Purine

Purine is a bicyclic organic compound. Two of the bases in nucleic acids, adenine? and guanine, are purine derivatives. In DNA, these bases form hydrogen bonds with their complementary pyrimidines.

 purine  pyrimidine

   A         T

   G         C

In RNA, instead of T, U complements A:

 purine  pyrimidine

   A         U

   G         C

These hydrogen bonding modes are for classical [Watson-Crick base pair]?ing. Other hydrogen bonding modes are available in both DNA and RNA, although the additional 3'-hydroxyl group of RNA expands the configurations through which RNA can form hydrogen bonds.

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