While there is redudency in the
genetic code of
DNA which codes for proteins in organisms, not all codes are used equally. The triplet codes which are used most often are also those whose corresponding
tRNAs are most abundant in the cell. It has been hypothesized that codon bias has
evolved by
natural selection for
translational efficiency. In other words, the most abundant
tRNAs are used because they will allow a protein the be translated quickest. This is supported by the observation that
genes which are highly
transcribed have the greatest degree of codon bias.
Codon bias has been envoked as evidence for the
nearly neutral theory.
For a technical review, check out: http://www.genetics.org/cgi/content/abstract/146/1/295