DNA Structure

Abstract

Deoxyribonucleic 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 identified in 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.

Keywords: deoxyribonucleic acid; base pair; gene; double helix; Watson–Crick; unusual DNA structures

Figure 1.

Structure of DNA and base pairs. Note that bond lengths are not proportional and some have been exaggerated for clarity. Broken lines represent hydrogen bonds in each base pair. (a) Chemical structure of DNA. The nomenclature for each base and its corresponding nucleoside is indicated. Atoms are numbered for one sugar, one purine base and one pyrimidine base. A single phosphodiester linkage is shown between adjacent nucleosides on each strand. Arrows highlight the antiparallel orientation of each polynucleotide strand in a duplex. The major and minor groove edges of each base pair are indicated. (b) Hoogsteen and reverse Hoogsteen base pairs of dA·dT and dG·dC.

Figure 2.

Three‐dimensional space‐filling models of B‐, A‐ and Z‐form helices. Major (M) and minor (m) grooves are indicated for each double helix.

Figure 3.

Three‐dimensional space‐filling models and hydrogen‐bonding patterns for triplexes and quadruplexes. Broken lines indicate hydrogen bonds and ‘R groups’ represent the continuation of polynucleotide structure through the phosphate and sugar backbone.

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Sayre A (1975) Rosalind Franklin and DNA. New York: Norton.

Watson JD (1997) The Double Helix: A Personal Account of the Discovery of the Structure of DNA. London: Weidenfeld & Nicolson.

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Bowater, Richard P, and Waller, Zoë AE(Apr 2014) DNA Structure. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0006002.pub2]