DNA thermal stability — If a cell’s DNA has Tm = 40 °C at 20% GC content, what happens to Tm if GC% rises to 60% (qualitative trend)?

Difficulty: Easy

Correct Answer: Increases

Explanation:


Introduction:
The melting temperature (Tm) of DNA is the point at which half of the duplex molecules are denatured. It depends strongly on base composition because GC pairs have three hydrogen bonds and stronger stacking than AT pairs. This question asks for the qualitative direction of Tm change when GC% increases markedly.


Given Data / Assumptions:

  • Initial Tm is 40 °C at 20% GC.
  • GC% is increased to 60% (substantially higher).
  • We are asked for the trend, not an exact recalculated temperature.


Concept / Approach:
Higher GC content typically raises Tm. A common approximation for short DNAs is Tm ≈ 2 °C * (A + T) + 4 °C * (G + C). While exact values depend on length, salt, and sequence context, the direction is unambiguous: more GC → higher Tm.


Step-by-Step Solution:

1) Recognize that GC base pairs contribute greater stability via three hydrogen bonds and stacking.2) Increasing GC from 20% to 60% increases the stabilizing fraction.3) Therefore, the temperature needed to melt half the duplex (Tm) increases.


Verification / Alternative check:
Empirical nearest-neighbor models show positive correlation between GC fraction and Tm across ionic strengths; calibration curves confirm the trend.


Why Other Options Are Wrong:

a) Stability changes with composition; Tm is not constant.c) More GC does not lower Tm.d) Although exact values require conditions, the trend is clear.e) Tm is a thermodynamic property, not a cyclic variable.


Common Pitfalls:
Ignoring salt concentration and length when computing exact Tm; here only the qualitative direction is required.


Final Answer:
Increases.

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