Watson-crick base pairing
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.  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.
Watson-Crick base pairing is of very great importance as it is a deciding factor in DNA Replication. 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.
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