DNA structure – Which statement about base pairing in the DNA double helix is incorrect?

Introduction:
Watson–Crick base pairing underpins the stability, fidelity, and information storage capacity of DNA. Knowing which bases pair in standard B-form DNA is fundamental for understanding replication, transcription, and hybridization technologies.

Given Data / Assumptions:

• DNA is a double helix with antiparallel strands.
• Complementary base pairing occurs via hydrogen bonds.
• A pairs with T; G pairs with C in canonical pairing.

Concept / Approach:
In standard B-DNA, adenine (A) forms specific hydrogen bonds with thymine (T), whereas guanine (G) pairs with cytosine (C). These pairings enforce a uniform helix geometry and enable predictable melting behavior. Any statement contradicting A–T or G–C pairing is incorrect for canonical DNA under physiological conditions.

Step-by-Step Solution:

Verification / Alternative check:
Thermodynamic parameters and structural data consistently show stability of A–T and G–C pairs, with mismatches (e.g., A–C) destabilizing duplexes and used diagnostically in SNP genotyping assays.

Why Other Options Are Wrong:

• Strands held by base interactions: correct (hydrogen bonding plus stacking).
• A with T and G with C: both correct.
• “None of the above are incorrect”: incorrect because A–C is wrong.

Common Pitfalls:
Confusing RNA pairing (A–U) with DNA; in DNA, thymine replaces uracil.

Final Answer:
Adenine normally pairs with cytosine in standard B-DNA.