Which nitrogenous base is NOT found in RNA?

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

Answer: (A)

Verified step by step guidance

1. Recall that nucleic acids DNA and RNA are composed of nitrogenous bases paired with a sugar-phosphate backbone. The four main bases in DNA are Adenine (A), Thymine (T), Guanine (G), and Cytosine (C).
2. Understand that RNA differs from DNA in one key base: instead of Thymine, RNA contains Uracil (U). This means that Uracil replaces Thymine in RNA molecules.
3. List the nitrogenous bases found in RNA: Adenine (A), Uracil (U), Guanine (G), and Cytosine (C). Notice that Thymine is absent from this list.
4. Therefore, identify that the nitrogenous base NOT found in RNA is Thymine, which is unique to DNA.
5. Summarize that the presence of Uracil instead of Thymine is a fundamental difference between RNA and DNA, helping to distinguish their structures and functions.