Purine

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

 purine  pyrimidine
   A         T
   G         C

In RNA, the complement of A is uracil instead of thymine:

 purine  pyrimidine
   A         U
   G         C

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