A quantitative measure of error minimization in the genetic code.

We have calculated the average effect of changing a codon by a single base for all possible single-base changes in the genetic code and for changes in the first, second, and third codon positions separately. Such values were calculated for an amino acid's polar requirement, hydropathy, molecular volume, and isoelectric point. For each attribute the average effect of single-base changes was also calculated for a large number of randomly generated codes that retained the same level of redundancy as the natural code. Amino acids whose codons differed by a single base in the first and third codon positions were very similar with respect to polar requirement and hydropathy. The major differences between amino acids were specified by the second codon position. Codons with U in the second position are hydrophobic, whereas most codons with A in the second position are hydrophilic. This accounts for the observation of complementary hydropathy. Single-base changes in the natural code had a smaller average effect on polar requirement than all but 0.02% of random codes. This result is most easily explained by selection to minimize deleterious effects of translation errors during the early evolution of the code.