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 guanyl nucleotides. The activation of certain receptors activates the G protein, which in turn activates adenylate cyclase. G protein activation involves binding a GTP molecule in place of a GDP molecule.
Gametangium (gam i tan´ gee um) [Gr. gamos: marriage + angeion: vessel or reservoir] • Any plant or fungal structure within which a gamete is formed.
Gamete (gam´ eet) [Gr. gamete: wife, gametes: husband] • The mature sexual reproductive cell: the egg or the sperm.
Gametocyte (ga meet´ oh site) [Gr. gamete: wife, gametes: husband + kytos: cell] • The cell that gives rise to sex cells, either the eggs or the sperm. (See oocyte and spermatocyte.)
Gametogenesis (ga meet´ oh jen´ e sis) [Gr. gamete: wife, gametes: 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 haploid phase that produces the gametes. (Contrast with sporophyte.)
Ganglion (gang´ glee un) [Gr.: tumor] • A group or concentration of neuron cell bodies.
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 channel (membrane protein) that opens and closes in response to binding of specific molecules or to changes in membrane potential.
Gel electrophoresis (jel ul lec tro for´ eesis) • A semisolid matrix suspended in a salty buffer in which molecules can be separated on the basis of their size and change when current is passed through the gel.
Gene [Gr. gen: to produce] • A unit of heredity. Used here as the unit of genetic function which carries the information for a single polypeptide.
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 pool • All of the genes in a population.
Gene therapy • Treatment of a genetic disease by providing patients with cells containing wild type alleles for the genes that are nonfunctional in their bodies.
Generative nucleus • In a pollen tube, a haploid nucleus that undergoes mitosis to produce the two sperm nuclei that participate in double fertilization. (Contrast with tube nucleus.)
Genet • The genetic individual of a plant that is composed of a number of nearly identical but repeated units.
Genetic drift • Changes in gene frequencies from generation to generation in a small population as a result of random processes.
Genetic stochasticity • Variation in the frequencies of alleles and genotypes in a population over time.
Genetics • The study of heredity.
Genetic structure • The frequencies of alleles and genotypes in a population.
Genome (jee´ nome) • The genes in a complete haploid set of chromosomes.
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 • A reproductive cell or gamete of a multicellular organism.
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) [L. gibberella: hunchback (refers to shape of a reproductive structure of a fungus that produces gibberellins)] • One of 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 very 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.: glue] • Cells, found only in the nervous system, which 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, the linking component of phospholipids and triglycerides.
Glycogen (gly´ ko jen) • A branched-chain polymer of glucose, similar to starch (which is less branched and may be of lower molecular weight). Exists mostly in liver and muscle; the principal storage carbohydrate of most animals and fungi.
Glycolysis (gly kol´ li sis) [from glucose + Gr. lysis: loosening] • The enzymatic breakdown of glucose to pyruvic acid. One of the oldest energy-yielding machanisms in living organisms.
Glycosidic linkage • The connection in an oligosaccharide or polysaccharide chain, formed by removal of water during the linking of monosaccharides.by root pressure.
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. Plays a role in the production and release of secretory materials such as the digestive enzymes manufactured in the pancreas. First described by Camillo Golgi (1844-1926).
Gonad (go´ nad) [Gr. gone: seed, that which produces 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 South America, Africa, India, Australia, and Antarctica.
Gram stain • A differential stain useful in characterizing bacteria.
Granum • 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 radiated 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 (gwan´een) • A nitrogen-containing base found in DNA, RNA and GTP.
Guard cells • In plants, paired epidermal cells which surround and control the opening of a stoma (pore).
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 (plural: gyri) • The raised or ridged portion of the convoluted surface of the brain. (Contrast to sulcus.)