Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

Unevenness in the use of the individual codons for a certain aminoacid is an interesting consequence of the existence of internal gene periodicity.Although a single aminoacid can be encoded by up to six differentcodons, in actual genes, the individual codons for a single aminoacid occur with different frequencies – some are very common while others are practically absent.

            Originally, it was assumed that those codons whose tRNA occurs most frequently in the cell are used most often to ensure the greatest rate of protein synthesis (Sharp et al. 1995).However, further research showed that different codons are used most frequently in various genes of the same cell (Kepler 1997).In bacteria, this is most often a consequence of horizontal transfer of genes between various species; however, the same situation is also encountered in multicellular organisms.It is, of course, quite possible that the uneven numbers of codons serves not to maximize the rate of synthesis of proteins, but rather to optimize its rate (the optimum rate can be different for each protein).However, in a great many cases, unevenness in the use of the individual synonymous codons need not have any functional importance and could only be a consequence of the fact that the genes originated as tandem repetition of short oligonucleotides (Ohno 1988).It is apparent that, in this case, certain sequence motifs, for example some triplets, occur very frequently while others occur only very rarely or not at all.