gene segments In immunology: short DNA sequences that are joined together during B cell and T cell development to produce the coding sequences for immunoglobulins and T cell receptors, respectively. (Figure 24–28) general transcription factor   Any of the proteins whose assembly at all promoters of a given type is required for the binding and activation of RNA polymerase and the initiation of transcription. (Table 6–3, p. 311) genetic code   The set of rules specifying the correspondence between nucleotide triplets (codons) in DNA or RNA and amino acids in proteins. (Figure 6–48) genetic instability   Abnormally increased spontaneous mutation rate, such as occurs in cancer cells.
genetic screen   Procedure for discovery of genes affecting a specific phenotype by surveying large numbers of mutagenized individuals.
genetics The study of the genes of an organism on the basis of heredity and variation.
genome The totality of genetic information belonging to a cell or an organism; in particular, the DNA that carries this information.
genome annotation   Process attempting to mark out all the genes (protein-coding and noncoding) in a genome and ascribing functions to each.
genomic imprinting Phenomenon in which a gene is either expressed or not expressed in the offspring depending on which parent it is inherited from. (Figure 7–49) genomic library Collection of cloned DNA molecules representing an entire genome.
genotype Genetic constitution of an individual cell or organism. The particular combination of alleles found in a specific individual. (Panel 8–2, p. 486) germ cell A cell in the germ line of an organism, which includes the haploid gametes and their specified diploid precursor cells. Germ cells contribute to the formation of a new generation of organisms and are distinct from somatic cells, which form the body and leave no descendants.
germ layer   One of the three primary tissue layers (endoderm, mesoderm, and ectoderm) of an animal embryo. (Figure 21–3) glial cell Supporting non-neural cell of the nervous system. Includes oligodendrocytes and astrocytes in the vertebrate central nervous system and Schwann cells in the peripheral nervous system.
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. (Figure 2–51 and Panel 2–4, pp. 96–97) glycolipid   Lipid molecule with a sugar residue or oligosaccharide attached. (Panel 2–5, pp. 98–99) glycolysis Ubiquitous metabolic pathway in the cytosol in which sugars are incompletely degraded with production of ATP. Literally, “sugar splitting.” (Figure 2–46 and Panel 2–8,  pp. 104–105) glycoprotein   Any protein with one or more saccharide or 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, hyaluronan, and heparin. (Figure 19–32) glycosylphosphatidylinositol anchor (GPI anchor) Lipid linkage by which some membrane proteins are bound to the membrane. The protein is joined, via an oligosaccharide linker, to a phosphatidylinositol anchor during its travel through the endoplasmic reticulum. (Figure 12–52)