymes required for the catabolism of carbohydrates other than glucose.

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catalase A ubiquitous heme protein that catalyzes the dismutation of hydrogen peroxide into molecular oxygen and water.

catalysis by approximation Enhancing the rate of a reaction by bringing multiple substrates together along a single binding surface of an enzyme.

catalytic antibodies Antibodies generated by using transition-state analogs of a particular reaction as antigens. Such antibodies often function as catalysts for the reaction. Also called abzymes.

catalytic group An amino acid or cofactor at an enzymes active site that directly participates in the making or breaking of covalent bonds.

catalytic RNA One of a class of RNA molecules that display enzymatic activity.

catalytic triad A constellation of three residues， found in many proteolytic enzymes， in which two of the residues convert the remaining residue， usually a serine or cysteine， into a potent nucleophile.

CD4 A protein present on the surface of helper T cells that， along with the T-cell receptor， binds to class II MHC proteins on antigen-presenting cells. CD4 is the source of the specificity of helper T cells for class II MHC interactions.

CD8 A cell-surface protein expressed by cytotoxic T cells that， in conjunction with the T cell receptor， recognizes class I MHC-peptide complexes. CD8 binds to the MHC protein itself.

cDNA DNA complementary to an mRNA sequence.

CDP-alcohol Activated form of an alcohol that can be used in the synthesis of phosphoglycerides by combining with diacylglycerol at the free hydroxyl group on glycerol; for instance， CDP-choline can react with diacylglycerol to form the phospholipid phosphatidylcholine.

CDP-diacylglycerol (cytidine diphosphodiacylglycerol) Formed from phosphatidate and cytidine triphosphate (CTP); activated diacylglycerol used in the synthesis of phosphoglycerides such as phosphatidyl serine.

cell type Cells within an organism that have the same genetic makeup but have different properties owing to differences in gene expression.

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cellular immune response A system for cellular recognition of foreign substances that employs cellattached T-cell receptors to eliminate cells infected by a pathogen or to elicit a particular antigenic response by stimulating B-lymphocyte antibody production.

cellulose An unbranched homopolysaccharide in plants， composed of glucose residues in α-1，4 linkage; the major structural polysaccharide in plants.

centromere The attachment site of mitotic spindles in chromosomes.

ceramide (N-acyl sphingosine) A sphingosine with a long-chain acyl group attached to the amino group.

cerebroside A sphingolipid in which glucose or galactose is linked to the terminal hydroxyl group of a ceramide.

cGMP phosphodiesterase An enzyme that hydrolyzes cyclic GMP to GMP; in the visual system， this hydrolysis leads to the generation of an action potential.

cGMP-gated calcium channel A channel in rod cells that opens in response to cGMP to allow entry of calcium and sodium ions into the cell; closing of the channel in response to cGMP hydrolysis initiates the visual signal-transduction pathway.

channel A protein passage that is continuous and that allows ions to flow rapidly through a eukaryotic membrane from a compartment of higher to a compartment of lower concentration. Channels (also known as pores in bacteria) are generally composed of four to six subunits， or domains， and are gated by membrane potential， allosteric effectors， or covalent modification.

chaperone proteins Slow ATPases that bind newly synthesized proteins and assist their proper folding.

chemiosmotic hypothesis The idea that electron transfer in the respiratory chain is used to pump protons across the inner mitochondrial membrane， establishing a proton gradient; this gradient (the proton-motive force) drives the synthesis of ATP by ATP synthase.

chemoattractants Substances， such as glucose， that， when present in the form of a gradient， cause bacteria to swim toward the source of the gradient.

chemorepellants Potentially harmful substances， such phenol， that， when present in the form of a gradient， cause bacteria to swim away from the source of the gradient.

chemotrophs Organisms that obtain energy by the oxidation of foodstuffs. See also phototrophs.

CheY A protein in the bacterial chemotaxis signaling pathway that， when phosphorylated， leads to clockwise rotation of the flagellum， causing tumbling， and that， when dephosphorylated， leads to counterclockwise rotation and smooth swimming.

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chimeric DNA A recombinant DNA molecule containing unrelated genes.

chloramphenicol An antibiotic that inhibits the peptidyl transferase activity of the prokaryotic 50S ribosomal subunit.

chlorophyll A substituted tetrapyrrole that is the principal photoreceptor in plants.

chloroplast The plant organelle in which photosynthesis takes place.

cholera toxin A toxin that consists of a catalytic domain and a membrane-penetration domain; the catalytic subunit ADP-ribosylates the G stimulatory protein， persistently activating it， leading to activation of ion pumps and subsequent efflux of large amounts of Na+ and water into the gut.

cholesterol A sterol that is an important constituent of eukaryotic membranes as well as lipoproteins; also a precursor of steroid hormones.

chromatin Nucleoprotein chromosomal material consisting mainly of DNA and histones.

chromatin immunoprecipitation (ChIP) A technique for identifying the binding sites of DNAbinding proteins. The protein is cross-linked to DNA to which it is bound in chromatin， and the DNA is fragmented into small pieces. Antibodies to the bound protein are used to isolate the chromatin fragments with the bound protein. The cross-linking is reversed， and the DNA is isolated and characterized.

chromatin remodeling machine A complex of proteins that contain domains homologous to helicases and use the energy of ATP hydrolysis to shift the positions of nucleosomes and induce other conformational changes in chromatin.

chromophore A light-absorbing group， such as 11-cis-retinal in rhodopsin.

chromosome walking A technique for analyzing long stretches of DNA by sequential subcloning and rescreening of overlapping segments.

chylomicrons Lipoprotein particles that transport dietary triacylglycerols from the intestine to other tissues; apolipoprotein B-48 is a protein component of chylomicrons.

cilia Hairlike organelles made of microtubules that protrude from the surface of the cell and move liquid past the cell surface; responsible for movement of many single-celled organisms.

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citrate synthase An enzyme that catalyzes the condensation of acetyl CoA with oxaloacetate to form citrate， initiating the citric acid cycle.

citric acid cycle A cyclic series of metabolic reactions that completely oxidize acetyl units to carbon dioxide. Also known as the tricarboxylic acid cycle (after citrate) or the Krebs cycle， after Hans Krebs， who elucidated the cyclic nature of the pathway.

class I MHC proteins Membrane proteins that tightly bind proteolytic fragments of cellular proteins and present them to the scrutiny of T cells. A foreign protein presented in a class I MHC protein provokes attack by killer T cells that initiate apoptosis in the target cell.

class II MHC proteins Proteins expressed only in antigen-presenting cells; Class II MHC proteins display peptides derived by the destruction of proteins internalized by endocytosis.

class switching A step in the differentiation of an antibody-producing cells in which the cells switch from producing IgM antibodies to producing one or the other classes of antibodies while maintaining the same antigen specificity.

clathrin A protein that coats the cytosolic side of coated pits and can form a lattice around the pit， excising it from the membrane to form a coated vesicle.

coated pits Specialized regions of the plasma membrane containing localized cell-membrane receptors. The cytosolic side of these indentations is coated with the protein clathrin.

cobalamin (vitamin B12) A complex taking part in a number of reactions including the formation of deoxyribonucleosides from ribonucleosides.

coding strand The strand of DNA that has the same sequence as the RNA transcript except it contains thymine (T) in place of uracil (U).

codon Nucleotide triplet in mRNA that encodes for a particular amino acid.

coenzyme A small organic molecule required for the activity of many enzymes; vitamins are often components of coenzymes.

coenzyme A (CoA) A coenzyme consisting of an adenine nucleotide linked to pantothenate， which is itself linked to mercaptoethylamine; universal carrier of acyl groups， which are bound to the mercaptoethylamine unit.

coenzyme Q A mobile electron carrier that is a component of the respiratory chain; it shuttles between the oxidized ubiquinone form to the reduced ubiquinol form through a semiquinone intermediate; accepts electrons from NADH-Q reductase as well as succinate-Q reductase.

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cohesive ends In double-stranded DNA molecules， complementary single-stranded ends produced by staggered cuts. Specific cohesive ends in DNA can be produced by certain restriction enzymes， such as EcoRI， and can then be used to join unrelated DNA molecules by annealing and joining with DNA ligase.

colchicine An alkaloid from the autumn crocus that prevents polymerization of tubulin to form microtubules; as a consequence， it inhibits the cell cycle at metaphase.

combinatorial chemistry The process of producing large populations of molecules en masse and then selecting for a particular biochemical property.

combinatorial control A means of controlling gene expression in eukaryotes in which each transcription factor， rather than acting on