Which statement about purines and pyrimidines is true?

Which statement about purines and pyrimidines is true?

• A. Purines have double rings; pyrimidines have single rings.
• B. Purines have single rings; pyrimidines have double rings.
• C. Purines have double rings; pyrimidines have double rings.
• D. Purines have single rings; pyrimidines have no rings.

Answer: (A)

Purines and pyrimidines differ by ring structure: purines have two fused rings, while pyrimidines have a single ring. This difference in size and shape helps them pair across the DNA double helix (purines with pyrimidines) to keep the helix width uniform. Adenine and guanine are purines; cytosine pairs with either thymine in DNA or uracil in RNA as pyrimidines. Therefore, the statement that purines have double rings and pyrimidines have single rings is true. The other options assign incorrect ring counts to the two classes.

Purines and pyrimidines differ by ring structure: purines have two fused rings, while pyrimidines have a single ring. This difference in size and shape helps them pair across the DNA double helix (purines with pyrimidines) to keep the helix width uniform. Adenine and guanine are purines; cytosine pairs with either thymine in DNA or uracil in RNA as pyrimidines. Therefore, the statement that purines have double rings and pyrimidines have single rings is true. The other options assign incorrect ring counts to the two classes.