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

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