Optimisation of Asymmetric Mutational Pressure and Selection Pressure Around the Universal Genetic Code

One of hypotheses explaining the origin of the genetic code assumes that its evolution has minimised the deleterious effects of mutations in coded proteins. To estimate the level of such optimization, we calculated optimal codes for genes located on differently replicating DNA strands separately assuming the rate of amino acid substitutions in proteins as a measure of code’s susceptibility to errors. The optimal code for genes located on one DNA strand was simultaneously worse than the universal code for the genes located on the other strand. Furthermore, we generated 20 million random codes of which only 23 were better than the universal one for genes located on both strands simultaneously while about two orders of magnitude more codes were better for each of the two strands separately. The result indicates that the existing universal code, the mutational pressure, the codon and amino acid compositions are highly optimised for the both differently replicating DNA strands.