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. (See Panel 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-b 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 GTPbinding
proteins; these are commonly referred to as monomeric GTPases. guanine nucleotide exchange factor (GEF) Protein that binds to a GTPbinding
protein and activates it by stimulating it to release its tightly bound
GDP, thereby allowing it to bind GTP in its place.
