A double-stranded DNA sample has 50% G–C content. Approximately at what temperature will it melt (denature) under standard laboratory ionic conditions?

Introduction / Context:
Melting temperature (Tm) is the midpoint of duplex denaturation where half of the base pairs are disrupted. Approximate formulas relate Tm to base composition and salt concentration for long DNA molecules.

Given Data / Assumptions:

• Double-stranded DNA with 50% G–C content.
• Moderate salt typical of standard buffers.
• Long polymer behavior (not short oligos which use nearest-neighbor rules).

Concept / Approach:
A commonly used empirical relationship for long DNA is Tm ≈ 69 + 0.41*(%GC) under moderate ionic strength. Substituting %GC = 50 gives a value close to 90 °C, consistent with many textbook approximations.

Step-by-Step Solution:

Verification / Alternative check:
UV absorbance melting curves for genomic DNA with 50% GC typically center around 88–92 °C depending on salt, aligning with 90 °C as an acceptable estimate.

Why Other Options Are Wrong:

• 60 or 70 °C: too low for GC50% long DNA at normal salt.
• 100 °C: overly high; most DNA melts below boiling under lab conditions.
• 120 °C: exceeds water’s boiling point at 1 atm and is not a typical Tm.

Common Pitfalls:
Applying short-oligo rules to long polymers or ignoring salt effects that shift Tm modestly but do not overturn the estimate.

Final Answer:
90 °C