In DNA structure, which molecule is NOT found as a nitrogenous base in DNA?

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

Answer: (D)

Verified step by step guidance

1. Recall that DNA is composed of four nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G). These bases pair specifically (A with T, and C with G) to form the rungs of the DNA double helix.
2. Understand that uracil (U) is a nitrogenous base found in RNA, not DNA. In RNA, uracil replaces thymine and pairs with adenine.
3. Identify each option given in the problem and classify whether it is a DNA or RNA base: guanine, adenine, and thymine are DNA bases, while uracil is an RNA base.
4. Conclude that the molecule not found as a nitrogenous base in DNA is uracil, because it is exclusive to RNA.
5. Summarize that the key difference between DNA and RNA nitrogenous bases is the presence of thymine in DNA and uracil in RNA.