Pyrimidines have how many rings?

• A. Two-ring
• B. Three-ring
• C. One ring
• D. Four rings

Answer: (C)

Pyrimidines are defined by a single six-membered ring that contains two nitrogen atoms. This single-ring structure distinguishes them from purines, which have two fused rings. In DNA and RNA, pyrimidines include cytosine and thymine (and uracil in RNA). Because they are one ring, that makes the correct statement that pyrimidines have one ring. Two rings describe purines like adenine and guanine, while three or four rings do not occur for the standard nucleobases.