Three-nucleotide genetic coding system

Several important features of the three-nucleotide genetic coding system are illustrated in Table 1.1. First, the code is redundant but lacks ambiguity. For example, both TTC and TTT code for phenylalanine, thus the code is redundant however, neither TTC or TTT code for any other amino acid, thus the code is not ambiguous. 

It is clear that no genetic system could have been based on these isomeric nucleotides, in which one does not see the association of two strands to transfer and utilize the genetic coding, only three strand association of a special sort. Such studies— related to those by Eschenmoser [7] in which ribose was substituted by other sugars— give us insight into the special chemical properties of natural substances such as DNA that allow them to perform their biological functions. 

In DNA the genetic code is found in the sequential bases, more precisely in the succession of groups of three nucleotides (triplets). As discussed in a latter section the genetic code was completely elucidated by comparing the template properties of synthetic polyribonucleotides in cell-free systems. However, first evidence that the c ing groups (codons) were triplets came from genetic studies. 

An obvious answer is that there is not one base, but rather a combination of bases coding for each amino acid. If the code consists of nucleotide pairs, there are 4 = 16 combinations this is a more extensive code, but it is still not extensive enough to code for 20 amino acids. If the code consists of nucleotides in groups of three, there are 4 = 64 combinations more than enough to code for the primary structure of a protein. This appears to be a very simple solution to the problem for a system that must have taken eons of evolutionary trial and error to develop. Yet proof now exists, from comparisons of gene (nucleic acid) and protein (amino acid) sequences, that nature does indeed use a simple three-letter or triplet code to store genetic information. A triplet of nucleotides is called a codon. 

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