Watson-crick base pairing

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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 by Figure 1.1 below.

   Figure 1.1 A-T and G-C base pairs                                         


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
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