Description of the Genetic Triplet Code

Even before the experimental analysis of Crick's group, there was speculation as to what the code might turn out to be. The guiding principle in this speculation was, how does one arrive biochemically at the specification of twenty distinct amino acids. If one assumes that successive base pairs on the rungs of the DNA ladder code in some way for each amino acid, i.e., that some type of codon exists in the transcription process, one could then speculate on the length of a codon. If, for example, each base on the linear DNA chain coded specifically for an amino acid (codon of length one) this scheme would only account for four amino acids, since there are just four bases in the DNA molecule. If on the other hand, pairs of successive bases were codons, then there would be 4 × 4 = 16 distinct base pairs (four choices of base at each of the two locations in the codon), which is still not adequate to specify twenty amino acids. On the other hand, the number of distinct triplets of 4 bases is 4 × 4 × 4 = 64, which gives an overspecification for twenty amino acids, leading to a degeneracy problem. There were also problems from a purely theoretical point of view, whether the code might be overlapping (54), and whether it was read off just one side of the DNA ladder or off of both sides. Of course, as we discussed in Chapter 5, Crick and his associates were able to show experimentally that the code is, in fact, a triplet code, that there is no overlapping of codons but that there is degeneracy (55). Moreover, experiment established that the process of transcription from DNA to mRNA is directional, proceeding from the 5′ end of the DNA chain to the 3′ end, (where the 5′ and 3′ labels correspond to a standard numbering of carbon atoms in the nucleotides; recall Fig. 3)…