Complementary Base Pairing in DNA

Steps:

• DNA contains four bases: adenine (A), thymine (T), cytosine (C), guanine (G).
• A pairs with T via two hydrogen bonds; C pairs with G via three hydrogen bonds.
• Review options: 1 (A-C) mismatches; 2 (A-G) mismatches; 3 (C-G) matches; 4 (A-T) matches; 5 (C-T) mismatches.
• Correct pairs are 3 and 4 only.

Why D is correct:

• D identifies 3 (C-G) and 4 (A-T) as the standard Watson-Crick base pairs that hold DNA strands together.

Why the others are wrong:

• A includes non-pairs (1,2,5) and nonexistent 6.
• B selects mismatches 2 (A-G) and 5 (C-T).
• C provides no valid selection.

Final answer: D