15. What is the correct base pairing between two DNA strands?

Question

BlackTom AI · Accepted Answer

Adenine forms two hydrogen bonds with thymine, and cytosine forms three hydrogen bonds with guanine — these are the canonical Watson–Crick base pairs that define DNA complementarity.

Relevant knowledge points:
- DNA is double-stranded and antiparallel; bases on opposite strands pair by hydrogen bonding.
- Watson–Crick base pairing: A pairs with T (A–T) via two hydrogen bonds; C pairs with G (C–G) via three hydrogen bonds.
- Base-pair complementarity ensures accurate replication and stable helical structure; mismatched or noncanonical pairs (e.g., A–C, G–T) destabilize the helix or occur only rarely.

Step-by-step reasoning:
1. Identify the two correct canonical pairs: adenine (A) with thymine (T), and cytosine (C) with guanine (G).
2. Note the bonding detail: the A–T pair uses two hydrogen bonds; the C–G pair uses three hydrogen bonds. These bonding patterns are specific because of the molecular shapes and hydrogen-bond donors/acceptors on each base.
3. Compare options: if option A states A–T and option C states C–G, then both are correct statements about standard DNA base pairing.
4. Option D, “Both A and C,” therefore correctly combines the two correct pairings into the complete answer for how two DNA strands pair.

Why the correct answer is correct:
D is correct because DNA base pairing requires both types of complementary pairs to fully describe standard A–T and C–G pairing. The double helix depends on both pair types for sequence-specific complementarity and stable helical geometry.

Why other options are incorrect:
- Any option that lists only one pairing (A only or C only) is incomplete because it omits the other canonical pair.
- Any option proposing noncanonical pairs (e.g., A–C, G–T, or pairing with uracil in DNA) is chemically incorrect for standard DNA and would not accurately represent base complementarity.

Conclusion: The correct answer is D. Both A and C, which matches the provided answer.

15. What is the correct base pairing between two DNA strands?Single choice

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