DNA or Deoxyribonucleic acid is a double-stranded helical molecule and codes the genetic information necessary for life. Each DNA molecule consists of four nitrogen containing nucleobases:
Adenine (designated as A)
Guanine (designated as G)
Thymine (designated as T) and,
Cytosine (designated as C).
Of these four bases, A and G are Purines, and are five and six-membered heterocyclic compounds. The remaining bases, C and T are known as Pyrimidines, and have six-membered ring structures.
The double-helix structure of DNA is possible only because of complementary base pairing, in which one base on a strand will only pair with a specific base on the other strand, through hydrogen bonds. For example, adenine pair with thymine through two hydrogen bonds and cytosine only pairs with guanine through three hydrogen bonds. The complementary pairing of nucleobases enables the DNA replication and is crucial for life.
There is another nucleobase, Uracil (designated as U) that is present only in RNA and replaces thymine.
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