In biochemistry, the primary structure of an unbranched biopolymer, such as a molecule of DNA, RNA or protein, is the specific nucleotide or peptide sequence from the beginning to the end of the molecule. The primary structure, in other words, identifies a biopolymer's exact chemical composition and the sequence of its monomeric subunits.
A protein primary structure is a chain of amino acids.
The primary structure of a biological polymer to a large extent determines the three-dimensional shape known as the tertiary structure, but nucleic acid and protein folding are so complex that knowing the primary structure often doesn't help either to deduce the shape or to predict localized secondary structure, such as the formation of loops or helices. However, knowing the structure of a similar homologous sequence (for example a member of the same protein family) can unambiguously identify the tertiary structure of the given sequence. Sequence families are often determined by sequence clustering, and structural genomics projects aim to produce a set of representative structures to cover the sequence space of possible non-redundant sequences.
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