 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 <unk> . 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 <unk> and — in certain archaea — <unk> . Shortly after or even during synthesis , the residues in a protein are often chemically modified by post @-@ <unk> 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 <unk> . Proteins can also work together to achieve a particular function , and they often associate to form stable protein complexes . 
