The genetic code is the set of correspondences between the nucleotide sequences of nucleic acids such as DNA (deoxyribonucleic acid), and the amino acid sequences of polypeptides. These correspondences enable the information encoded in the chemical components of the DNA to be transferred to the ribonucleic acid messenger (mRNA), and then to be used to establish the correct sequence of amino acids in the polypeptide. The elements of the encoding system, the nucleotides, differ by only four different bases. These are known as adenine (A), guanine, (G), thymine (T) and cytosine (C), in DNA or uracil (U) in RNA. Thus, RNA contains U in the place of C and the nucleotide sequence of DNA acts as a template for the synthesis of a complementary sequence of RNA, a process known as transcription. For historical reasons, the term genetic code in fact refers specifically to the sequence...