n. (Molecular biology) a sequence of three nucleotides in a genome or a DNA or messenger RNA molecule, which specifies the incorporation of one amino acid or is a stop signal, during the biosynthesis of proteins. Codons occur within the protein-coding segments of the DNA or RNA genome of living organisms. The amino acid sequence of proteins synthesized on ribosomes is thus determined by the sequence of the nucleotides in the genome. [ PJC ]

☞ Outside of coding regions, there is no direct correspondence of the nucleotide sequence with protein sequence, and certain signalling functions of nucleic acids are not specified by three-nucleotide codons. Certain types of polypeptide synthesized in living organisms are not synthesized on ribosomes, and the sequences of these polypeptides do not have a corresponding nucleotide sequence in the genome. [ PJC ]