Biomolecular structure is the intricate folded, three-dimensional shape that is formed by a protein, DNA, or RNA molecule, and which is important to its function. The structure of these molecules is frequently decomposed into primary structure, secondary structure, tertiary structure, and quaternary structure. The scaffold for this structure is provided by secondary structural elements which are hydrogen bonds within the molecule. This leads to several recognizable domains of protein structure and nucleic acid structure, including secondary structure like hairpin loops, bulges and internal loops for nucleic acids, and Alpha helix|alpha helices and beta sheets for proteins.
The terms primary, secondary, tertiary, and quaternary structure were first coined by Kaj Ulrik Linderstrøm-Lang in his 1951 Lane Medical Lectures at Stanford University.