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| 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 [[standard residues|nucleotides]] covalently linked by ribose (or deoxyribose)-phosphodiester bonds, e.g. either [[DNA]] or [[RNA]].
| | #REDIRECT [[Chains and Chain IDs]] |
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| 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]], or less commonly by other types of [[protein crosslinks]].
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| Each protein chain has '''two ends''', an amino terminus (positively charged) and a carboxy terminus (negatively charged). The first residue in a protein chain becomes the amino terminus, with new amino acids being added at the carboxy terminus. The sequence of [[amino acids]] is specified by messenger RNA, which is a copy of the sequence of codons in the template strand of the DNA gene. The first residue in a nucleic acid chain becomes the 5' (phosphate) terminus, with new nucleotides being added at the 3' (hydroxy) terminus.
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| 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]].
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| In a protein molecule consisting of multiple chains, the chains are usually held together by non-covalent bonds, but sometimes by covalent bonds, usually [[disulfide bonds]]. See [[Protein primary, secondary, tertiary and quaternary structure|quaternary structure]].
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