Pyrimidine is a heterocyclic aromatic organic compound, which is similar to benzene and pyridine and that contains two nitrogen atoms at positions 1 and 3 of the six-membered ring.

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

purine  pyrimidine
   A         T
   G         C

In RNA, the complement of A is U instead of T:

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 2'-hydroxyl group of RNA expands the configurations through which RNA can form hydrogen bonds.

See also

• Pyrazine, an analog with the nitrogen atoms in positions 1 and 4.
• Simple aromatic rings