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. 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 a peptide bond (example needed). Polypeptide chains may be covalently linked together, most commonly by disulfide bonds.

Protein molecules may consist of one or more polypeptide chains, 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.