| opal codon | <molecular biology> The codon UGA, one of the three that causes termination of protein synthesis. (18 Nov 1997) |
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| opal | <chemical> A mineral consisting, like quartz, of silica, but inferior to quartz in hardness and specific gravity. The precious opal presents a peculiar play of colours of delicate tints, and is highly esteemed as a gem. One kind, with a varied play of colour in a reddish ground, is called the harlequin opal. The fire opal has colours like the red and yellow of flame. Common opal has a milky appearance. Menilite is a brown impure variety, occurring in concretions at Menilmontant, near Paris. Other varieties are cacholong, girasol, hyalite, and geyserite. Origin: L. Opalus: cf. Gr, Skr. Upala a rock, stone, precious stone: cf. F. Opale. Source: Websters Dictionary (01 Mar 1998) |
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| opal mutation | <molecular biology> Mutation that changes any codon to the termination codon UGA. (18 Nov 1997) |
| opal suppressor | <molecular biology> A gene that codes for an altered tRNA so that its anticodon can recognise the opal codon and thus allows the continuation of protein synthesis. A suppressor of an opal mutation is a tRNA that is charged with the amino acid corresponding to the original codon or a neutral substitute. Some eukaryote cells normally synthesise opal suppressor tRNAs. The function of these is not clear and they usually do not prevent normal termination of protein synthesis at an opal codon. (18 Nov 1997) |
| amber codon | <molecular biology> One of the three terminator codons. Its sequence is UAG. See: ochre codon, opal codon. (18 Nov 1997) |
| ambiguous codon | <molecular biology> A codon that codes for more than one amino acid. (09 Oct 1997) |
| codon | <molecular biology> The coding unit of DNA that specifies the function of the corresponding messenger RNA. A triplet of bases recognised by anticodons on transfer RNA and hence specifying an amino acid to be incorporated into a protein sequence. The code is degenerate, i.e. Each amino acid has more than one codon. The stop codon determines the end of a polypeptide. (18 Nov 1997) |
| codon, initiator | Any codon that directs initiation of genetic translation (translation, genetic) by stimulating the binding of initiator trna. In prokaryotes, the codons aug or gug can act as initiators while in eukaryotes, aug is the only initiator codon. (12 Dec 1998) |
| codon, nonsense | A codon that is not assigned to an amino acid or to any stop signal. It was originally thought that nonsense codons were synonymous with terminator codons (codon, terminator) in that they signaled termination of a protein chain. Recent studies have found this not to be true. (12 Dec 1998) |
| codon, terminator | Any codon that signals the termination of genetic translation (translation, genetic) by triggering the hydrolysis of the aminoacyl bond connecting the completed polypeptide to the trna. Terminator codons (uaa, uag, and uga) do not specify amino acids. (12 Dec 1998) |
| punctuation codon | <molecular biology> The three codons, UAA known as ochre, UAG as amber and UGA as opal, that do not code for an amino acid but act as signals for the termination of protein synthesis. They are not represented by any tRNA and termination is catalysed by protein release factors. There are two release factors in E. Coli, RF1 recognises UAA and UAG, RF2 recognises UAA and UGA. Eukaryotes have a single GTP requiring factor, eRF. See: ochre suppressor, amber suppressor. (13 Jan 1998) |
| start codon | The codon 5' AUG in mRNA, at which polypeptide synthesis is started. It is recognised by formylmethionyl tRNA in bacteria and by methionyl tRNA in eukaryotes. (18 Nov 1997) |
| stop codon | <molecular biology> The three codons, UAA known as ochre, UAG as amber and UGA as opal, that do not code for an amino acid but act as signals for the termination of protein synthesis. They are not represented by any tRNA and termination is catalysed by protein release factors. There are two release factors in E. Coli, RF1 recognises UAA and UAG, RF2 recognises UAA and UGA. Eukaryotes have a single GTP requiring factor, eRF. See: ochre suppressor, amber suppressor. (13 Jan 1998) |
| nonsense codon | <molecular biology> The three codons, UAA (known as ochre), UAG (amber) and UGA (opal), that do not code for an amino acid but act as signals for the termination of protein synthesis. Any mutation that causes a base change which produces a nonsense codon results in premature termination of protein synthesis and probably a nonfunctional or nonsense protein. (13 Nov 1997) |
| initiating codon | The trinucleotide AUG (or sometimes GUG) that codes for the first amino acid in protein sequences, formylmethionine; the latter is often removed post-transcriptionally. Synonym: start codon. (05 Mar 2000) |
| initiation codon | The codon 5' AUG in mRNA, at which polypeptide synthesis is started. It is recognised by formylmethionyl tRNA in bacteria and by methionyl tRNA in eukaryotes. (18 Nov 1997) |