What is the primary structural difference between purines and pyrimidines in nitrogenous bases?

• A. Purines are found only in $\mathrm{DNA}$, while pyrimidines are found only in $\mathrm{RNA}$.
• B. Purines contain a single carbon-nitrogen ring, while pyrimidines contain two carbon-nitrogen rings.
• C. Purines are composed of only $C$ and $H$, while pyrimidines contain $N$.
• D. Purines have a double-ring structure, while pyrimidines have a single-ring structure.

Answer: (D)

Verified step by step guidance

1. Identify the two types of nitrogenous bases: purines and pyrimidines.
2. Understand that purines and pyrimidines are both found in DNA and RNA, not exclusively in one or the other.
3. Recognize that purines and pyrimidines are composed of carbon, hydrogen, and nitrogen atoms.
4. Learn that purines have a double-ring structure, consisting of a six-membered and a five-membered nitrogen-containing ring fused together.
5. Understand that pyrimidines have a single-ring structure, consisting of a six-membered nitrogen-containing ring.