G

G protein:  A GTP binding protein that relays signals from a plasma membrane signal receptor, known as a G-protein linked receptor, to other signal transduction proteins inside the cell.
Go phase: A nondividing state in which the cell has left the cell cycle.
G1 phase: The first growth phase of the cell cycle, consisting of the portion of interphase before DNA synthesis begins.
G2 phase: The second growth phase of the cell cycle, consisting of the portion of interphase after DNA synthesis occurs.
Gallbladder: An organ that stores bile and releases it as needed into the small intestine.
Galls: Structure induced in a plant by a parasite (e.g. bacterium or an insect) that nurture that parasite.
Gametangium: Multicellular plant structures in which gametes are formed. Female gametangia are called archegonia, and male gametangia are called antheridia.
Gamete: A haploid cell, such as an egg or sperm. Gametes unite during sexual reproduction to produce a diploid zygote.
Gametogenesis: The process by which gametes are produced in the mammalian body.
Gametophore: The mature gamete-producing structure of a gametophyte body of a moss.
Gametophyte: In plants, the haploid structure that produces gametes. It contrasts with sporophyte.
Gamma Aminobutyric acid (GABA): An amino acid that functions as a CNS neurotransmitter.
Ganglion: A cluster, functional group, of nerve cell bodies in a central nervous system.
Ganglion cell: A type of neuron in the retina that synapses with bipolar cells and transmit action potentials to the brain via axon in the optic nerve.
Gap junction: A type of intercellular junction in animal cells that allow the passage of material or current between cells.
Gas exchange: The uptake of molecular oxygen from the environment and the discharge of carbon dioxide to the environment.
Gas hydrate: Frozen deposits rich in hydrocarbons that occur in the deep ocean basins.
Gastric juice: A digestive fluid secreted by the stomach.
Gastrovascular cavity:  An extensive pouch that serves as the site of extracellular digestion and a passage to disperse materials throughout most of an animal’s body.
Gastrula: The three-layered cup-shaped embryonic stage.
Gastrulation: The formation of a gastrula from a blastula.
Gated channel: A protein channel in a cell membrane that opens or closes in response to a particular stimulus.
Gated ion channel: A gated channel for a specific ion, when open or close it alters the membrane potential of the cell.
Gel electrophoresis: The separation of nuclei acids or proteins, on the basis of their size and electrical charge, by measuring the rate of movement through an electrical field in a gel.
Gene: A stretch of DNA (or in some viruses RNA) sequence that codes for a protein or RNA molecule, together with associated elements.
Genealogy: The tree-like ancestral relationship that connects a set of genes at a single genetic locus.
Gene cloning: The production of multiple copies of a gene.
Gene conversion: A meiotic process in which nonreciprocal exchange of genetic information occurs as a result of heteroduplex formation between non-sister chromatids. Thus, heterozygous sites are converted to sites homozygous for one or the other allele.
Gene diversity: The probability that two randomly chosen genes will carry different alleles.
Gene dosage: The number of copies of a gene within an individual.
Gene flow: The movement of genes from place to place. The term usually refers to movement in space but also refers to movement between microhabitats or to introgression between distinct populations or species.
Gene-for-gene recognition: A widespread form of plant disease resistance involving recognition of pathogen-derived molecules by the protein products of specific plant disease resistance genes.
Gene pool: The total aggregate of genes in a population at any one time.
Gene therapy: A treatment in which functional genes are introduced into the patient.
Genetic annealing: The production of a new genome through the transfer of part of the genome of one organisms to another organism.
Genetic assimilation: The process in which a phenotype, normally expressed only in a specific environment, through selection shows increase expression in that environment.
Genetic background: The set of genes with which a gene of interest is associated. Recombination moves genes from one genetic background to another.
Genetic code: The code that translates 64 possible triplet codons into amino acids and translation stop signals.
Genetic correlation: A correlation between the breeding values for different traits. This may be due to linkage disequilibrium between genes affecting the different traits or to pleiotropy, in which alleles affect both traits.
Genetic distance: A measure of the difference in allele frequencies between populations.
Genetic drift: Unpredictable fluctuations in allele frequencies from one generation to the next because of a population’s finite size.
Genetic engineering: The manipulation of organisms by the artificial introduction of DNA sequence in order to change their characteristics.
Genetic load: The loss of mean fitness relative to some ideal fitness. It includes mutation, substitution, recombination, and segregation loads.
Genetic map: A map of the linear order of genes constructed by measuring the rates of recombination between them.
Genetic marker: A polymorphic locus that is used to observe genetic variation but that is not itself of primary interest.
Genetic polymorphism: The existence of two or more distinct alleles at a given locus in a population’s gene pool.
Genetic recombination: General term for the production of offspring that combine traits of the two parents.
Genetic modified organisms (GMO): An organism that has acquired one or more genes by artificial means; also known as transgenic organism.
Genetic system: The system immediately responsible for transmission of genetic information.
Genetics: The scientific study of heredity and hereditary variation.
Genome: A genome is an organism’s complete set of DNA, including all of its genes. Each genome contains all of the information needed to build and maintain that organism. In humans, a copy of the entire genome—more than 3 billion DNA base pairs—is contained in all cells that have a nucleus.
Genome size (C-value): The size of a single haploid genome. It is sometimes expressed as the mass of a single haploid genome measured in picograms and corresponding to approximately 1000Mb of DNA.
Genomic imprinting: Phenomenon in which expression of an allele in offspring depends on whether the allele is inherited from the male of female parent
Genomic library: A set of thousands of DNA segments from a genome, each carried by a plasmid, phage or other cloning vector.
Genomics: The study of whole sets of genes and their intercations.
Genotype: The set of alleles carried by an individual.
Genus: A class composed of species. The subdivision above species, for example: Homo (genus) sapiens (species).
Geographical variation:  Differences between the gene pools of separate populations or population subgroups.
Geological record: The division of Earth’s history into time periods, grouped into three eras: Archean, Proterozoic, and Phanerozoic.
Geometric isomer: One of several organic compounds that have the same molecular formula but differ in the spatial arrangement of its atoms.
Germ layers: Three main layers, ectoderm, endoderm, and mesoderm, that from the various tissues and organs of an animal body.
Germ line: In a multicellular organism, the lineage of cells that will generate gametes via meiosis. It contrasts with soma.
Germinal centers: Sites in secondary lymphoid organs (lymph nodes, spleen, tonsils) where high affinity antibody-secreting plasma cells and memory B cells are generated during immune responses.
Gestation: Pregnancy; the state of carrying developing young within the female reproductive tract.
Gibberellins: A class of related plant hormones that stimulates growth in the stem and leaves, trigger the germination of seeds and breaking of bud dormancy, and stimulates fruit development with auxin.
Gill: A localized  extension of the body surface of many aquatic animals, specialized for gas exchange.
Gigantism: The quality of exceeding the usual or expected (as in size, force, or prominence).
Glandular epithelium: An epithelium that absorbs and secretes chemical solutions.
Glaucocystophyte: Member of a phylum of eukaryotes that contains single-celled photosynthetic organisms.
Glia: Supporting cells that are essential for the structural integrity of the nervous system and for the normal functioning of neurons.
Glomeromycete: A member of the fungal phylum Glomeromycota, characterized by forming a distinct branching form of endomycorrhizae called arbuscular mycorrhizae that forms a symbiotic relationship with plants.
Glomerulus: A ball of capillaries surrounding the Bowman’s capsule in the nephron and serving as the site of filtration in the vertebrate kidney.
Glucagon: A hormone secreted by pancreatic alpha cells that raises blood glucose levels. It promotes glycogen breakdown and the release of glucose by the liver.
Glucocorticoid:  A steroid hormone secreted by the adrenal cortex that influences glucose metabolism and immune function.
Glutamate:  An amino acid that functions as a CNS neurotransmitter.
Glyceraldehyde-3-phosphate (G3P): The carbohydrate produced  directly from the Calvin cycle.
Glycine: An amino acid that functions as a CNS neurotransmitter.
Glycogen: An extensively branched glucose storage polysaccharide found in the liver and muscle of animals. The animal equivalent of starch.
Glycolipid: A lipid covalently attached to a carbohydrate.
Glycolysis: The splitting of glucose  into pyruvate. Glycolysis is the one metabolic pathway that occurs in all living cells, serving as a starting point for fermentation or aerobic respiration.
Glycoprotein: A protein covalently attached to a carbohydrate.
Glycosidic linkage: A covalent bond formed between two monosaccharides by a dehydration reaction.
Gnathostome: Member of the vertebrate subgroup possessing jaws.
Golden algae: A chrysophyte; a typically unicellular, biflagellated alga with yellow and brown  carotenoid pigments.
Golgi apparatus: An organelle in eukaryotic cells consisting of stacks of flat membranous sacs that modify, store, and route products of the endoplasmic reticulum.
Gonadotropin: A hormone that stimulates the activities of the testes and ovaries. Follicle-stimulating hormone and luteinizing hormone are gonadotropins.
Gonads: The female and male sexual organs; the gamete-producing organs in most animals
Good genes: Genes that increase fitness for reasons that are not due to sexual selection on a single trait. Such components of fitness are most conveniently thought of as viability, but fertility and mating success could also be included.
G-protein-linked receptor: A signal receptor protein in the plasma membrane that responds to the binding signal molecule by activating a G protein.
Gracile: Graceful, slender, and delicate. The term is used to describe the large rear teeth of certain Australopithecenes. It contrasts with robust.
Grade: Group of animal species that share the same level of organizational complexity.
Grade potential:  A local voltage change in a neuron membrane induced by stimulation of a neuron, with strength proportional to the strength of the stimulus and lasting about a millisecond.
Gradualism: A view of Earth’s history that attributes profound change to the cumulative product of slow but continuous processes.
Graft versus host reaction: An attack against a patients’ body cells by lymphocytes received in a bone marrow transplant.
Gram negative: Describing a group of bacteria with a cell wall that is structurally more complex and contains less peptidoglycan than that of a gram-positive bacteria.
Gram positive: Describing a group of bacteria with a cell wall that is structurally less complex and contains more peptidoglycan than that of a gram-negative bacteria.
Gram stain: A stain that specifically detects the type of cell wall and membrane structure found in certain bacterial species. These are known as “gram positive” species.
Granum: A stacked portion of the thylakoid membrane in the chloroplast. Grana function in the light reactions of photosynthesis.
Gravitropism: A response of a plant or animal to gravity.
Gray crescent:  A light-gray region of cytoplasm located near the equator of the egg on the side opposite the sperm entry.
Gray matter: Regions of dendrites and clusters of neuron cell bodies within the CNS.
Graze: to feed on growing grass or pasturage.
Great oxidation event (GOE): The initial rise of oxygen ca. 2.5 billion years ago.
Green algae: A unicellular, colonial, or multicellular photosynthetic protist that has grass-green  chloroplasts. Green algae is closely related to true plants.
Greenhouse effect: The warming of planet Earth due to the atmosphere accumulation of carbon dioxide, which absorbs reflected infrared radiation and re-reflects some of it back towards Earth.
Green world hypothesis: The conjecture that terrestrial herbivores consume relatively little plant material because they are held in check by several factors including predators, parasites, and disease.
Gross primary production (GPP): The total primary production of an ecosystem.
Ground tissue system: Plant tissue that are neither vascular nor dermal, fulfilling a variety of functions, such as storage, photosynthesis, and support.
Group selection: Selection of traits that increase survival and proliferation of groups of individuals.
Growth cone: Responsive region at the leading edge of a growing axon.
Growth factor: A protein that must be present in the extracellular environment (culture medium or animal body) for the growth and normal development of certain types of cells; a local regulator that acts on nearby cells to stimulate cell proliferation and differentiation.
Growth hormone (GH): A hormone produced and secreted by the anterior pituitary that has both direct (non tropic) effects and tropic effects on a wide variety of tissues.
Guard cell: The two cells that flank the stomatal pore and regulate the opening and closing of the pore.
Gustatory receptor: Taste receptor.
Guttation: The exudation of water droplets, cause by root pressure in certain plants.
Gymnosperm: A vascular plant that bears naked seeds; seeds not enclosed in specialized chambers.
Gynodioecious: A population that contains both females and hermaphrodites.