Fig. 30.4. Genetic code dictionary.

It should also be realized that although the genetic code in vivo is unambiguous (no codon coding for more than one amino acid) there is an element of ambiguity with GUG, since if present at initiating position, it would code for methionine, but would otherwise always code for valine.

As shown in Figure 30.4, UAA (also known as ochre), UAG (also known as amber) and UGA (also known as opal) are three codons which whenever present in mRNA, would bring about termination of polypeptide chain which would then be released from the ribosome. Till the function of these codons (UAA, UAG, UGA) was discovered, they were called nonsense codons, since these codons did not code for any of the 20 essential amino acids. Now that we know, that they have a definite function of termination to perform, it would not be correct to call them nonsense codons.

Several Roles for UGA Codon
Of the above three termination codons, UGA has served a variety of distinct roles including the following : (i) termination codon in the universal genetic code, (ii) tryptophan codon in mitochondria and mycoplasma, (iii) an inefficiently read tryptophan codon in B. subtilis and (presumably) in E. coli, (iv) cysteine codon in Euplotes octocarinatus and (v) selenocysteine (SeCys) codon in E. coli, mammals, higher plants and fungi. The last function of coding for selenocysteine is actually as universal as its termination function, so that UGA, in the universal code, serves both as a selenocysteine codon and as a termination codon (see later in this section).