Gowen crossover suppressor
GMO an acronym for genetically modified organ- isms (i.e., transgenic bacteria, plants, or animals). See Bt designer plants, promoter 35S, Roundup, trans- formation. gnotobiosis the rearing of laboratory animals in a germ-free state or containing only microorganisms known to the investigator. gnotobiota the known microfauna and microflora of a laboratory animal in gnotobiosis (q.v.). goiter a chronic enlargement of the thyroid gland that is due to hyperplasia, not neoplasia. Golgi apparatus (or body or complex or material) a cell organelle identified in electron micrographs as a complex made up of closely packed broad cister- nae and small vesicles. The Golgi apparatus is distin- guished from the endoplasmic reticulum by the ar- rangement of the membranous vesicles and by the lack of ribosomes.
The Golgi apparatus functions to collect and sequester substances synthesized by the endoplasmic reticulum. The Golgi apparatus in a typical animal cell appears as a stack of six to eight flattened membranous sacs. Plant Golgi often have 20 or more of these cisternae. The Golgi apparatus is differentiated into a cis face, the receiving surface, which is closest to the ER. Protein-rich vesicles bud off the ER and fuse with the cis face of the Golgi. While in the medial region of the Golgi, the carbo- hydrate units of glycoproteins are chemically modi- fied in various ways that target them to their final destinations. Eventually the finished proteins are ex- ported from the cisterna farthest from the ER. They are enclosed in vesicles that bud off the trans face and function as secretory vesicles (q.v.) or are re- tained as lysosomes (q.v.). See Appendix C, 1898, Golgi; 1954, Dalton and Felix; dictyosome, protein sorting signal peptide. gonad a gamete-producing organ of an animal; in the male, the testis; in the female, the ovary. gonadotropic hormones pituitary hormones (such as LH and FSH, q.v.) that stimulate the gonads; also called gonadotropins.
gonadotropin-releasing hormone (GnRH) a neu- rohormone from the hypothalamus that stimulates release of luteinizing hormone (LH) and follicle- stimulating hormone (FSH) from the pituitary gland. gonochorism a sexual system in which each indi- vidual is either a male or a female. Compare with hermaphrodite, monoecy. gonophore 1. in sessile coelenterates, the bud pro- ducing the reproductive medusae. 2. in higher ani- mals, any accessory sexual organ, such as an oviduct, or a sperm duct. 3. in plants, a stalk that bears sta- mens and pistils. gonomery the separate grouping of maternal and paternal chromosomes during the first few mitoses following fertilization as occurs in some insect em- bryos. gonotocont an auxocyte (q.v.). goosecoid a gene that maps to 14q32.1 in hu- mans. It encodes a homeodomain transcription fac- tor with a DNA-binding specificity identical to the morphogen encoded by the Drosophila gene bicoid (q.v.).
Microinjection of gsc mRNAs into the ventral sides of Xenopus embryos leads to the formation of an additional complete body axis. Therefore the product of the gsc gene can mimic the natural orga- nizer (q.v.). Genes homologous to gsc have been iso- lated from the mouse, chick, and zebrafish. See ho- meobox, Spemann-Mangold organizer. Gorilla gorilla the gorilla, a primate with a hap- loid chromosome number of 24.
About 40 biochem- ical markers have been found to be distributed among 22 linkage groups. See Hominoidea. Gossypium a genus of plants composed of about 50 diploid and tetraploid species, four of which have been domesticated and produce most commercial cotton worldwide. These species are G. herbaceum (African-West Asian cotton), G. arboreum (Paki- stani-Indian cotton), G. hirsutum (Mexican cotton) and G. barbadense (South American cotton). Of these, G. hirsutum and G. barbadense are the pre- dominant species for commercial cotton production. The comparative genetics of these species has been intensively studied.
See Appendix A, Plantae, Tra- chaeophyta, Angiospermae, Dicotyledonae, Mal- vales; Appendix E. gout a hereditary disorder of purine metabolism characterized by increased amounts of uric acid in the blood and recurrent attacks of acute arthritis. Gowen crossover suppressor a recessive gene on the third chromosome of Drosophila melanogaster symbolized by c(3)G. It was discovered in 1917 by Marie and John Gowen, who showed that crossing over was suppressed in homozygous females. Later it was observed that mutant oocytes lacked synapto- nemal complexes (q.v.), and the wild-type allele was found to encode a protein component of that organ- elle. C(3)G and the Zip 1 gene of Saccharomyces cer- evisiae are orthologs. See centromeric coupling.
GPCRs G protein-coupled receptors (q.v.). G6PD glucose-6-phosphate dehydrogenase (q.v.). G6PD deficiency See glucose-6-phosphate dehy- drogenase deficiency. G0, G1, G2 phases See cell cycle. G proteins guanine nucleotide-binding regulatory proteins. They are activated by the binding of a sig- naling ligand, such as a hormone, to a transmem- brane receptor protein. This interaction causes the receptor to change its shape so that it can now react with a G protein. G proteins are heterotrimeric mol- ecules made up of alpha, beta, and gamma chains. G proteins are active when GTP is bound to them and inactive when GDP is present instead. Activated G proteins dissociate from their receptors and acti- vate effector proteins that control the level of second messengers (q.v.).
If, for example, adenylcyclases were the effector proteins, cAMP would be gener- ated. See Appendix C, 1970, Rodbell and Birm- baumer; 1977, Ross and Gilman; cellular signal trans- duction, cholera, diabetes insipidus, Gprotein-coupled receptors (GPCRs). G protein-coupled receptors (GPCRs) integral proteins belonging to a large superfamily of cell sur- face receptors, each of which contains 7 membrane- spanning, alpha helix (q.v.)
domains and a bound G protein. GPCRs bind ligands, such as hormones, odorants, growth factors, and neurotransmitters on the cell surface, and transmit signals to the interior of the cell to regulate cell functions, such as hor- monal regulation, olfactory perception, cell growth, and transmission of nerve impulses. See cellular signal transduction, G proteins, growth factor, hormone, in- tegral protein, ligand, odorant, odorant receptor, seven transmembrane domain (7 TM) receptor, taste receptor gene. G quartet See guanine quartet model. Graafian follicle a fluid-filled spherical vesicle in the mammalian ovary, containing an oocyte attached to its wall. See Appendix C, 1657, de Graaf. grade a stage of evolutionary advance.
A level reached by one or more species in the development of a structure, physiological process, or behavioral character. Different species may reach the same grade because they share genes that respond in the same way to an environmental change.
When two species that do not have a common ancestry reach the same grade, the term convergence is used to de- scribe this evolutionary parallelism. gradient a gradual change in some quantitative property of a system over a specific distance (e.g., a clinal gradient; density gradient). gradualism a model explaining the mechanism of evolution that represents an updating of the original ideas set forth by Charles Darwin. According to this model, those individuals with hereditary traits that best adapt them to their habitat are most likely to survive and to transmit these adaptive genes to their offspring. As a result, with the passage of time the frequencies of beneficial genes rise in the popula- tion, and when the composition of the gene pool of the evolving population becomes sufficiently differ- ent from that of the original population, a new spe- cies will have arisen. Since a beneficial mutation must spread through an entire population to pro- duce detectable evolutionary changes, speciation will be a gradual and continuous process. Contrast with punctuated equilibrium.
Graffi leukemia virus a virus that induces myeloid leukemia in mice and rats. graft 1. a relatively small piece of plant or animal tissue implanted into an intact organism. 2. the act of transferring a part of an organism from its normal position to another position in the same or another organism. In the case of relatively giant cells, cyto- plasmic regions of characteristic morphology can be transplanted. Grafts between species of Acetabularia (q.v.) are examples. See allograft, autograft, hetero- graft, homograft, scion, stock, transplantation, xeno- graft.
graft hybrid a plant made up of two genetically distinct tissues due to fusion of host and donor tis- sues after grafting. graft rejection a cell-mediated immune response to transplanted tissue that causes destruction of a graft. Rejection is evoked by the histocompatibility antigens (q.v.) of the foreign cells. graft-versus-host reaction a syndrome arising when an allograft, containing immunocompetent cells, mounts an immune response against a host that is unable to reject it because the host is immu- nologically immature or immunologially compro- mised or suppressed (e.g., by radiation or drugs); synonymous with allogeneic disease, runt disease (q.v.). gram atomic weight the quantity of an element that has a mass in grams numerically equal to its atomic weight.