Boldface terms within a definition are also defined in this glossary.

Figures and tables that illustrate defined terms are noted in parentheses.

cadherins A family of dimeric cell-adhesion molecules that aggregate in adherens junctions and desmosomes and mediate Ca²⁺-dependent cell-cell homophilic interactions. (Figure 20-2)

calmodulin A small cytosolic regulatory protein that binds four Ca²⁺ ions. The Ca²⁺/calmodulin complex binds to many proteins, thereby activating or inhibiting them. (Figure 3-33)

Calvin cycle See carbon fixation.

cancer General term denoting any of various malignant tumors, whose cells grow and divide more rapidly than normal, invade surrounding tissue, and sometimes spread (metastasize) to other sites.

capsid The outer proteinaceous coat of a virus, formed by multiple copies of one or more protein subunits and enclosing the viral nucleic acid.

CAP site The DNA sequence in bacteria bound by catabolite activator protein, also known as the cyclic AMP regulatory protein. (Figure 9-4)

carbohydrate General term for certain polyhydroxyaldehydes, polyhydroxyketones, or compounds derived from these usually having the formula (CH₂O)_n. Primary type of compound used for storing and supplying energy in animal cells. (Figure 2-18)

carbon fixation The major metabolic pathway that fixes CO₂ into carbohydrates during photosynthesis; also called the Calvin cycle. It is indirectly dependent on light but can occur both in the dark and light. (Figure 12-48)

carcinogen Any chemical or physical agent that can cause cancer when cells or organisms are exposed to it.

caretaker gene Any gene whose encoded protein helps protect the integrity of the genome by participating in the repair of damaged DNA. Loss of function of a caretaker gene leads to increased mutation rates and promotes carcinogenesis.

caspases A class of vertebrate protein-degrading enzymes (proteases) that function in apoptosis and work in a cascade with each type activating the next. (Figures 21-35 and 21-40)

catabolism Cellular degradation of complex molecules to simpler ones usually accompanied by the release of energy. Anabolism is the reverse process in which energy is used to synthesize complex molecules from simpler ones.

catalyst A substance that increases the rate of a chemical reaction without undergoing a permanent change in its structure. Enzymes are proteins with catalytic activity, and ribozymes are RNAs that can function as catalysts. (Figure 3-22)

cation A positively charged ion.

Cdc14 phosphatase A dual-specificity protein phosphatase that triggers mitotic CDK inactivation at the end of mitosis.

Cdc25 phosphatase A protein phosphatase that dephosphorylates CDKs on threonine 14 and tyrosine 15, thereby activating CDKs.

CDK-activating kinase (CAK) Phosphorylates CDKs on a threonine residue near the active site. This phosphorylation is essential for CDK activity.

CDK inhibitor (CKI) Binds to cyclin-CDK complex and inhibits its activity.

cDNA (complementary DNA) DNA molecule copied from an mRNA molecule by reverse transcriptase and therefore lacking the introns present in the DNA of the genome.

cell-adhesion molecules (CAMs) Proteins in the plasma membrane of cells that bind similar proteins on other cells, thereby mediating cell-cell adhesion. Four major classes of CAMs include the cadherins, IgCAMs, integrins, and selectins. (Figures 20-1 and 20-2)

cell-adhesion proteins See cell adhesion molecules (CAMs).

cell cycle Ordered sequence of events in which a eukaryotic cell duplicates its chromosomes and divides into two. The cell cycle normally consists of four phases: G₁ before DNA synthesis occurs; S when DNA replication occurs; G₂ after DNA synthesis; and M when cell division occurs, yielding two daughter cells. Under certain conditions, cells exit the cell cycle during G₁ and remain in the G₀ state as nondividing cells. (Figures 1-16 and 19-1)

cell division Separation of a cell into two daughter cells. In higher eukaryotes, it involves division of the nucleus (mitosis) and of the cytoplasm (cytokinesis); mitosis often is used to refer to both nuclear and cytoplasmic division.

G-5

cell junctions Specialized regions on the cell surface through which cells are joined to each other or to the extracellular matrix. (Figure 20-11; Table 20-3)

cell line A population of cultured cells, of plant or animal origin, that has undergone a genetic change allowing the cells to grow indefinitely. (Figure 4-1b)

cell polarity The ability of cells to organize their internal structure, resulting in changes of cell shape and generating regions of the plasma membrane with different protein and lipid compositions.

cell strain A population of cultured cells, of plant or animal origin, that has a finite life span and eventually dies, commonly after 25–50 generations. (Figure 4-1a)

cell-surface receptor Protein embedded in the plasma membrane that has an extracellular domain that binds an extracellular molecule(s), called a ligand. Many cell surface receptors bind to extracellular signaling molecules; such binding induces conformational changes in the receptor, altering the activity of the receptor’s intracellular domain, which transmits the signal to the interior of the cell.

cellular communication The transfer of information via signaling molecules or ions from one cell to another.

cellular respiration See respiration.

cellulose A structural polysaccharide made of glucose units linked together by β(1 → 4) glycosidic bonds. It forms long microfibrils, which are the major component of the cell wall in plants.

cell wall A specialized, rigid extracellular matrix that lies next to the plasma membrane, protecting a cell and maintaining its shape; prominent in most fungi, plants, and prokaryotes but absent in most multicellular animals. (Figure 20-41)

centriole Either of two cylindrical structures within the centrosome of animal cells and containing nine sets of triplet microtubules; structurally similar to a basal body. (Figure 18-6)

centromere DNA sequence required for proper segregation of chromosomes during mitosis and meiosis; the region of mitotic chromosomes where the kinetochore forms and that appears constricted. (Figure 6-39)

centrosome (cell center) Structure located near the nucleus of animal cells that is the primary microtubule-organizing center (MTOC); it contains a pair of centrioles embedded in a protein matrix and duplicates before mitosis, with each centrosome becoming a spindle pole. (Figures 18-6 and 18-35)

centrosome disjunction Describes the process of centrosome segregation during prophase.

channels Membrane proteins that transport water, ions, or small hydrophilic molecules across membranes down their concentration or electric potential gradients.

chaperone Collective term for two types of proteins—molecular chaperones and chaperonins—that prevent misfolding of a target protein or actively facilitate proper folding of an incompletely folded target protein, respectively. (Figures 3-17 and 3-18)

chaperonin See chaperone.

checkpoint Any of several points in the eukaryotic cell cycle at which progression of a cell to the next stage can be halted until conditions are suitable.

checkpoint pathway Surveillance mechanism that prevents initiation of each step in cell division until earlier steps on which it depends have been completed and mistakes that occurred during the process have been corrected.

chemical equilibrium The state of a chemical reaction in which the concentration of all products and reactants is constant because the rates of the forward and reverse reactions are equal.

chemical potential energy The energy stored in the bonds connecting atoms in molecules.

chemiosmosis Process whereby an electrochemical proton gradient (pH plus electric potential) across a membrane is used to drive an energy-requiring process such as ATP synthesis; also called chemiosmotic coupling. See proton-motive force. (Figure 12-2)

chemokine Any of numerous small, secreted proteins that function as chemotatic cues for leukocytes.

chemotaxis Movement of a cell or organism toward or away from certain chemicals.

chlorophylls A group of light-absorbing porphyrin pigments that are critical in photosynthesis. (Figure 12-39)

chloroplast A specialized organelle in plant cells that is surrounded by a double membrane and contains internal chlorophyll-containing membranes (thylakoids) where the light-absorbing reactions of photosynthesis occur. (Figure 12-37)

cholesterol A lipid containing the four-ring steroid structure with a hydroxyl group on one ring; a component of many eukaryotic membranes and the precursor of steroid hormones, bile acids, and vitamin D. (Figure 7-8c)

chromatid One copy of a replicated chromosome, formed during the S phase of the cell cycle, that is joined to the other copy; also called sister chromatid. During mitosis, the two chromatids separate, each becoming a chromosome of one of the two daughter cells. (Figure 8-35)

chromatin Complex of DNA, histones, and nonhistone proteins from which eukaryotic chromosomes are formed. Condensation of chromatin during mitosis yields the visible metaphase chromosomes. (Figures 8-23 and 8-25)

chromatography, liquid Group of biochemical techniques for separating mixtures of molecules (e.g., different proteins) based on their mass (gel filtration chromatography), charge (ion exchange chromatography), or ability to bind specifically to other molecules (affinity chromatography). (Figure 3-38)

chromosome In eukaryotes, the structural unit of the genetic material consisting of a single, linear double-stranded DNA molecule and associated proteins. In most prokaryotes, a single, circular double-stranded DNA molecule constitutes the bulk of the genetic material. See also chromatin and karyotype.

cilium (pl. cilia) Short, membrane-enclosed structure extending from the surface of eukaryotic cells and containing a core bundle of microtubules. Cilia usually occur in groups and beat rhythmically to move a cell (e.g., single-celled organism) or to move small particles or fluid along a surface (e.g., trachea cells). See also axoneme and flagellum.

cisterna (pl. cisternae) Flattened membrane-bounded compartment, as found in the Golgi complex and endoplasmic reticulum.

G-6

citric acid cycle A set of nine coupled reactions occurring in the matrix of the mitochondrion in which acetyl groups are oxidized, generating CO₂ and reduced intermediates used to produce ATP; also called Krebs cycle and tricarboxylic acid (TCA) cycle. (Figure 12-16)

clathrin A fibrous protein that with the aid of assembly proteins polymerizes into a lattice-like network at specific regions on the cytosolic side of a membrane, thereby forming a clathrin-coated pit that buds off to form a vesicle. (Figure 14-18; Table 14-1)

cleavage In embryogenesis, the series of rapid cell divisions that occurs following fertilization and with little cell growth, producing progressively smaller cells; culminates in formation of the blastocyst in mammals or blastula in other animals. Also used as a synonym for the hydrolysis of molecules. (Figure 21-3)

cleavage furrow Indentation in the plasma membrane that represents the initial steps in cytokinesis.

cleavage/polyadenylation complex Large, multiprotein complex that catalyzes the cleavage of pre-mRNA at a 3' poly(A) site and the initial addition of adenylate (A) residues to form the poly(A) tail. (Figure 10-15)

clone (1) A population of genetically identical cells, viruses, or organisms descended from a common ancestor. (2) Multiple identical copies of a gene or DNA fragment generated and maintained via DNA cloning.

co-activator A protein or protein complex required for transcription activation that does not bind directly to DNA. In contrast, an activator has a DNA-binding domain and binds directly to a DNA transcription control sequence.

codon Sequence of three nucleotides in DNA or mRNA that specifies a particular amino acid during protein synthesis; also called triplet. Of the 64 possible codons, three are stop codons, which do not specify amino acids and cause termination of synthesis. (Table 5-1)

cohesin Protein complex that holds the replicated sister chromatids together.

cohesin complex Protein complex that establishes cohesion between sister chromatids.

coiled-coil A protein structural motif marked by amphipathic α helical regions that can self-associate to form stable, rodlike structures in proteins; commonly found in fibrous proteins and certain transcription factors. (Figure 3-10a)

collagen A triple-helical glycoprotein rich in glycine and praline that is a major component of the extracellular matrix and connective tissues. The numerous subtypes differ in their tissue distribution and the extracellular components and cell-surface proteins with which they associate. (Figure 20-24; Table 20-4)

complement A group of constitutive serum proteins that bind directly to microbial or fungal surfaces, thereby activating a proteolytic cascade that culminates in formation of the cytolytic membrane attack complex. (Figure 23-5)

complementary (1) Referring to two nucleic acid sequences or strands that can form perfect base pairs with each other. (2) Describing regions on two interacting molecules (e.g., an enzyme and its substrate) that fit together in a lock-and-key fashion.

complementary DNA (cDNA) See cDNA.

complementation See genetic complementation and functional complementation.

concentration gradient In cell biology, a difference in the concentration of a substance in different regions of a cell or embryo or on different sides of a cellular membrane.

condensin Protein complex that promotes chromosome condensation.

condensin complex Protein complex related to cohesins that compacts chromosomes and is necessary for their segregation during mitosis.

conformation The precise shape of a protein or other macromolecule in three dimensions resulting from the spatial location of the atoms in the molecule. (Figure 3-8)

connexins A family of transmembrane proteins that form gap junctions in vertebrates. (Figure 20-21)

constitutive Referring to the continuous production or activity of a cellular molecule or the continuous operation of a cellular process (e.g., constitutive secretion) that is not regulated by internal or external signals.

contractile bundles Bundles of actin and myosin in nonmuscle cells that function in cell adhesion (e.g., stress fibers) or cell movement (contractile ring in dividing cells).

contractile ring Composed of actin and myosin; located beneath the plasma membrane. During cytokinesis, its contraction pulls the membrane inward, eventually closing the connection between the two daughter cells.

contractile vacuole A vesicle found in many protozoans that takes up water from the cytosol and periodically discharges its contents through fusion with the plasma membrane.

coordinately regulated Genes whose expression are induced and repressed at the same time, as for the genes in a single bacterial operon. (Figure 5-13)

co-oriented Indicates that sister kinetochores attach to microtubules emanating from the same spindle pole rather than from opposite spindle poles.

COPI A class of proteins that coat transport vesicles in the secretory pathway. COPI-coated vesicles move proteins from the Golgi to the endoplasmic reticulum and from later to earlier Golgi cisternae. (Table 14-1)

COPII A class of proteins that coat transport vesicles in the secretory pathway. COPII-coated vesicles move proteins from the endoplasmic reticulum to the Golgi. (Table 14-1)

co-repressor A protein or protein complex required for transcription repression that does not bind directly to DNA. In contrast, a repressor has a DNA-binding domain and binds directly to a DNA transcription control sequence.

cotranslational translocation Simultaneous transport of a secretory protein into the endoplasmic reticulum as the nascent protein is still bound to the ribosome and being elongated. (Figure 13-6)

cotransport Protein-mediated movement of an ion or small molecule across a membrane against a concentration gradient driven by coupling to movement of a second molecule down its concentration gradient in the same (symport) or opposite (antiport) direction. (Figure 11-2, [3B, C]; Table 11-1)

G-7

covalent bond Stable chemical force that holds the atoms in molecules together by sharing of one or more pairs of electrons. See also noncovalent interaction. (Figures 2-2 and 2-6)

CpG islands Regions in vertebrate DNA ∼100 to ∼1000 bp that have an unusually high occurrence of the sequence CG. Many CpG islands function as promoters for transcription initiation, usually in both directions.

CRISPR Named for clustered regularly interspaced short palindromic repeats, a mechanism used by many bacterial cells to protect against invasion by foreign DNA. The mechanism has been utilized in the lab to edit the genomic DNA of metazoan organisms.

cristae Sheet-like and tubelike invaginations that extend from the boundary membrane into the center of the mitochondrion.

critical cell size The cell size at which cells can enter the cell cycle. The critical cell size varies with nutrient availability.

cross-exon recognition complex Large assembly including RNA-binding SR proteins and other components that helps delineate exons in the pre-mRNAs of higher eukaryotes and ensure correct RNA splicing. (Figure 10-13)

crossing over Exchange of genetic material between maternal and paternal chromatids during meiosis to produce recombined chromosomes. See also recombination. (Figure 6-10)

cyclic AMP (cAMP) A second messenger, produced in response to hormonal stimulation of certain G protein–coupled receptors, that activates protein kinase A. (Figure 15-6; Table 15-2)

cyclic GMP (cGMP) A second messenger that opens cation channels in rod cells and activates protein kinase G in vascular smooth muscle and other cells. (Figures 15-6, 15-23, and 15-38)

cyclin Any of several related proteins whose concentrations rise and fall during the course of the eukaryotic cell cycle. Cyclins form complexes with cyclin-dependent kinases, thereby activating and determining the substrate specificity of these enzymes.

cyclin-dependent kinase (CDK) A protein kinase that is catalytically active only when bound to a cyclin. Various cyclin-CDK complexes trigger progression through different stages of the eukaryotic cell cycle by phosphorylating specific target proteins. (Table 19-1)

cytochromes A group of colored, heme-containing proteins, some of which function as electron carriers during cellular respiration and photosynthesis. (Figure 12-20a)

cytokine Any of numerous small, secreted proteins (e.g., erythropoietin, G-CSF, interferons, interleukins) that bind to cell-surface receptors on blood and immune-system cells to trigger their differentiation or proliferation.

cytokine receptor Member of major class of cell-surface signaling receptors, including those for erythropoietin, growth hormone, interleukins, and interferons. Ligand binding leads to activation of cytosolic JAK kinases associated with the receptor, thereby initiating intracellular signaling pathways. (Figures 16-6 and 16-13)

cytokinesis The division of the cytoplasm following mitosis to generate two daughter cells, each with a nucleus and cytoplasmic organelles. (Figure 17-35)

cytoplasm Viscous contents of a cell that are contained within the plasma membrane but, in eukaryotic cells, outside the nucleus.

cytoskeletal proteins Proteins that confer cell strength and rigidity, including microtubules, microfilaments, and intermediate filaments.

cytoskeleton Network of fibrous elements, consisting primarily of microtubules, microfilaments, and intermediate filaments, found in the cytoplasm of eukaryotic cells. The cytoskeleton provides organization and structural support for the cell and permits directed movement of organelles, chromosomes, and the cell itself. (Figures 17-1, 17-2, and 18-1)

cytosol Unstructured aqueous phase of the cytoplasm excluding organelles, membranes, and insoluble cytoskeletal components.

cytosolic face The face of a cell membrane directed toward the cytosol. (Figure 7-5)