G cap • A chemically modified GTP added to the 5´ end of mRNA; facilitates binding of mRNA to ribosome and prevents mRNA breakdown.

G₁ phase • In the cell cycle, the gap between the end of mitosis and the onset of the S phase.

G₂ phase • In the cell cycle, the gap between the S (synthesis) phase and the onset of mitosis.

G protein • A membrane protein involved in signal transduction; characterized by binding GDP or GTP.

Gametangium (gam uh tan´ gee um) [Gr. gamos: marriage + angeion: vessel] • Any plant or fungal structure within which a gamete is formed.

Gamete (gam´ eet) [Gr. gamete/gametes: wife, husband] • The mature sexual reproductive cell: the egg or the sperm.

Gametogenesis (ga meet´ oh jen´ e sis) [Gr. gamete/gametes: wife, husband + genesis: source] • The specialized series of cellular divisions that leads to the production of sex cells (gametes). (Contrast with oogenesis and spermatogenesis.)

Gametophyte (ga meet´ oh fyte) • In plants and photosynthetic protists with alternation of generations, the multicellular haploid phase that produces the gametes. (Contrast with sporophyte.)

Ganglion (gang´ glee un) [Gr.: tumor] • A group or concentration of neuron cell bodies.

Gap genes • During insect development, the first step of segmentation genes to act organizing the anterior-posterior axis.

Gap junction • A 2.7-nanometer gap between plasma membranes of two animal cells, spanned by protein channels. Gap junctions allow chemical substances or electrical signals to pass from cell to cell.

Gas exchange • In animals, the process of taking up oxygen from the environment and releasing carbon dioxide to the environment.

Gastrovascular cavity • Serving for both digestion (gastro) and circulation (vascular); in particular, the central cavity of the body of jellyfish and other cnidarians.

Gastrula (gas´ true luh) [Gr. gaster: stomach] • An embryo forming the characteristic three cell layers (ectoderm, endoderm, and mesoderm) which will give rise to all of the major tissue systems of the adult animal.

Gastrulation • Development of a blastula into a gastrula.

Gated channel • A membrane protein that opens and closes in response to binding of specific molecules or to changes in membrane potential. When open, it allows specific ions to move across the membrane.

Gene [Gr. genes: to produce] • A unit of heredity. Used here as the unit of genetic function which carries the information for a single polypeptide or RNA.

Gene amplification • Creation of multiple copies of a particular gene, allowing the production of large amounts of the RNA transcript (as in rRNA synthesis in oocytes).

Gene cloning • Formation of a clone of bacteria or yeast cells containing a particular foreign gene.

Gene family • A set of identical, or once-identical, genes, derived from a single parent gene; need not be on the same chromosomes; classic example is the globin family in vertebrates.

Gene flow • The exchange of genes between different species (an extreme case referred to as hybridization) or between different populations of the same species caused by migration following breeding.

Gene frequency • See Allele frequency.

Gene library • All of the cloned DNA fragments generated by action of a restriction endonuclease on a genome or chromosome.

Gene pool • All of the alleles of all of the genes in a population.

Gene therapy • Treatment of a genetic disease by providing patients with cells containing functioning alleles of the genes that are nonfunctional in their bodies.

Generative cell • In a pollen tube, a haploid nucleus that undergoes mitosis to produce the two sperm nuclei that participate in double fertilization. (Contrast with tube cell.)

Genetics • The study of the structure, functioning, and inheritance of genes, the units of hereditary information.

Genetic drift • Changes in gene frequencies from generation to generation in a small population as a result of random (chance) processes.

Genetic screening • The application of medical tests to determine whether an individual carries a specific allele.

Genetic stochasticity • Random variation in the frequencies of alleles and genotypes in a population over time. (Compare with demographic stochasticity.)

Genome (jee´ nome) • All the genes in a complete haploid set of chromosomes. (Compare with proteome.)

Genomics • The study of entire sets of genes and their interactions.

Genomic imprinting • When a given gene's phenotype is determined by whether that gene is inherited from the male or the female parent.

Genotype (jean´ oh type) [Gr. gen: to produce + typos: impression] • An exact description of the genetic constitution of an individual, either with respect to a single trait or with respect to a larger set of traits. (Contrast with phenotype.)

Genus (jean´ us) (plural: genera) [Gr. genos: stock, kind] • A group of related, similar species.

Geotropism • See gravitropism.

Germ cell [L. germen: to beget] • A reproductive cell or gamete of a multicellular organism. Contrast with somatic cell.

Germ layers • The three embryonic tissue layers formed during gastrulation (ectoderm, mesoderm, endoderm).

Germination • The sprouting of a seed or spore.

Gestation (jes tay´ shun) [L. gestare: to bear] • The period during which the embryo of a mammal develops within the uterus. Also known as pregnancy.

Gibberellin (jib er el´ lin) • A class of plant growth substances playing roles in stem elongation, seed germination, flowering of certain plants, etc. Named for the fungus Gibberella.

Gill • An organ for gas exchange in aquatic organisms.

Gill arch • A skeletal structure that supports gill filaments and the blood vessels that supply them.

Gizzard (giz´ erd) [L. gigeria: cooked chicken parts] • A muscular port of the stomach of birds that grinds up food, sometimes with the aid of fragments of stone.

Gland • An organ or group of cells that produces and secretes one or more substances.

Glans penis • Sexually sensitive tissue at the tip of the penis.

Glia (glee´ uh) [Gr. glia: glue] • Cells, found only in the nervous system, that do not conduct action potentials.

Glomerulus (glo mare´ yew lus) [L. glomus: ball] • Sites in the kidney where blood filtration takes place. Each glomerulus consists of a knot of capillaries served by afferent and efferent arterioles.

Glucocorticoids • Steroid hormones produced by the adrenal cortex. Secreted in response to ACTH, they inhibit glucose uptake by many tissues in addition to mediating other stress responses.

Glucagon • A hormone produced and released by cells in the islets of Langerhans of the pancreas. It stimulates the breakdown of glycogen in liver cells.

Gluconeogenesis • The biochemical synthesis of glucose from other substances, such as amino acids, lactate, and glycerol.

Glucose (glue´ kose) [Gr. gleukos: sweet wine mash for fermentation] • The most common sugar, one of several monosaccharides with the formula C₆H₁₂O₆.

Glycerol (gliss´ er ole) • A three-carbon alcohol with three hydroxyl groups; a component of phospholipids and triglycerides.

Glycogen (gly´ ko jen) • An energy storage polysaccharide found in animals and fungi; a branched-chain polymer of glucose, similar to starch.

Glycolysis (gly kol´ li sis) [Gr. gleukos: sugar + lysis: break apart] • The enzymatic breakdown of glucose to pyruvic acid. One of the evolutionarily oldest of the cellular energy-yielding mechanisms.

Glycosidic linkage • The bond between sugar molecules through an intervening oxygen atom (-O-).

Glyoxysome (gly ox´ ee soam) • An organelle found in plants, in which stored lipids are converted to carbohydrates.

Golgi apparatus (goal´ jee) • A system of concentrically folded membranes found in the cytoplasm of eukaryotic cells; functions in secretion from cell by exocytosis.

Gonad (go´ nad) [Gr. gone: seed] • An organ that produces sex cells in animals: either an ovary (female gonad) or testis (male gonad).

Gonadotropin • A hormone that stimulates the gonads.

Gondwana • The large southern land mass that existed from the Cambrian (540 mya) to the Jurassic (138 mya). Present-day remnants are South America, Africa, India, Australia, and Antarctica.

Graft • A bud or stem segment from one plant artificially and viably attached to another plant. A form of asexual reproduction.

Gram stain • A differential purple stain useful in characterizing bacteria.

Granum (plural: grana) • Within a chloroplast, a stack of thylakoids.

Gravitropism • A directed plant growth response to gravity.

Grazer • An animal that eats the vegetative tissues of herbaceous plants.

Green gland • An excretory organ of crustaceans.

Greenhouse effect • The heating of Earth's atmosphere by gases that are transparent to sunlight but opaque to heat.

Greenhouse gases • Gases that contribute climate warming (water vapor, carbon dioxide, and methane) because they are transparent to light but opaque to heat.

Gross primary production • The total energy captured by plants growing in a particular area.

Ground meristem • That part of an apical meristem that gives rise to the ground tissue system of the primary plant body.

Ground tissue system • Those parts of the plant body not included in the dermal or vascular tissue systems. Ground tissues function in storage, photosynthesis, and support.

Group transfer • The exchange of atoms between molecules.

Growth • Irreversible increase in volume (probably the most accurate definition, but at best a dangerous oversimplification).

Growth factors • A group of proteins that circulate in the blood and trigger the normal growth of cells. Each growth factor acts only on certain target cells.

Guanine (G) (gwan´ een) • A nitrogen-containing base found in DNA, RNA, and GTP.

Guard cells • In plants, specialized, paired epidermal cells that surround and control the opening of a stoma (pore). See stoma.

Gut • An animal's digestive tract.

Guttation • The extrusion of liquid water through openings in leaves, caused by root pressure.

Gymnosperm (jim´ no sperm) [Gr. gymnos: naked + sperma: seed] • A plant, such as a pine or other conifer, whose seeds do not develop within an ovary (hence, the seeds are "naked").

Gyrus • The raised or ridged portion of the convoluted surface of the brain. (Contrast to sulcus.)