Group-transfer reactions

29 Mar

Group-transfer reactions

gram equivalent weight the mass of an acid or a base that will release or neutralize one gram mole- cule (mole) of hydrogen ion. A 1-mole solution of H2SO4 contains 2 gram equivalents. A thousandth of a gram equivalent weight is a millequivalent. See normal solution. gramicidin S a cyclic antibiotic synthesized by Bacillus brevis. The molecule has the structure [ d-phe-pro-val-orn-leu ] leu-orn-val-pro-d-phe and it contains amino acids not usually found in pro- teins—namely, ornithine (orn) and d-phenylalanine (rather than the usual l-isomer).

The synthesis of gramicidin S constitutes one of the best understood examples of a polypeptide that is not synthesized on a ribosome. Two enzymes are required (E1 and E2), which are bound together, forming a unit called gramicidin synthetase. One molecule each of pro- line, valine, ornithine, and leucine bind in that se- quence to sulfhydryl groups of E1. E2 functions to isomerize l- to d-phenylalanine and to transfer it to the proline attached to E1. The two identical poly- peptides are then joined head to tail to form a deca- peptide. This rare type of polypeptide synthesis is very uneconomical and cannot generate molecules greater than 20 amino acids long. gram molecular weight the quantity of a com- pound that has a mass in grams numerically equal to its molecular weight.

Gram molecular weight is of- ten shortened to gram-mole or mole. Gram staining procedure a staining technique that allows bacteria to be divided into two groups, Gram-positive (which stain deep purple) and Gram- negative (which stain light pink). The staining differ- ences lie in the permeability properties of the cell walls of the two groups of bacteria. Agrobacterium, Escherichia, Haemophilus, Salmonella, Serratia, Shi- gella, and Vibrio are all Gram-negative; Bacillus, Myco- bacterium, Staphylococcus, and Streptococcus are exam- ples of Gram-positive bacterial genera. See Appendix C, 1884, Gram. grana (singular granum) long columns of dense discs found in chloroplasts (q.v.).

Each disc contains a double layer of quantasomes (q.v.). grandchildless genes genes that cause female mutants to produce progeny that are sterile and of- ten show other developmental abnormalities. See maternal effect gene, pole plasm. granulocytes white blood cells possessing distinct cytoplasmic granules and a multilobed nucleus; in- cludes basophils, eosinophils, and neutrophils; also called polymorphonuclear leukocytes. grass any species of monocotyledon belonging to the family Gramineae. Such species are character- ized by leaves with narrow, spear-shaped blades, and flowers borne in spikelets of bracts. gratuitous inducer a compound not found in na- ture that acts as an inducer although it cannot be metabolized. See IPTG, ONPG. gravid an animal that is swollen from accumulated eggs or embryos. Gray (Gy) a unit that defines that energy absorbed from a dose of ionizing radiation equal to one joule per kilogram. 1 Gy = 100 rad. green fluorescent protein (GFP) a protein pro- duced by the jellyfish Aequorea victoria. It is made up of 238 amino acids and produces a green emis- sion when it is excited by blue light.

This green fluorescence is stable and shows very little photo- bleaching. GFP provides an excellent means for cy- tologically localizing the product from any foreign gene that can be spliced to the GFP open reading frame. The fused protein is often fully functional and can be localized to its normal site in the cell by its green fluorescence. See Appendix C, 1994, Chalfie et al.. Compare with luciferase. grid 1. a network of uniformly spaced horizontal and vertical lines. 2. a specimen screen used in elec- tron microscopy. griseofulvin an antibiotic synthesized by certain Penicillium species that is used as a fungicide. gRNA guide RNA. See RNA editing. groove See DNA grooves. Gross mouse leukemia virus (GMLV) a filterable RNA virus discovered by Ludwig Gross in 1951.

It caused leukemia when injected into newborn mice. group selection natural selection acting upon a group of two or more individuals by which traits are selected that benefit the group rather than the indi- vidual. See Hamilton’s genetical theory of social be- havior. group-transfer reactions chemical reactions in- volving the exchange of functional groups between molecules (excluding oxidations or reductions and excluding water as a participant). The enzymes that catalyze group-transfer reactions are called transfer- ases or synthetases. For example, activation of an amino acid involves transfer of an adenosine mono- phosphate group from ATP to the COO− group of the amino acid.

growth curve in microbiology, a curve showing the change in the number of cells in a growing cul- ture as a function of time. growth factor a specific substance that must be present in the growth medium to permit cell multi- plication. growth hormone See human growth hormone. growth hormone deficiencies See hereditary growth hormone deficiencies. GSH reduced glutathione. GT-AG rule intron junctions start with the dinu- cleotide GT and end with the dinucleotide AG, cor- responding to the left and right ceptor”) splicing sites, respectively. GTP guanosine triphosphate (q.v.). guanine See bases of nucleic acids.

guanine deoxyriboside See nucleoside. guanine-7-methyl transferase See methylated cap. guanine quartet model a three-dimensional ar- rangement of guanine molecules which explains the interactions occurring within DNA strands that con- tain repeating units rich in guanine, such as telome- ric repeats. One, two, or four DNA strands can fold into a compact unit containing a planar array of four guanines, as shown above. Each guanine serves both as a proton donor and acceptor with its neighbors, and so the quartet is held together by eight hydrogen bonds. The quartets can stack one above another.

This stacking is facilitated by axially located mono- valent cations. For example, Na+ fits exactly in the central cavity, and the slightly larger potassium ion can fit by binding in the cavity between adjacent quartets. See Appendix C, 1989, Williamson, Rag- huraman, and Czech; hydrogen bond, telomere. guanine tetraplex See guanine quartet model. guanosine See nucleoside. guanosine triphosphate an energy-rich molecule (analogous to ATP) that is required for the synthesis of all peptide bonds during translation. guanylic acid See nucleotide. guanylyl transferase See methylated cap. guide RNA See RNA editing. guinea pig See Cavia porcellus. guppy See Lebistes reticularis.

Guanine quartet model

gustatory receptor (GR) genes genes which en- code in chemosensory neuron proteins that function as taste receptors. For example, in Drosophila mela- nogaster there are male-specific GR genes that are expressed on appendages including the labial palps of the proboscis, the taste bristles of the fore tarsi, and the maxillary palps. They perceive pheromones (q.v.) that are utilized during mating displays. See courtship rituals. Guthrie test a bacterial assay for phenylalanine de- veloped by R. Guthrie. He not only developed this methodology of newborn screening but also pro- moted the passage of state laws that mandated screening.

Newborn screening and dietary treatment that begins within the first weeks of life have virtu- ally eliminated mental retardation from phenylke- tonuria (q.v.) in the United States. See Appendix A, 1961, Guthrie. Gy abbreviation for gray (q.v.). gymnosperm a primitive plant having naked seeds (conifers, cycads, ginkgos, etc.). gynander synonymous with gynandromorph (q.v.). gynandromorph an individual made up of a mo- saic of tissues of male and female genotypes. The fruit fly illustrated on page 191 is a bilateral gynan- dromorph, with the right side female and the left side male. The zygote was ++/w m. Loss of the X chromosome containing the dominant (+) genes oc- curred at the first nuclear division. The cell with the single X chromosome containing the recessive marker genes gave rise to the male tissues. There-

Gyrase

fore, the left eye is white and the left wing is minia- ture. Note the male abdominal pigmentation and the sex comb. gynodioecy a sexual dimorphism in plants having both bisexual and separate female individuals. gynoecium a collective term for all the carpels of a flower. gynogenesis 1. reproduction by parthenogenesis requiring stimulation by a spermatozoan for the acti- vation of the egg; synonymous with pseudogamy (q.v.). 2.

production of a diploid embryo having two sets of maternal chromosomes by nuclear transfer (q.v.). Compare with androgenesis. gynogenote a cell or embryo produced by gyno- genesis. Compare with androgenote. GYPA See glycophorin A. gyrase the colloquial name for a type II topoiso- merase (q.v.) of E. coli that converts relaxed closed- circular, duplex DNA to a negatively superhelical form both in vitro and in vivo. This enzyme prepares the DNA for proteins that require unwinding of the duplex or single-stranded regions in order to partici- pate in such processes as replication, transcription, repair, and recombination. Several drugs are known to inhibit gyrase, including adriamycin, naladixic acid, and novobiocin. See Appendix C, 1976, Gellert et al.; replisome.

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