Proteins are large organic molecules that are involved in all aspects of cell structure and function. Proteins are made up of polypeptide chains, each constructed from a basic set of 20 amino acids, covalently linked in specific sequences. Each amino acid is coded by three successive nucleotide residues in deoxyribonucleic acid (DNA); the sequence of amino acids in a polypeptide chain, which determines the structure and function of the protein molecule, is thus specified by a sequence of nucleotide residues in DNA.