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 pairing. 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.