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

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

gamete Specialized haploid cell (in animals either a sperm or an egg) produced by meiosis of precursor germ cells; in sexual reproduction, union of a sperm and an egg initiates the development of a new individual.

gap junction Protein-lined channel connecting the cytoplasms of adjacent animal cells that allows passage of ions and small molecules between the cells. See also plasmodesmata. (Figure 20-21)

gene Physical and functional unit of heredity, which carries information from one generation to the next. In molecular terms, it is the entire DNA sequence—including exons, introns, and transcription-control regions—necessary for production of a functional polypeptide or RNA. See also transcription unit.

gene conversion A type of DNA recombination in which one DNA sequence is converted to the sequence of a second homologous DNA sequence in the same cell.

gene control All of the mechanisms involved in regulating gene expression. Most common is regulation of transcription, although mechanisms influencing the processing, stabilization, and translation of mRNAs help control expression of some genes.

gene expression Overall process by which the information encoded in a gene is converted into an observable phenotype (most commonly production of a protein).

gene family Set of genes that arose by duplication of a common ancestral gene and subsequent divergence due to small changes in the nucleotide sequence. (Figure 8-21)

gene knockout Selective inactivation of a specific gene by replacing it with a nonfunctional (disrupted) allele in an otherwise normal organism.

genetic code The set of rules whereby nucleotide triplets (codons) in DNA or RNA specify amino acids in proteins. (Table 5-1)

genetic complementation Restoration of a wild-type function in diploid heterozygous cells generated from haploid cells, each of which carries a mutation in a different gene whose encoded protein is required for the same biochemical pathway. Complementation analysis can determine if recessive mutations in two mutants with the same mutant phenotype are in the same or different genes. (Figure 6-7)

genome Total genetic information carried by a cell or organism.

genome maintenance genes Genes that detect or repair DNA damage.

genome-wide association study (GWAS) A statistical method based on linkage disequilibrium of identifying genes for human diseases or other traits that exhibit genetic heterogeneity or may be polygenic.

genomics Comparative analyses of the complete genomic sequences from different organisms and determination of global patterns of gene expression; used to assess evolutionary relations among species and to predict the number and general types of RNAs produced by an organism.

genotype Entire genetic constitution of an individual cell or organism, usually with emphasis on the particular alleles at one or more specific loci.

germ cell In sexually reproducing organisms, any cell that can potentially contribute to the formation of offspring including gametes and their immature precursors; also called germ-line cell. See also somatic cell.

glia Supporting cells of nervous tissue that, unlike neurons, do not conduct electrical impulses; also called glial cells. Of the four types, Schwann cells and oligodendrocytes produce myelin sheaths, astrocytes function in synapse formation, and microglia make trophic factors and serve in immune responses. (Figure 22-17)

glucagon A peptide hormone produced in the cells of pancreatic islets that triggers the conversion of glycogen to glucose by the liver; acts with insulin to control blood glucose levels.

GLUT proteins A family of transmembrane proteins, containing 12 membrane-spanning α helices, that transport glucose (and a few other sugars) across cell membranes down its concentration gradient. (Figure 11-5)

GLUT4 storage vesicle (GSV) An intracellular vesicle with GLUT4 transporters in its membrane. Upon insulin stimulation, GSVs fuse with the cell membrane, exposing GLUT4s to the extracellular space, from which they can transport glucose into the cytosol.

glycogen A very long, branched polysaccharide, composed exclusively of glucose units, that is the primary storage carbohydrate in animals; found primarily in liver and muscle cells.

glycolipid Any lipid to which a short carbohydrate chain is covalently linked; commonly found in the plasma membrane.

glycolysis Metabolic pathway in which sugars are degraded anaerobically to lactate or pyruvate in the cytosol with the production of ATP. (Figure 12-3)

glycoprotein Any protein to which one or more oligosaccharide chains are covalently linked. Most secreted proteins and many membrane proteins are glycoproteins.

glycosaminoglycan (GAG) A long, linear, highly charged polymer of a repeating disaccharides in which many residues often are sulfated. GAGs are major components of the extracellular matrix, usually as components of proteoglycans. (Figure 20-29)

glycosidic bond The covalent linkage between two monosaccharide residues formed when a carbon atom in one sugar reacts with a hydroxyl group on a second sugar with the net release of a water molecule (dehydration). (Figure 2-13)

G protein–coupled receptor (GPCR) Member of a large class of cell-surface signaling receptors, including those for epinephrine, glucagon, and yeast mating factors. All GPCRs contain seven transmembrane α helices. Ligand binding leads to activation of a coupled trimeric G protein, thereby initiating intracellular signaling pathways. (Figures 15-14 and 15-15)

Golgi complex Stacks of flattened, interconnected membrane-bounded compartments (cisternae) in eukaryotic cells that function in processing and sorting of proteins and lipids destined for other cellular compartments or for secretion; also called Golgi apparatus. (Figure 4-32)

granulocyte colony–stimulating factor (G-CSF) A cytokine that induces a granulocyte progenitor cell in the bone marrow to divide and differentiate into granulocytes.

G-12

growth factor An extracellular polypeptide molecule that binds to a cell-surface receptor, triggering an intracellular signaling pathway generally leading to cell proliferation.

growth hormone (GH) A cytokine secreted by the anterior pituitary gland that stimulates proliferation of a variety of cells.

GTPase superfamily Group of intracellular switch proteins that cycle between an inactive state with bound GDP and an active state with bound GTP. Includes the Gα subunit of trimeric (large) G proteins, monomeric (small) G proteins (e.g., Ras, Rab, Ran, and Rac), and certain elongation factors used in protein synthesis. (Figure 3-34)

haploid Referring to an organism or cell having only one memberof each pair of homologous chromosomes and hence only one copy (allele) of each gene or genetic locus. Gametes and bacterial cells are haploid. See also diploid.

Hedgehog (Hh) A family of secreted signaling proteins that are important regulators of the development of most tissues and organs in diverse animal species. Mutations in Hh signal transduction components are implicated in human cancer and birth defects. The receptor is the Patched transmembrane protein. (Figures 16-32, 16-33 and 16-34)

helicase (1) Any enzyme that moves along a DNA duplex using the energy released by ATP hydrolysis to separate (unwind) the two strands; required for DNA replication. (2) Activity of certain initiation factors that can unwind the secondary structures in mRNA during initiation of translation.

helix-loop-helix, basic (bHLH) A conserved DNA-binding structural motif, consisting of two α helices connected by a short loop, that is found in many dimeric eukaryotic transcription factors. (Figure 9-30d)

helix-turn-helix A structural motif in which two alpha helices are connected by a short stretch of connecting residues, sometimes also called a “loop.” Helix-turn-helix/helix-loop-helix structural motifs can perform various functions, including binding calcium and binding DNA.

HER family Group of receptors, belonging to the receptor tyrosine kinase (RTK) class, that bind to members of the epidermal growth factor (EGF) family of signaling molecules in humans. Overexpression of HER2 protein is associated with some breast cancers. (Figure 16-17)

heredity The transfer of genetically determined characteristics from one generation to the next.

heterochromatin Regions of chromatin that remain highly condensed and transcriptionally inactive during interphase. (Figure 8-28a)

heterotrimeric G proteins A class of GTPase switch proteins, composed of alpha, beta, and gamma polypeptides, that bind to and are activated by certain cell-surface receptors. When activated, heterotrimeric G proteins release GDP and bind GTP.

heterozygous Referring to a diploid cell or organism having two different alleles of a particular gene.

hexose A six-carbon monosaccharide.

high-energy bond Covalent bond that releases a large amount of energy when hydrolyzed under the usual intracellular conditions. Examples include the phosphoanhydride bonds in ATP, thioester bond in acetyl CoA, and various phosphate ester bonds.

Hippo pathway A signal transduction pathway that controls cell growth in the context of tissues.

histone One of several small, highly conserved basic proteins, found in the chromatin of all eukaryotic cells, that associate with DNA in the nucleosome. (Figure 8-24)

Holliday structure An intermediate in DNA recombination with four DNA strands. (Figure 5-42)

homeodomain Conserved DNA-binding structural motif (a helix-turn-helix) found in many developmentally important transcription factors.

homologous See homologs.

homologous chromosome One of the two copies of each morphologic type of chromosome present in a diploid cell; also called homolog. Each homolog is derived from a different parent.

homologous recombination See recombination.

homolog A protein that shares a common ancestor, and therefore is similar in sequence and/or structure, with another protein.

homologs Maternal and paternal copies of each morphologic type of chromosome present in a diploid cell; also called homologues.

homology Similarity in characteristics (e.g., protein and nucleic acid sequences or the structure of an organ) that reflects a common evolutionary origin. Proteins or genes that exhibit homology are said to be homologous and sometimes are called homologs. In contrast, analogy is a similarity in structure or function that does not reflect a common evolutionary origin.

homozygous Referring to a diploid cell or organism having two identical alleles of a particular gene.

hormone Generally, any extracellular substance that induces specific responses in target cells; specifically, those signaling molecules that circulate in the blood and mediate endocrine signaling.

hyaluronan A large, highly hydrated glycosaminoglycan (GAG) that is a major component of the extracellular matrix; also called hyaluronic acid and hyaluronate. It imparts stiffness and resilience as well as a lubricating quality to many types of connective tissue. (Figure 20-29a)

hybridization, nucleic acid Association of two complementary nucleic acid strands to form double-stranded molecules, which can contain two DNA strands, two RNA strands, or one DNA and one RNA strand. Used experimentally in various ways to detect specific DNA or RNA sequences.

hybridoma A clone of hybrid cells that are immortal and produce monoclonal antibody; formed by fusion of a normal antibody-producing B cell with a myeloma cell. (Figure 4-6)

hydrocarbon Any compound containing only carbon and hydrogen atoms.

hydrogen bond A noncovalent interaction between an atom (commonly oxygen or nitrogen) carrying a partial negative charge and a hydrogen atom carrying a partial positive charge. Important in stabilizing the conformation of proteins and in formation of base pairs between nucleic acid strands. (Figure 2-8)

G-13

hydrophilic Interacting effectively with water. See also polar.

hydrophobic Not interacting effectively with water; in general, poorly soluble or insoluble in water. See also nonpolar.

hydrophobic effect The tendency of nonpolar molecules or parts of molecules to associate with each other in aqueous solution so as to minimize their direct interactions with water; commonly called a hydrophobic interaction or bond. (Figure 2-11)

hyperpolarization Increase in the magnitude of the cytosolic-face negative electric potential that normally exists across the plasma membrane of a cell at rest, resulting in a more negative membrane potential.

hypertonic Referring to an external solution whose solute concentration is high enough to cause water to move out of cells due to osmosis.

hypotonic Referring to an external solution whose solute concentration is low enough to cause water to move into cells due to osmosis.