Chain

The term chain, in biochemistry, usually denotes either a polypeptide chain or a polynucleotide chain. A polypeptide chain is a sequence of amino acids covalently linked by peptide bonds. The chain structures of proteins are most easily visualized with backbone representations. A short polypeptide consisting of 50 or fewer amino acids is termed a peptide. A polynucleotide chain is a sequence of nucleotides covalently linked by ribose (or deoxyribose)-phosphate bonds, e.g. either DNA or RNA.

Polypeptide (protein) chains are linear, with rare exceptions where a side-chain forms an isopeptide bond. Polypeptide chains may be covalently linked together, most commonly by disulfide bonds.

Protein molecules may consist of one or more polypeptide chains. Those with more than one chain may be termed homo-oligomers or hetero-oligomers, homo-multimers or hetero-multimers. The functional form of the molecule, termed the biological unit, often contains a different number of chains than does the crystallographic asymmetric unit. Examples are given in the article on biological units.