Which base is not a pyrimidine base?

Which base is not a pyrimidine base?

• A. Cytosine
• B. Uracil
• C. Thymine
• D. Adenine

Answer: (D)

Think about the two classes of nucleotide bases: purines, which have two fused rings, and pyrimidines, which have a single ring. Cytosine, uracil, and thymine are all pyrimidines, each built from one ring. Adenine, in contrast, is a purine with two rings. Since the question asks which base is not a pyrimidine, adenine is the correct choice because its structure places it in the purine category. (In DNA thymine pairs with adenine, and in RNA uracil pairs with adenine, reflecting the purine–pyrimidine pairing pattern.)

Think about the two classes of nucleotide bases: purines, which have two fused rings, and pyrimidines, which have a single ring. Cytosine, uracil, and thymine are all pyrimidines, each built from one ring. Adenine, in contrast, is a purine with two rings. Since the question asks which base is not a pyrimidine, adenine is the correct choice because its structure places it in the purine category. (In DNA thymine pairs with adenine, and in RNA uracil pairs with adenine, reflecting the purine–pyrimidine pairing pattern.)