Pyrimidine is an organic compound, similar to benzene, but with a heterocyclic ring: 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 of the nucleic acids, namely cytosine, thymine, and uracil, are pyrimidine derivatives. In DNA, these bases form hydrogen bonds with their complementary purines.
purine pyrimidineIn RNA, the complement of A is U instead of T:A T
G C
purine pyrimidineThese 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.A U
G C