Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 199

In the genetic code, more than one codon often encodes the same amino acid. This feature of the code is called redundancy. There are 64 (4 x 4 x 4) possible codons that can be made of 4 different nucleotides taken 3 at a time. Thus, without redundancy, there would be far too many codons for only 20 amino acids. As shown in Table 2.2, some amino acids are encoded by a single codon (e.g., tryptophan), while others use as many as six different codons (e.g., arginine).

Figure 2.32 trans-Translation by transfer-messenger RNA (tmRNA). (A) A ribosome translating an mRNA that lacks a termination codon will stall, because a release factor is unable to bind and release the translation complex. (B) tmRNA, which has features of both a tRNA and an mRNA, enters the A site. (C) The ribosome switches from translation of the mRNA to translation of the coding sequence in the tmRNA, which results in addition of a short polypeptide tag to the carboxy terminus of the nascent polypeptide. (D) The ribosome and mRNA are released, and the tmRNA-encoded tag targets the polypeptide for degradation by the Clp protease system.