Which statement about purine bases is true?

Which statement about purine bases is true?

• A. They have a single-ring structure.
• B. They are not nitrogenous bases.
• C. They have a double-ring structure.
• D. They pair with cytosine.

Answer: (C)

Purine bases are defined by their double-ring structure, a fused six-membered and five-membered ring. This distinguishes them from pyrimidines, which have a single ring. The two-ring design helps keep the width of the DNA/RNA ladder uniform when purines pair with pyrimidines. In DNA, adenine pairs with thymine and guanine pairs with cytosine (three hydrogen bonds with G–C, two with A–T). Therefore, the statement about having a double-ring structure is true. The other options don’t fit: one ring is characteristic of pyrimidines, purines are nitrogenous bases, and purines pair with pyrimidines (not with cytosine alone).

Purine bases are defined by their double-ring structure, a fused six-membered and five-membered ring. This distinguishes them from pyrimidines, which have a single ring. The two-ring design helps keep the width of the DNA/RNA ladder uniform when purines pair with pyrimidines. In DNA, adenine pairs with thymine and guanine pairs with cytosine (three hydrogen bonds with G–C, two with A–T). Therefore, the statement about having a double-ring structure is true. The other options don’t fit: one ring is characteristic of pyrimidines, purines are nitrogenous bases, and purines pair with pyrimidines (not with cytosine alone).