<P> Proteins are large biomolecules, or macromolecules, consisting of one or more long chains of amino acid residues . Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, responding to stimuli, and transporting molecules from one location to another . Proteins differ from one another primarily in their sequence of amino acids, which is dictated by the nucleotide sequence of their genes, and which usually results in protein folding into a specific three - dimensional structure that determines its activity . </P> <P> A linear chain of amino acid residues is called a polypeptide . A protein contains at least one long polypeptide . Short polypeptides, containing less than 20--30 residues, are rarely considered to be proteins and are commonly called peptides, or sometimes oligopeptides . The individual amino acid residues are bonded together by peptide bonds and adjacent amino acid residues . The sequence of amino acid residues in a protein is defined by the sequence of a gene, which is encoded in the genetic code . In general, the genetic code specifies 20 standard amino acids; however, in certain organisms the genetic code can include selenocysteine and--in certain archaea--pyrrolysine . Shortly after or even during synthesis, the residues in a protein are often chemically modified by post-translational modification, which alters the physical and chemical properties, folding, stability, activity, and ultimately, the function of the proteins . Sometimes proteins have non-peptide groups attached, which can be called prosthetic groups or cofactors . Proteins can also work together to achieve a particular function, and they often associate to form stable protein complexes . </P>

A protein is made up of a chain of