Question

Most amino acids are coded for by a set of similar codons. Provide an explanation for why amino acids are associated with such closely related codons?

Answer 1

There are more codons than there are amino acids.  Except methionine and tryptophan almost all amino acids are represented by more than one codon. Codons that have the same meaning are called synonyms. Codons representing the same or related amino acids are similar in sequence. The four codons differing only in the third base represent the same amino acid, showing the base in the third position of a codon is insignificant.

The codons used in the mRNA of one species have the same meaning for the ribosomes and tRNAs of other species. Very early in evolution the code started in a primitive form in which a small number of codons were used to represent fewer amino acids. More precise codon meanings and additional amino acids could have been added later. At first only two of the three bases in each codon could have been used, discrimination at the third position may have evolved later. Evolution of the code may have stopped at a point at which the system had become too complex that any changes in codon meaning would have disrupted the existing proteins by substituting unacceptable amino acids. This must have happened at such an early stage that all living organisms are descended from a single pool of primitive cells in which this occurred.

Effects of mutations are minimized when similar amino acids are represented by related codons. It increases the probability that a single random base change will result in no amino acid substitution or in one involving amino acids of similar character.