Identify the base: which purine bears an amino (NH2) group at the 6-position on the ring? Choose the correct heterocycle based on its substituent pattern.

Introduction / Context:
Nucleobases are distinguished by ring system (purine vs pyrimidine) and functional groups. Correctly identifying substituents is essential for understanding base pairing, tautomerism, and chemical reactivity.

Given Data / Assumptions:

• Purines: adenine and guanine (fused imidazole–pyrimidine ring).
• Pyrimidines: cytosine, thymine, uracil (single six-membered ring).
• Question targets an amino group at C6 of a purine.

Concept / Approach:
Adenine is 6-aminopurine (NH2 at C6). Guanine is 2-amino-6-oxo purine (amino at C2, carbonyl at C6). Cytosine, thymine, and uracil are pyrimidines and do not match the description.

Step-by-Step Solution:
Classify bases: identify purines among options (adenine, guanine).Check substituents: adenine → NH2 at C6; guanine → NH2 at C2 and =O at C6.Select adenine as the correct base bearing NH2 at the 6-position.

Verification / Alternative check:
Standard chemical structures in biochemistry references annotate adenine as 6-aminopurine.

Why Other Options Are Wrong:
Pyrimidines do not have a purine C6 position; guanine’s 6-position is carbonyl, not amino.

Common Pitfalls:
Confusing numbering schemes between rings and mixing up the 2-amino group of guanine with the 6-amino of adenine.

Final Answer:
Adenine.