Which nucleic acid sample would require a higher temperature to denature (melt) under comparable ionic conditions?

Introduction / Context:
Thermal denaturation (melting) breaks hydrogen bonds and base-stacking interactions in nucleic acids. The base composition significantly influences the melting temperature (Tm).

Given Data / Assumptions:

• We compare samples under similar salt and pH conditions.
• Double-stranded polymers exhibit cooperative melting behavior.
• G–C pairs have three hydrogen bonds; A–T (or A–U) pairs have two.

Concept / Approach:
Higher G–C content strengthens duplex stability through extra hydrogen bonds and stronger stacking, raising Tm. While RNA duplexes can be very stable, the question asks for the general sample that requires higher temperature among typical choices and conditions.

Step-by-Step Solution:

Verification / Alternative check:
Empirical formulas (e.g., Tm ≈ 69 + 0.41*%GC for long DNA at moderate salt) predict larger Tm for GC-rich DNA, matching experimental melting curves.

Why Other Options Are Wrong:

• High A–T DNA: fewer hydrogen bonds and weaker stacking lower Tm.
• High G–C RNA: can be stable, but the canonical, straightforward answer in comparative DNA contexts is GC-rich DNA.
• High A–U RNA: generally lower Tm than GC-rich duplexes.
• Single-stranded DNA: lacks duplex to melt cooperatively.

Common Pitfalls:
Equating base-pair count only with hydrogen bonds and forgetting base stacking; ignoring salt effects that shift absolute Tm but not the ordering.

Final Answer:
Double-stranded DNA with high G–C content