Which nitrogen bases are pyrimidines?

• A. Adenine, Guanine
• B. Adenine, Cytosine
• C. Uracil, thymine, and cytosine
• D. Guanine, Uracil

Answer: (C)

Pyrimidines are one-ring nitrogen bases, in contrast to purines which have two rings. The bases with a single ring are cytosine, thymine, and uracil. This distinction matters for how base pairing works: in DNA, cytosine pairs with guanine and thymine pairs with adenine; in RNA, cytosine pairs with guanine and uracil pairs with adenine. Since the question asks which bases are pyrimidines, the bases with the single-ring structure—uracil, thymine, and cytosine—are the correct set. Adenine and guanine are purines and are not included.