Codons assignments

Table 38-1. The genetic code (codon assignments in mammalian messenger RNA)J...

Note Presented here is a summary of data from one of the early experiments designed to elucidate the genetic code. A synthetic RNA containing only A and C residues in a 5 1 ratio directed polypeptide synthesis, and both the identity and the quantity of incorporated amino acids were determined, Based on the relative abundance of A and C residues in the synthetic RNA, and assigning the codon AAA (the most likely codon) a frequency of 100, there should be three different codons of composition A2C, each at a relative frequency of 20 three of composition AC2, each at a relative frequency of 4,0 and CCC at a relative frequency of 0.8. The CCC assignment was based on information derived from prior studies with poly(C), Where two tentative codon assignments are made, both are proposed to code for the same amino acid. 

TABLE 1 Known Variant Codon Assignments in Mitochondria... 

Table 5-5 shows the codon assignments, as we now know them, for each of the 20 amino acids. Table 5-6 shows the same 64 codons in a rectangular array. 

In addition to those codons assigned to specific amino acids, three are designated as chain termination codons UAA, UAG, and UGA. These are frequently referred to as "nonsense" codons. The termination codons UAA and UAG are also known as ochre and amber, respectively, although these names have no scientific significance.41 The codons AUG (methionine)... 

Degeneracy of the code is very obvious from an examination of the codon assignments shown in Tabic 17.1. For example, there are six codons for leucine. It should be noted that the nucleotide components of each triplet... 

In addition to limited initiation sites for transcription and the presence of overlapping reading frames, mtDNA has other distinctive features, including a codon assignments that differ from those of nDNA. 

The genetic code (codon assignments in messenger RNA. (Reproduced, with permission, from Murray RK, Granner DK, Mayes PA, Rodwell VW Harper s Biochemistry, 25/e. New York, McGraw-Hill, 2000 453.)... 

The adaption hypothesis assumes that the problem of codon assignment is a typical optimization issue with an error minimization scheme stating that near codons should encode similar amino acids. Thus, upon mutating one of the three bases of the codon, the encoded amino acid should be synonymous to the one encoded by the unmutated codon. This minimizes structural and functional deleterious effects of this mutation on the resulting protein (Di Giulio 2005). 

Recently, an asymmetric codon assignment rule was proposed. Evidence was found for a rapid early evolution of the Code via successive binary choices of 16 X X2N codons. It was claimed that the other scenarios (listed above in points 1-5) could have played a role in different periods of the evolving Genetic Code. 

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