A single codon is a sequence of three nucleotides (a trinucleotide) that forms a unit of genomic information encoding a particular amino acid or signaling the termination of protein synthesis (stop signals) . With some exceptions, a three-nucleotide codon in a nucleic acid sequence specifies a single amino acid. Methionine and tryptophan are the only two amino acids that are coded for by just a single codon (AUG and UGG, respectively) .