DNA double helix — What interactions hold the two antiparallel DNA strands together in the duplex?

Introduction:
The stability and specificity of the DNA double helix depend on defined interactions between bases on opposing strands. This question asks which interaction is responsible for holding the two strands together in the Watson–Crick model.

Given Data / Assumptions:

• Strands are antiparallel and complementary (A pairs with T, G pairs with C).
• Backbone consists of covalently linked sugar-phosphate units.
• Base pairing provides sequence-specific recognition.

Concept / Approach:
Differentiate the types of bonds: covalent bonds form the backbone within each strand; hydrogen bonds and base stacking stabilize pairing and helix formation between strands. The direct strand-to-strand connection comes from hydrogen bonds between bases.

Step-by-Step Solution:

Verification / Alternative check:
Thermal denaturation disrupts hydrogen bonds, separating strands; the process is reversible upon cooling, consistent with non-covalent pairing.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing backbone phosphodiester bonds (within a strand) with base pairing (between strands); overlooking the contribution of stacking as auxiliary stability.

Final Answer:
Hydrogen bonds between complementary base pairs.