The two purines in DNA are adenine and guanine. Within the structure of DNA, purines are paired with complementary pyrimidine bases, forming base pairs that are bonded by hydrogen bonds. These two purines are also found in RNA.
The main chemical structure of purines consists of a six-membered ring made of carbon and nitrogen fused together with a five-membered ring constructed out of the same elements. In contrast to purines, pyrimidines have a single-ring structure. To form base pairs, adenine, which is a six-amino purine, pairs with thymine through two hydrogen bonds. Guanine, which is a two-amino-six-oxy purine, bonds with cytosine through thee hydrogen bonds. In RNA, uracil is the pyrimidine base that pairs with adenine instead of thyamine.