Identifying Pyrimidine Bases in DNA

Steps:

• Recall thymine and cytosine are single-ring pyrimidines: thymine has a methyl group, cytosine an amino group.
• Examine diagrams for six-membered rings with N at positions 1 and 3, plus specific substituents.
• Eliminate options with double-ring purines or mismatched groups.
• Select the option matching both structures.

Why C is correct:

• C depicts thymine's methyl-substituted pyrimidine and cytosine's amino-keto pyrimidine, per standard nucleotide base definitions.

Why the others are wrong:

• A shows purine bases (adenine, guanine) with fused rings.
• B displays thymine paired with uracil, not cytosine.
• D illustrates pyrimidine dimers, not individual bases.

Final answer: C