Nucleobase functional groups: Which pyrimidine base possesses an amino (–NH2) group at the carbon-4 position on the ring?

Introduction / Context:
Knowing the functional groups on nucleobases is central to understanding base pairing, mutagenesis, and recognition by enzymes such as polymerases and deaminases.

Given Data / Assumptions:

• Pyrimidines include cytosine, thymine, and uracil.
• We focus on substituents at the C4 ring position.
• Standard canonical structures (not tautomers) are considered.

Concept / Approach:
Cytosine is a pyrimidine with an amino group at C4 and a keto at C2. Thymine and uracil are 2,4-diketo pyrimidines lacking an amino at C4. Adenine is a purine, not a pyrimidine, and thus is irrelevant to the asked position.

Step-by-Step Solution:
List pyrimidines: C, T, U.Recall structural motifs: cytosine has –NH2 at C4; thymine and uracil have carbonyl (keto) groups at C4.Therefore, the base with an amino at C4 is cytosine.

Verification / Alternative check:
Base-pairing logic: cytosine pairs with guanine via hydrogen bonds partly supported by its C4 amino.

Why Other Options Are Wrong:

• Thymine/Uracil: both have carbonyl at C4, not amino.
• Adenine: a purine with different ring numbering and groups.

Common Pitfalls:
Mixing up ring numbering or confusing purines with pyrimidines.

Final Answer:
Cytosine