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)-phosphodiester 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.
