DNA Base Pairing Rules

Steps:

• Purines are adenine and guanine; pyrimidines are cytosine and thymine.
• Specific pairs in DNA: adenine with thymine (2 H-bonds), guanine with cytosine (3 H-bonds).
• Verify each option's base types and pairing accuracy.
• Only options matching both classification and pairing are correct.

Why A is correct:

• Adenine (purine) pairs specifically with thymine (pyrimidine) via two hydrogen bonds, per Watson-Crick model.

Why the others are wrong:

• B: Cytosine is pyrimidine (not purine); guanine is purine (not pyrimidine).
• C: This statement is also correct, as guanine (purine) pairs with cytosine (pyrimidine).
• D: Guanine pairs with cytosine, not thymine.

Final answer: A and C