GBVAlike agents GBVAlikeagents

Unassigned species in the family Flaviviridae, found in New World monkeys. Their significance in natural host populations is not yet known. See GB virus A.

G+C content The total guanine (G) and cytosine (C) content of a double-stranded DNA is usually expressed as a percentage of the total content of bases. The triple hydrogen bond between G and C is more stable than the double hydrogen bond between adenine (A) and thymine (T). Thus the G+C content affects the physical properties of the DNA: both the melting point and the density of the molecule are proportional to the G+C content. Most mammalian cell DNAs have a G+C content of about 44%; some viral DNAs are similar but some are as high as 70%.

GD VII virus A strain of mouse poliovirus.

gecko lung-1 (GL1) cells (CCL 111) A cell line derived from the lung of an adult male Tokay gecko, Gekko gekko; susceptible to a number of mammalian viruses at 36°C.

gecko virus A possible species in the family Iridoviridae. The virus has been seen in the erythrocytes of reptiles, Gehyra variegata. Virion diameter was 220nm, with a core of 120-160nm. Transmission between animals has been reported.

Stehbens WE and Johnston MRL (1966) J Ultrastruct Res 15, 543

gel electrophoresis Electrophoresis of macromolecules in a matrix of polyacry-lamide, agarose or similar gel.

gel filtration A type of column chromatog-raphy which separates molecules on the basis of size. The higher molecular weight molecules pass through the column first, the smaller molecules entering pores in the gel making up the column and thus being retarded. Usually the gels are made from sugar polymers that are flexible (Sephadex) or rigid (Sepharose). The pore sizes are determined by the degree of cross-linking of the gel.

GeLu cells (CCL 100) A fibroblast-like cell line derived from the normal lung tissue of a 403-day-old Mongolian gerbil, Meriones unguiculatus.

gene

gene A sequence of nucleotides along a nucleic acid molecule which can determine the composition of one polypeptide. A protein of 500 amino acids, each one represented by one codon of three nucleotides in the mRNA, thus requires 1500 nucleotides of genetic information. Many eukaryotic and some viral DNA genes are discontinuous and the sequence of nucleotides is interrupted by introns, which are removed after transcription by splicing, to form the mature mRNA.

gene cloning The isolation and amplification of the nucleic acid sequence of a gene. This usually involves insertion into a suitable plasmid vector that replicates in bacterial or yeast cells, generating multiple copies of the sequence.

gene enhancers Short cis-acting nucleotide sequences that increase transcriptional activity of genes.

gene expression The multistep process by which the protein product of a gene is synthesized.

genetic code The sequence of nucleotides that specifies the amino acid sequences of proteins and the start and stop signals for protein synthesis. Each of the 20 amino acids is specified by one or more codons, each consisting of three nucleotides. The code is degenerate, since most amino acids can be specified by more than one codon. The signals for initiation of protein synthesis (start codons) are AUG (methionine) in eukaryotes, and either AUG or GUG in bacteria. The major signals for termination of protein synthesis (stop codons) are UAA (ochre), UAG (amber) and UGA (opal). Although the code is 'universal', a number of deviations are known, particularly in mitochondrial DNA where UGA, for example, specifies tryptophan rather than acting as a stop codon. It has also been suggested that UGA may specify insertion of a 21st amino acid, selenocysteine, during expression of certain virus genomes in eukaryotic cells.

genetic marker A mutation in a gene which allows its genotypic or phenotypic identification.

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