Watson-Crick base pairing

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James Watson and Francis Crick
Figure 1.1 A-T and G-C base pairs

DNA consists of two types of bases namely; purines and pyrimidines. There are two types of purines: adenine and guanine as well as two types of Pyrimidines: cytosine and thymine. In the Watson-Crick DNA base pairing model a purine always binds with a pyrimidine, however, each purine binds to one particular type of pyrimidine.

Adenine (A) binds to thymine (T) whilst, guanine (G) binds to cytosine (C); although in RNA unracil (U) is substituted for thymine (T). This base pairing is referred to as complementary, hence the base pairs are called complementary base pairs [1].  The base pairs are bound by hydrogen bonds, although the number of H-bonds differs between base pairs. G-C base pairs are bound by three (3) hydrogen bonds whilst, A-T base pairs are bound by two (2) hydrogen bonds as illustrated in the figure on the right.



Watson-Crick base pairing is of very great importance as it is a deciding factor in DNA Replication [2]. It ensures that pairs form between complementary bases only. The formation of base pairs between two non-complementary bases results in gene mutations which can be detrimental to development of an organism.


  1. Hartl D., Ruvolo M. (2012) Genetics; Analysis of Genes and Genomes, 8th edition, Burlington: Jones and Barlett.
  2. Genetic Science Learning Center (2011) Build a DNA Molecule. Learn.Genetics. Available at: http://learn.genetics.utah.edu/content/begin/dna/builddna/. [Last assessed: 26/11/2011] University of Utah


In 1953, James D Watson and Francis Crick discovered the structure of DNA using X ray Crystallography. They worked out that DNA was a double helix using Rosalind Franklin's X ray diffraction pattern [1]. At first, it was thought that DNA was made up of many chemicals, which proved too difficult to analyse, but the researchers persisitence led to the discovery of complementary base pairing.


It has been found that DNA contains four bases namly adenine (A), thymine(T), guanine(G) and cytosine(C). A and T occur in same amounts and C and G occur in same amounts, thus the two possible base pair combinations [2].


  1. BBC NEWS. Science/Nature Available: http://news.bbc.co.uk/1/hi/sci/tech/2804545.stmfckLR[accessed 2 December 2011]
  2. DNA tutorial Available :http://www.dnatutorial.com/BasePairing.shtml [accessed 2 December 2011]
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