DNAStructure
Authors: Bowater Richard P , Waller Zoë AE 
Published: April 2014

AbstractDeoxyribonucleic acid (DNA)is a polymer of nucleotides that provides the chemical basis for inheritable characteristics of all cellular organisms. The genetic information in DNA is defined by the sequence of individual bases, which are the pyrimidines, cytosine and thymine and the purines, guanine and adenine. Hydrogen bonds form between appropriately positioned donors and acceptors on the bases of each strand, such that A pairs with T and G pairs with C. In the cell, DNA usually adopts a double‐stranded helical form, with complementary base pairing holding the two strands together. The most stable double‐stranded conformation is called B‐form DNA. A high degree of flexibility in DNA molecules means that a wide range of other structures can occur under specific conditions, including some that involve more than two strands of DNA.Key Concepts:Deoxyribonucleic acid (DNA) is the genetic material of all cellular organisms and provides the chemical basis for inheritable characteristics.DNA is a polymer of nucleotides, each being the phosphate ester of one of four different nucleosides that consist of a five‐carbon sugar and a nitrogen‐containing base; the presence of different chemical groups at opposing ends of the nucleotide means that it has polarity, with sequence details usually defined in the 5′ to 3′ direction.The genetic information contained in DNA is defined by the sequence of individual bases, which are the pyrimidines – cytosine (C) and thymine (T) – and the purines – guanine (G) and adenine (A).Double‐stranded DNA has a 1:1 ratio of purine to pyrimidine bases, known as Chargaff's rules; hydrogen bonds are formed between appropriately positioned donors and acceptors on the bases of each strand, such that A pairs with T and G pairs with C.Base‐pairing rules mean that the sequence of one strand dictates the sequence of the second strand – a fundamental property of DNA in copying this information into new DNA molecules (replication) and for directing the synthesis of RNA molecules (transcription).In 1953 the structure of DNA was shown to consist of two twisted backbone chains of alternating units of phosphoric acid and deoxyribose, linked by crosspieces of purine and pyrimidine bases.In addition to base pairing, DNA helices are stabilised by base‐stacking interactions that occur between neighbouring bases in order to reduce the area of these hydrophobic heterocycles that are exposed to solvent.In Watson–Crick base pairs, the two sugars linked to each base are located on the same side of the helix, meaning that the gap between these sugars forms asymmetric, continuous grooves in the surface, referred to as ‘major’ and ‘minor’.DNA has a remarkably supple structure that can adopt a variety of bends, twists and altered helical and nonhelical conformations that are typically stabilised by the many different hydrogen‐bonding schemes that can form.Many unusual conformations of DNA have been identifiedin vitro, including some involving more than two strands of DNA, such as triplexes (three strands) and quadruplexes (four strands), although the significance for such structures in biological functions is not yet fully clear.