Pyrimidine is a heterocyclic aromatic organic compound similar to benzene and pyridine, containing two nitrogen atoms at positions 1 and 3 of the six-member ring. It is isomeric with two other forms of diazine.
Three nucleobases found in nucleic acids, cytosine (C), thymine (T), and uracil (U), are pyrimidine derivatives:
In DNA and RNA, these bases form hydrogen bonds with their complementary purines. Thus, the purines adenine (A) and guanine (G) pair up with the pyrimidines thymine (T) and cytosine (C), respectively.
In RNA, the complement of A is U instead of T and the pairs that form are adenine:uracil and guanine:cytosine.
Very rarely, thymine can appear in RNA, or uracil in DNA. Other than the three major pyrimidine bases presented, some minor pyrimidine bases can also occur in nucleic acids. These minor pyrimidines are usually methylated versions of major ones and are postulated to have regulatory functions. 
These hydrogen bonding modes are for classical Watson-Crick base pairing. Other hydrogen bonding modes ("wobble pairings") 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.
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