A polypeptide chain is a series of amino acids that are joined by peptide bonds. Each amino acid in a polypeptide chain is called a residue. It also has polarity because its ends are different. The backbone or main chain is the part of the polypeptide chain that consists of a regularly repeated portion and is rich in hydrogen bonding potential. There is also a variable part, which includes the distinct side chain. Each residue in the chain has a carbonyl group, which is a good hydrogen bond acceptor, and an NH group, which is a good hydrogen bond donor. The groups interact with the functional groups of the side chains and with each other to stabilize the structures. Proteins are polypeptide chains containing 500 to 2,000 amino acid residues. Oligopeptides, or peptides, are made up of a small number of amino acids. Each protein has a unique, precisely defined amino acid sequence, called a primary structure. The amino acid sequences of proteins are determined by the nucleotide sequences of genes, because the nucleotides in DNA specify a complementary sequence in RNA, which specifies the amino acid sequence. Amino acid sequences determine the 3D structures of proteins. Alteration of the amino acid sequence can produce disease and abnormal function. All the different ways