Codon usage bias

Codons are triplets of nucleotides that together specify an amino acid residue in a polypeptide chain. Most organisms use 20 or 21 amino acids to make their polypeptides, which are proteins or protein precursors. Because there are four possible nucleotides, adenine (A), guanine (G), cytosine (C) and thymine (T) in DNA, a triplet has the potential to distinguish between 64 amino acids. Due to this redundancy, many amino acids can be coded by more than one triplet. Organisms often show strong preference for one of the several codons that all encode the same given amino acid. Such preference may arise if the transfer RNA (tRNA) corresponding to the preferred codon translates faster than the alternative codons. In this scenario, the preferred codon would become more common in the genome and hence the concentration of preferred-codon tRNAs would be rate limiting. It would then be efficient for the preferred tRNA to be present at a higher concentration, and the less preferred tRNAs at a lesser concentration, reinforcing the preference for the preferred codon.