secondary structure

Localized three dimensional conformations adopted by macromolecules, in particular nucleic acids and polypeptides. These arise as a result of the action of non-covalent forces generated by interactions between residues which are brought into close contact with one another. Examples are alpha-helix regions and beta-pleated sheets in protein s, and hairpin loops in nucleic acids. See: primary structure, tertiary structure, quaternary structure.