How many rings does a pyrimidine base have?

A pyrimidine base has one ring.

Pyrimidines are single-ring nitrogen-containing bases. In DNA, the pyrimidine bases are cytosine and thymine; in RNA, uracil takes thymine’s place. Their one six-membered ring contrasts with purines, such as adenine and guanine, which contain two fused rings.

The ring structure helps determine how bases fit together in the DNA double helix. A purine pairs with a pyrimidine, keeping the distance between the two sugar-phosphate backbones relatively uniform. In standard Watson–Crick pairing, adenine pairs with thymine, while guanine pairs with cytosine. In RNA, adenine pairs with uracil.

The word “pyrimidine” can refer both to the parent chemical compound and to its biological derivatives. The parent molecule has the formula C4H4N2 and contains nitrogen atoms at positions 1 and 3 of its six-membered ring. A common mix-up is to count the total rings in a base pair or confuse pyrimidines with purines; the defining structural distinction is one ring versus two.