A sequence of three nucleotides ('triplet) that codes for one amino acid in a polypeptide. A sequence of codons will thus determine the type and sequence of amino acids in a polypeptide. There are about 20 different kinds of amino acids in proteins. Most amino acids are coded for by more than one codon: the *genetic code
is thus said to be degenerate. The term codon is given both to the sequence of triplets on DNA and also to the sequence on messenger RNA. However these are not identical but complementary copies of each other.