Some Features of the Code Dictionary

The complete code dictionary is given in Fig. 2.2. Of the 64 triplets termed codons, 61 are meaningful or sense ones: they code for 20 amino acids of natural polypeptides and proteins. Regularly three codons -UAG ("amber"), UAA ("ochre"), and UGA ("opal") - normally do not code for amino acids and therefore are sometimes called nonsense codons. The nonsense triplets play an important part in translation, since in mRNA these codons serve as signals for the termination of polypeptide chain synthesis; at present they are usually referred to as termination or stop codons.

At the same time UGA triplet may also code for the 21st amino acid of a number of proteins, selenocysteine (Chambers et al., 1986; Zinoni et al., 1987). This, however, requires the presence in mRNA of an additional structural element, either immediately adjacent to UGA from its 3'-side (in the case of Prokaryotes), or located beyond the coding sequence, in the 3'-proximal untranslated region of mRNA (in Eukaryotes) (see Chapter 10, Section 10.2.2).

As seen from Fig. 2.2, the degeneracy of the code does not extend to all 20 main amino acids. Two amino acids, methionine and tryptophan, are coded by one codon each, i.e. by AUG and UGG, respectively. On the contrary, three amino acids, specifically leucine, serine, and arginine, have six codons

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