The genetic code. The standard one-letter abbreviation for each amino acid is presented below its three-letter abbreviation (see Panel 2-5 for the full name of each amino acid and its structure). By convention, codons are always written with the 5?terminal nucleotide to the left. Note that most amino acids are represented by more than one codon, and that there are some regularities in the set of codons that specifies each amino acid. Codons for the same amino acid tend to contain the same nucleotides at the first and second positions, and vary at the third position. Three codons do not specify any amino acid, but act as termination sites (stop codons), signalling the end of the protein-coding sequence. One codon--AUG--acts both as an initiation codon, signalling the start of a protein-coding message, and also as the codon that specifies methionine