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Pyrimidine is an organic compound, similar to benzene but with two nitrogen atoms taking the place of carbon atoms at positions 1 and 3 relative to each other around the six-member ring.

Three bases in nucleic acids, namely cytosine, guanine, and uracil, are pyrimidine derivatives. In DNA, these bases form hydrogen bonds with their complementary purines.

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.