Backbone

Backbone, or main chain, is the general term used to describe the connecting chain in polymers. Different kinds of polymers have different chemical backbones. For example, in proteins the backbone is a polypeptide chain, but nucleic acids have a sugar phosphate backbone. The backbone of a polymer, which may adopt a regular structural conformation, is the constant or repeating part of the polymer, as opposed to the attached side chains, which can be variable.