DNA thermal stability: how do ionic strength and composition influence denaturation? Select the true statement regarding the melting behavior of the DNA duplex.

Introduction / Context:
DNA melting (denaturation) is the disruption of base pairing and stacking, producing single strands. The melting temperature (Tm) depends on sequence composition and solution conditions, especially ionic strength.

Given Data / Assumptions:

• DNA is a polyanion due to its phosphate backbone.
• Cations in solution shield electrostatic repulsion between strands.
• G–C pairs form three hydrogen bonds and often stronger stacking than A–T pairs.

Concept / Approach:
Increasing salt concentration screens negative charges on the backbone, reducing interstrand repulsion and stabilizing the helix, which raises Tm. Higher G–C content typically increases Tm. Hydrophobic base stacking contributes significantly to stability; it cannot be ignored.

Step-by-Step Solution:
Consider electrostatics: more salt → better charge screening → higher duplex stability → higher Tm.Consider composition: more G–C content → higher Tm due to three H-bonds and stacking.Reject statements that deny stacking contributions or sequence dependence.Choose the statement consistent with ionic strength increasing stability.

Verification / Alternative check:
Empirical formulas (for example, Tm ≈ 81.5 + 16.6 log[Na+] + 0.41(%G+C) − terms for mismatches) reflect positive dependence on salt and G–C content.

Why Other Options Are Wrong:
Claiming no dependence on stacking or G–C content contradicts established thermodynamics. Saying stability decreases with salt is opposite of observed behavior (within typical laboratory ranges). Tm is affected by pH and organic solvents.

Common Pitfalls:
Extrapolating beyond physiological ranges; extremely high salt or denaturants can introduce different effects, but the canonical trend in standard buffers is as stated.

Final Answer:
Increases with increasing salt concentration (ionic strength).