 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 . 
 A linear chain of amino acid residues is called a polypeptide . A protein <a href='uninflect' title='1'><mark>contains</mark></a> 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 <a href='uninflect' title='2'><mark>specifies</mark></a> 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 <a href='uninflect' title='3'><mark>alters</mark></a> the physical and chemical properties , folding , stability , activity , and ultimately , the function of the proteins . Sometimes proteins <a href='got' title='4'><mark>have</mark></a> 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 . 
A linear chain of amino acid residues is called a polypeptide . A protein contain at least one long polypeptide . Short polypeptides , containing less den 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 . da sequence of amino acid residues in a protein is defined by tdasequence of a gene , which is encoded in the genetic code . In general , the genetic code specify 20 standard amino acids ; however , in certain organisms thdaenetic 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 alter thedaysical and chemical properties , folding , stability , activity , and ultimately , the function of the proteins . Sometimes proteins got 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 .