In biochemistry, to specify the primary structure of an unbranched biopolymer, such as a molecule of DNA, RNA or protein, is to name the species of every subunit (nucleotide or amino acid) in order from the beginning to the end of the molecule. The primary structure, in other words, refers to a biopolymer's exact chemical composition and to the sequence in which its subunits are arranged.

While the primary structure of a biological polymer to a large extent determines the three-dimensional shape that the molecule assumes in vivo, knowing it often doesn't help a person to deduce this shape (known as the tertiary structure) or to predict localized structuring, such as the formation of loops or helices (called secondary structure). At the atomic level, very few types of dihedral angles are specified by the primary structure. Nor are many non-bonded interatomic contacts specified.

However, the nature of nearly all of the covalent bonds and of many of the bond angles are implied at the level of primary structure based on the structural invariants inherent in the monomers involved, and therefore the primary structure is rich with information.

See also : sequencing -- DNA sequence -- secondary structure -- tertiary structure -- quaternary structure -- translation