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- G -

G  Guanine residue in either DNA or RNA.

G cap  The 5´-terminal methylated guanine nucleoside that is present on many eukaryotic mRNAs; it is joined, after transcription, to the mRNA. See cap.

gall  A tumorous growth in plants.

gamete  A mature reproductive cell which is capable of fusing with a cell of similar origin but of opposite sex to form a zygote from which a new organism can develop. Gametes have a haploid chromosome content. In animals, a gamete is a sperm or egg; in plants, it is pollen, spermatic nucleous, or ovum.

gamete and embryo storage  Storage of ova, sperm or fertilized embryos outside their original source. Almost invariably this means cryopreservation in liquid nitrogen.

gametic (phase) disequilibrium  In relation to any two loci, the occurrence of haplotypes (gametes) with a frequency greater than or less than the product of the frequency of the two relevant alleles. See also gametic (phase) equilibrium.

gametic (phase) equilibrium  In relation to any two loci, the occurrence of haplotypes (gametes) with a frequency equal to the product of the frequency of the two relevant alleles, e.g., loci A and B are in linkage equilibrium if the frequency of the haplotype (gamete) AiBi equals the product of the frequencies of alleles Ai and Bi. a.k.a. linkage equilibrium, but this is a misleading term, because the concept applies equally to linked and unlinked loci.

gametoclone  A plant regenerated from a tissue culture originating from gametic tissue.

gametogenesis  The process of the formation of gametes.

gametophyte  That phase of the plant life cycle that bears the gamete producing organs; the cells have n chromosomes. In angiosperms, the pollen grain is the male gametophyte and the embryo sac is the female gametophyte.

gametophytic incompatibility  A phenomenon controlled by the complex S locus, in which a pollen grain cannot fertilize an ovule produced by a plant that carries the same S allele as the pollen grain. Thus S1 pollen cannot fertilize an ovule of a plant possessing the S1 allele.

gap 1. Period of time, during the cell cycle, between M and S phases.

    2. A missing section on one of the strands of double-stranded DNA. The DNA will therefore have a single-stranded region.

gapped DNA  A duplex DNA molecule with one or more internal single-stranded regions.

gas transfer  The rate at which gases are transferred from gas into solution. It is an important parameter in fermentation systems because it controls the rate at which the organism can metabolize. Gas transfer can be done by several methods, including use of small gas bubbles, that diffuse faster than larger ones, thanks to their larger surface area per unit of volume; or spreading the liquid out, for example in a thin sheet, or in a thin permeable tube, as in hollow fibre bioreactor.

gastrula  An early animal embryo consisting of two layers of cells; an embryological stage following the blastula.

G cap  See cap.

GC island  A segment of DNA that is rich in G=C base pairs and often precedes a transcribed gene in the genomes of vertebrate organisms.

GDP  Guanosine diphosphate, See guanosine.

gel  A lyophilic colloid that has coagulated to a rigid or jelly-like solid. It is used for the electrophoretic separation of nucleic acids or proteins, and for encapsulation. See encapsulating agent; gel electrophoresis.

gel electrophoresis  An analytical method for separating molecules according to their size. Samples are put at one end of a slab of polymer gel; an electric field across the gel pulls the molecules through it; the smaller molecules pass more easily and so move towards the other end faster; the various sizes of molecules end up at different positions according their size. Gels are made from different materials, but common combinations are:





Gel matrix:


agarose or polyacrylamide

polyacrylamide or starch

    Various chemicals can be included in the gel to help separation, such as the detergent sodium dodecyl sulphate (SDS) in protein gels to unfold proteins, or urea in DNA sequencing gels, which unfolds DNA. See iso-electric focusing gels; polyacrylamide gel electrophoresis (PAGE); pulsed field gel electrophoresis (PFGE).

gelatin  A glutinous, proteinaceous gelling and solidifying agent. Gelatin is produced by boiling animal connective tissues, which partially hydrolyses the collagen. Gelatin is used to gel or solidify nutrient solutions for tissue culture. cf agar.

gelatinization  Steam cooking of milled grain, a process that increases the surface area of the starch and converts the original mash to a material with a gel-like consistency.

GelriteTM  The brand name of a Pseudomonas-derived refined polysaccharide used as a gelling agent and agar substitute.

GEM  Genetically engineered micro-organism.

gene (Gr. gen, race, offspring). Conceptually, the unit of heredity transmitted from generation to generation during sexual or asexual reproduction. More generally, the term is used in relation to the transmission and inheritance of particular identifiable traits. Since the molecular revolution, it is now known that a gene is a segment of nucleic acid that encodes peptide or RNA.

    See polygenes; dominant gene; fusion gene; gene amplification; gene expression; gene flow; gene pool; gene splicing; gene translocation; recessive gene; regulatory gene.

gene addition  The addition of a functional copy of a gene to the genome of an organism.

gene amplification  The selective production of multiple gene copies without a proportional increase in others.

gene bank  1. The physical location where collections of genetic material in the form of seeds, tissues or reproductive cells of plants or animals are stored.

    2. Field gene bank: A facility established for the ex situ storage and maintenance, using horticultural techniques, of individual plants. Used for species whose seeds are recalcitrant (q.v.), or for clonally propagated species of agricultural importance, such as apple varieties).

    3. A collection of cloned DNA fragments from a single genome. Ideally the bank should contain cloned representatives of all the DNA sequences in the genome.

    4. A population of micro-organisms, each of which carries a DNA molecule that was inserted into a cloning vector. Ideally, all of the cloned DNA molecules together represent the entire genome of another organism. Also called gene library, clone bank, bank, library. This term is sometimes also used to denote all of the vector molecules, each carrying a piece of the chromosomal DNA of an organism, prior to the insertion of these molecules into a population of host cells.

gene cloning  The process of synthesizing multiple copies of a particular DNA sequence using a bacteria cell or another organism as a host. The gene of interest is inserted into a self-replicating DNA molecule (DNA vector, often a plasmid) and the resulting recombinant DNA molecule is amplified in an appropriate host cell. Used in genetic engineering. a.k.a. molecular cloning; cloning. See DNA; host.

gene conservation; genetic resources conservation  The conservation of species, populations, individuals or parts of individuals, by in situ or ex situ methods, to provide a diversity of genetic materials for present and future generations.

gene conversion  A process, often associated with recombination, during which one allele is replicated at the expense of another, leading to non-Mendelian segregation ratios. In whole tetrads, for example, the ratio may be 6:2 or 5:3 instead of the expected 4:4.

gene expression  The process by which a gene produces RNA and protein, and hence exerts its effects on the phenotype of an organism.

gene flow  The spread of genes from one breeding population to another (usually) related populations by migration, possibly leading to changes in allele frequency.

gene frequency   See allele frequency.

gene imprinting  The differential expression of a single gene according to its parental origin.

gene insertion  The incorporation of one or more copies of a gene into a chromosome.

gene interaction  Modification of gene action by a non-allelic gene.

gene library  Collection of cloned DNA fragments that ideally includes all the genetic information of an organism. If the original source of the DNA is the genomic DNA from an organism, then it is called a genomic library; if the DNA is from cDNA made by enzymatic copying of RNA, then the library includes representative fragments from all genes that were being expressed at the time the RNA was sampled, and would be called a cDNA library. a.k.a. gene bank; library; clone bank; bank; library.

gene linkage  See linkage.

gene modification  Chemical change to a gene's DNA sequence. See DNA.

gene pool  1. The total genetic information in all the genes in a breeding population at a given time.

    2. In PGR: Use is made of the concept of 1, 2 and 3 (primary, secondary and tertiary) gene pools. In general, members of a 1 gene pool are inter-fertile; those of the 2 gene pool can cross with the 1 gene pool under special circumstances; with the 3 gene pool, extreme techniques are required to achieve crossing.

    See germplasm.

gene probe  A single-stranded DNA or RNA fragment used in genetic engineering to search for a particular gene or other DNA sequence. The probe has a base sequence complementary to the target sequence and will thus attach to it by basepairing. By labelling the probe, it can be identified after subsequent separation and purification.

genera  See genus.

Generally Regarded as Safe (GRAS)  Designation given to foods, drugs, and other materials that have been used for a considerable period of time and have a history of not causing illness to humans, even though extensive toxicity testing has not been conducted. More recently, certain host organisms for recombinant DNA experimentation have been given this status.

generate  To propagate or (mass) proliferate. The process is generation or regeneration.

generation time  See cell generation time.

generative  See somatic cell; somatic embryo.

generative nucleous  One of the two male gametes in the pollen tube of angiosperms.

gene recombination  The appearance of gene combinations in the progeny that differ from the combinations present in the parents.

gene replacement  The incorporation of a transgene into a chromosome at its normal location by homologous recombination, thus replacing the copy of the gene originally present at the locus.

gene sequencing  The process of elucidating the nucleotide sequence of a gene. See DNA sequencing.

gene shears  See ribozyme.

gene splicing  A stage in the processing of mRNA, occurring only in eukaryote cells, in which intervening sequences (introns) are removed from the primary RNA transcript (hnRNA), and the coding exons are joined together to form the mature mRNA molecule.

gene therapy  The treatment of inherited diseases by introducing into the cells of affected individuals the wild-type copies of the defective gene causing the disorder. If reproductive cells are modified, the procedure is called germ-line or heritable gene therapy. If cells other than reproductive cells are modified, the procedure is called somatic-cell or non-inheritable gene therapy.

genetically engineered organism (GEO) sometimes used for genetically modified organism, q.v.

genetically modified organism (GMO)  An organism that has been modified by the application of recombinant DNA technology.

genetic assimilation  Botany: Eventual extinction of a natural species as massive pollen flow occurs from another related species and the natural species becomes more like the related species. cf gene flow.

genetic code  1. The set of 64 nucleotide triplets (codons) that specify the 20 amino acids and termination codons (UAA, UAG, UGA).

    2. The relationships between the nucleotide base-pair triplets of an mRNA molecule and the 20 amino acids that are the building blocks of proteins.

    See base pair; nucleic acid; nucleotide.

genetic complementation  When two DNA molecules that are in the same cell together produce a function that neither DNA molecule can supply on its own. a.k.a. complementation.

genetic disease  A disease that has its origin in changes to the genetic material. Usually refers to diseases that are inherited in a Mendelian fashion, although non-inherited forms may also result from DNA mutation.

genetic distance  In AnGR: A measure of the genetic similarity between any pair of populations. Such distance may be based on phenotypic traits, allele frequencies or DNA sequences. For example, genetic distance between two populations having the same allele frequencies at a particular locus, and based solely on that locus, is zero. The distance for one locus is maximum when the two populations are fixed for different alleles. When allele frequencies are estimated for many loci, the genetic distance is obtained by averaging over these loci. (Source: FAO, 1999)

genetic distancing  In AnGR: The collection of the data on phenotypic traits, marker allele frequencies or DNA sequences for two or more populations, and estimation of the genetic distances between each pair of populations. From these distances, the best representation of the relationships among all the populations may be obtained. (Source: FAO, 1999)

genetic diversity  The heritable variation within and among populations which is created, enhanced or maintained by evolutionary forces. See biodiversity; mutation; genetic drift; gene re-shuffling processes.

genetic drift  Change in allele frequency from one generation to another within a population, due to the sampling of finite numbers of genes that is inevitable in all real (finite) populations. The smaller the population, the greater is the genetic drift. Sooner or later (depending on the size of the population), genetic drift results in loss of alleles from a population, and hence leads to a loss of genetic variation. Because of this, the minimization of genetic drift is an important consideration for conservation of genetic resources. See genetic diversity.

genetic engineering  Changes in the genetic constitution of cells (apart from selective breeding) resulting from the introduction or elimination of specific genes through modern molecular biology techniques. This technology is based on the use of a vector for transferring useful genetic information from a donor organism into a cell or organism that does not possess it. See gene cloning.

    A broader definition of genetic engineering also includes selective breeding and other means of artificial selection.

genetic equilibrium  Condition in a group of interbreeding organisms in which the allele frequencies remain constant over time.

genetic fingerprinting  A technique in which an individual's DNA is analysed to reveal the pattern of repetition of particular nucleotide sequences throughout the genome. See DNA fingerprint.

genetic heterogeneity  The situation in which different mutant genes produce the same phenotype.

genetic immunization  Delivery to a host organism of a cloned gene that encodes an antigen. After the cloned gene is expressed, it elicits an antibody response that protects the organism from infection by a virus, bacterium or other disease-causing organism.

genetic information  Information contained in a nucleotide base sequence in chromosomal DNA or RNA.

genetic linkage  See linkage map; linkage.

genetic map  The linear array of genes on a chromosome, based on recombination frequencies (linkage map) or physical location (physical or chromosomal map). See mapping.

genetic mapping  Determining the linear order of genes and/or DNA markers along a chromosome. a.k.a. mapping.

genetic marker  A DNA sequence used to "mark" or track a particular location (locus) on a particular chromosome. cf marker gene.

genetic polymorphism  See polymorphism.

genetic resources conservation  See gene conservation

genetics  The science of heredity and variation.

genetic selection  The process of selecting genes, cells, clones, etc., within populations or between populations or species. Genetic selection usually results in differential success rates of different genotypes, reflecting many variables, including selection pressure and genetic variability in populations.

genetic transformation  The transfer of extracellular DNA among and between species by using bacterial or viral vectors.

genetic variation  Differences between individuals attributable to differences in genotypes.

gene tracking  Following the inheritance of a particular gene from generation to generation.

gene translocation  The movement of a gene from one chromosomal location to another.

genome  1. The entire complement of genetic material (genes + non-coding sequences) present in each cell of an organism, or in a virus or organelle.

    2. A complete set of chromosomes (hence of genes) inherited as a (haploid) unit from one parent.

genomic DNA library; genomic library  A collection of clones containing the genomic DNA sequences of an organism. Typically, these molecules are propagated in bacteria or phage. The library is an important tool used in the process of isolating genes. See library.

genotype (from gene + type)  1. The genetic constitution (gene makeup) of an organism.

    2. The pair of alleles at a particular locus, e.g., Aa or aa.

    3. The sum total of all pairs of alleles at all loci that contribute to the expression of a quantitative trait.

    cf phenotype.

genus (pl: genera)  A somewhat arbitrary group of closely related species, where perceived relationship is typically based on physical resemblance.

GEO  Genetically engineered organism. See GMO.

geotropism  (Gr. ge, earth + tropos, turning)  A growth curvature induced by gravity. a.k.a. gravitropism.

germ  1. In botany, a common name for a plant embryo.

    2. Colloquial: A disease-causing micro-organism.

germ cell  Any cell in the series of cells (the germ line) that eventually produces gametes. In mammals, germ cells are found in the germinal epithelium of the ovaries and testes.

germ cell gene therapy  The repair or replacement of a defective gene within the gamete-forming tissues, resulting in a heritable change in an organism's genetic constitution.

germicide  Any chemical agent used to control or kill any pathogenic and non-pathogenic micro-organisms.

germinal epithelium  1. A layer of epithelial cells on the surface of the ovary that are continuous with the mesothelium.

    2. The layer of epithelial cells lining the seminiferous tubules of the testis, which gives rise to spermatogonia. See spermatogenesis.

germination  (L. germinare, to sprout)  1. The initial stages in the growth of a seed to form a seedling. The embryonic shoot (plumule) and embryonic root (radicle) emerge and grow upwards and downwards respectively. Food reserves for germination come from endosperm tissue within the seed and/or from the seed leaves (cotyledons).

    2. The growth of spores (fungal; algal) and pollen grains.

germ layers; primary germ layers  The layers of cells in an animal embryo at the gastrula stage, from which the various organs of the animal's body will be derived.

germ line cells  Cells that produce gametes.

germ line gene therapy  The delivery of a gene or genes to a fertilized egg or an early embryonic cell. The transferred gene(s) is present in all or some of the nuclei of the cells of the mature individual, including possibly the reproductive cells, and alters the phenotype of the individual that develops.

germ line  A lineage of "generative" cells (= germ track) ancestral to the gametes (sperm and egg cells proper) which, during the development of an organism (animal or plant), are set aside as potential gamete-forming tissues. These ancestral cells, together with the gametes, are called germ cells, as opposed to somatic cells. Location, nature and time of formation of potential gamete-forming tissues are species specific, and may vary greatly from one species to another.

germplasm  1. The genetic material that forms the physical basis of hereditary and which is transmitted from one generation to the next by means of the germ cells.

    2. An individual or clone representing a type, species or culture, that may be held in a repository for agronomic, historic or other reasons.

gestation  The period in animals bearing live young (especially mammals) from fertilization of the egg to birth of the young (parturition).

GH  See growth hormone.

gibberellins  Plant growth regulators involved in elongation, enhancement of flower, fruit and leaf size, germination, vernalization and other processes.

gland  A group of cells or a single cell in animals or plants that is specialized to secrete a specific substance. In animals, there are two types of glands: endocrine glands discharge their products directly into the blood vessels; exocrine glands secrete through a duct or network of ducts into a body cavity or onto the body surface. In plants, glands are specialized to secrete certain substances produced by the plant. The secretions may be retained within a single cell, or secreted to the outside.

glaucus  A surface with a waxy, white coating. In most cases, this waxy covering can be rubbed off.

globulins  Common proteins in blood, eggs and milk, and as a reserve protein in seeds. Globulins are insoluble in water, but soluble in salt solutions. Alpha, beta and gamma globulins can be distinguished in blood serum. Gamma globulins are important in developing immunity to diseases.

GLP (Good Laboratory Practice); GMP (Good Manufacturing Practice)  They are codes of practice designed to reduce to a minimum the chance of accidents which could affect a research project or a manufactured product. The GLP and GMP prescriptions are quite voluminous, but boil down to a few key points. The essential point of both GLP and GMP is that everything is recorded, and that only established procedures are used and by people who have been trained to use them.

glucocorticoid  A steroid hormone that regulates gene expression in higher animals.

glucose invertase; glucose isomerase  Enzymes that catalyse the interconversion of the two sugars, glucose and fructose. As fructose is chemically more stable than glucose, a mixture of glucose and fructose with the enzyme will end up almost entirely as fructose. This is valuable for the food industry, as fructose is substantially sweeter than glucose, and so more sweetness per gram is achieved using fructose. The usual use for glucose isomerase is to take glucose made by hydrolysis of corn starch and turn it into a mixture of mostly fructose with some glucose. The corn starch is broken down using amylases. The result is called high-fructose corn syrup (HFCS). Invertase takes sucrose and turns it into glucose and fructose.

gluten  A mixture of two proteins, gliadin and glutenin, occurring in the endosperm of wheat grain. Their amino acid composition varies, but glutamic acid (33%) and proline (12%) predominate. The composition of wheat glutens determines the strength of the flour and whether or not it is suitable for biscuit or bread making. Sensitivity of the lining of the intestine to gluten occurs in coeliac disease, a condition that must be treated by a gluten-free diet.

glycolysis   (Gr. glycos, sugar (sweet) + lysis, dissolution)  Sequence of reactions that converts glucose into pyruvate, with the concomitant production of ATP.

glycosylation  The covalent addition of sugar or sugar-related molecules to proteins or polynucleotides.

GM food Food that comprises, in whole or in part, material that has been modified by the application of recombinant DNA technology.

GMO  See genetically modified organism.

GMP (good manufacturing practice)  See GLP/GMP.

GMP (guanosine monophosphate)  See guanosine.

gobar  See biogas.

Golgi apparatus  An assembly of vesicles and folded membranes within the cytoplasm of plant and animal cells that stores and transports secretory products (such as enzymes and hormones) and plays a role in formation of a cell wall (when this is present). It is named after its discoverer, the Italian cytologist Camillo Golgi (1843-1926).

gonad  Any of the usually paired organs in animals that produce reproductive cells (gametes). The most important gonads are the male testis, which produces spermatozoa, and the female ovary, which produces ova (egg cells). The gonads also produce hormones that control secondary sexual characteristics.

good laboratory practice  See GLP/GMP.

good manufacturing practice  See GLP/GMP.

G-proteins  Proteins with an important role in relaying signals in mammalian cells. The proteins occur on the inner surface of the plasma membrane and transmit signals from outside the membrane, via transmembrane receptors, to adenylate cyclase, which catalyses the formation of the second messenger, cyclic AMP, inside the cell. G-proteins derive their name from their ability to bind to guanine nucleotides, namely GTP and GDP; the GTP-protein complex is able to activate adenylate cyclase, whereas the GDP-protein complex is unable. G-proteins are activated when the signalling molecule (typically a hormone) binds to the transmembrane receptor.

graft (to). To place a detached branch (scion) in close cambial contact with a rooted stem (rootstock) in such a manner that scion and rootstock unite to form a single plant.

graft inoculation test  A test based on the use of a suspected viral carrier which is grafted to an indicator plant. If symptoms appear in the indicator plant, the viral assay is positive.

graft union  The point at which a scion from one plant is joined to a stock from another plant.

gram molecular weight  See mole.

Gram-negative organism  Any prokaryotic organism that does not retain the first stain (crystal violet) used in Gram's staining technique. It does retain the second stain (safranin O) and therefore has a pink colour when viewed under a light microscope. Retention of the stain is due to the structure of the cell wall.

Gram-positive organism  Any prokaryotic organism that retains the first stain used in the Gram technique, which gives a purple-black colour when viewed under a light microscope. Retention of the stain is due to the structure of the cell wall.

Gram staining  A technique to distinguish between two major bacterial groups, based on stain retention by their cell walls. Bacteria are heat-fixed, then stained with crystal violet, followed by iodine solution, and then rinsed with alcohol or acetone. Gram-positive bacteria are stained bright purple, while Gram-negative bacteria are decolourized.

grana  (L. granum, a seed)  Structures within chloroplasts, seen as green granules with the light microscope and as a series of parallel lamellae with the electron microscope. Disc- or sac-like structures found in chloroplasts composed of stacked membranes and containing the chlorophyll and carotenoid pigments directly involved in photosynthesis (singular: granum).

GRAS  See generally regarded as safe.

gratuitous inducer  A substance that can induce transcription of a gene or genes, but is not a substrate for the induced enzyme(s).

gravitropism  See geotropism.

Green Revolution  Name given by William Goud to the dramatic increase in crop productivity during the third quarter of the 20th century, as result of integrated advances in genetics and plant breeding, agronomy, and pest and disease control.

Gro-luxTM  A wide-spectrum fluorescent lamp suitable for plant growth purposes.

growth  An increase in cell size or cell number, or both, resulting in an increase in dry weight.

growth cabinet  A cupboard used for incubating tubes or culture vessels under controlled environmental conditions. The degree of control over temperature, light and humidity is a function of the quality of the cabinet. cf culture room; incubator.

growth curve  See growth phases.

growth factor  Any of various chemicals, particularly polypeptides, that have a variety of important roles in the stimulation of new cell growth and cell maintenance. They bind to the cell surface on receptors. Specific growth factors can cause new cell proliferation.

growth hormone (GH); somatotrophin; somatotropin  A hormone, secreted by the mammalian pituitary gland, that stimulates protein synthesis and growth of the long bones in the legs and arms. It also promotes the breakdown and use of fats as an energy source, rather than glucose. Production of growth hormone is greatest during early life. Its secretion is controlled by the opposing actions of two hormones from the hypothalamus: somatocrinin (growth-hormone-releasing hormone), which promotes its release; and somatostatin (growth-hormone-inhibiting hormone), that inhibits the release of growth hormone from the anterior pituitary gland.

growth inhibitor  Any substance inhibiting the growth of an organism. The inhibitory effect can range from mild inhibition (growth retardation) to severe inhibition or death (toxic reaction). Two plant growth regulators that may act as inhibitors are ethylene and abscisic acid. The concentration of the inhibitor, the length of exposure to it, and the relative susceptibility of the organisms exposed to the inhibitor, are all important factors which determine the extent of the inhibitory effect.

growth phases; growth phase curve  The characteristic periods in the growth of a bacterial culture, as indicated by the shape of a graph of viable cell number versus time, namely: lag phase; logarithmic (or exponential) phase ; stationary phase; death phase.

growth rate  Increase in mass per unit of time. See growth.

growth regulator  A synthetic or natural compound that at low concentrations elicits and controls growth responses in a manner similar to hormones.

growth retardant  A chemical that selectively interferes with normal hormonal promotion of growth and other physiological processes, but without appreciable toxic effects.

growth ring  Any of the rings that can be seen in a cross-section of a woody stem, such as a tree trunk. It represents the xylem formed in one year as a result of fluctuating activity of the vascular cambium.

growth substance  Any organic substance, other than a nutrient, that is synthesized by plants and regulates growth and development. They are usually made in a particular region, such as the shoot tip, and transported to other regions, where they take effect.

GTP  Guanosine triphosphate. See guanosine.

guanine  A purine derivative that is one of the major component bases of nucleotides and the nucleic acids, DNA and RNA.

guanosine  A nucleoside consisting of one guanine molecule linked to a D-ribose sugar molecule. The derived nucleotides, guanosine mono-, di-, and triphosphate (GMP, GDP, and GTP, respectively), are important in various metabolic reactions.

guard-cell  Specialized epidermal cell that occurs as a pair around a stoma and controls opening and closing of the stoma through changes in turgor. See stoma.

guide RNA  An RNA molecule that contain sequences that function as a template during RNA editing.

guide sequence  An RNA molecule (or a part of it) which hybridizes with eukaryotic mRNA and aids in the splicing of intron sequences. Guide sequences may be either external (EGS) or internal (IGS) to the RNA being processed and may hybridize with either intron or exon sequences close to the splice junction. See split gene; exon.

gymnosperm  Any plant whose ovules and the seeds into which they develop are borne unprotected, rather than enclosed in ovaries, as are those of the flowering plants (the term gymnosperm means naked seed).

gynandromorph  An individual in which one part of the body is female and another part is male; a sex mosaic.

gynogenesis  Female parthenogenesis; after fertilization of the ovum, the male nucleus is eliminated and the haploid individual (described as gynogenetic) so produced possesses the maternal genome only. cf androgenesis; parthenogenesis; anther culture.

gyrase  See DNA helicase.

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