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