Standard genetic code
The standard genetic code is the list of codons, made up of triplet bases on RNA, that provide instructions for which amino acid to add to a growing protein chain. The codon triplets are three sequential bases which can include adenine (A), cytosine (C), guanine (G) or uracil (U). The codon for the amino acid methionine is also the start codon in mRNA used to initiate protein production. These codons are made to interact with the corresponding anti-codon in transfer-RNA molecules (tRNA). Although some amino acids have only one codon, such as AUG for methionine, other amino acids are indicated by several codons. Thus, proline has four codons, CCU, CCC, CCA and CCG. This means there is a one-to-one correspondance in going from codon to amino acid, but not from an amino acid to codons. There is only one start codon, AUG, which serves as the codon for methionine, but there are three stop codons, UAA, UAG and UGA. The sequence of codons in mRNA is derived from the sequence present in the DNA from which the mRNA was transcribed.
- Biochemistry, 3rd Ed, Lubert Stryer, 1988, W. H. Freeman and Co, New York.