DNA melting temperature (Tm) — Within what approximate temperature range do most double-stranded DNA molecules denature under standard laboratory salt conditions?

Introduction:The melting temperature (Tm) of DNA is the temperature at which half of a population of double-stranded DNA molecules becomes single-stranded. This question tests fundamental knowledge of nucleic acid thermodynamics and typical laboratory conditions used in molecular biology.

Given Data / Assumptions:

• Standard buffer ionic strength (for example, moderate NaCl concentrations commonly used in the lab).
• Typical lengths from a few hundred base pairs to genomic fragments.
• No extreme GC content or unusual solvents unless stated.

Concept / Approach:DNA duplex stability depends on base stacking and hydrogen bonding. GC pairs contribute more to stability than AT pairs. Salt stabilizes the duplex by shielding negative charges on the phosphate backbone. Under common conditions, most DNA exhibits a cooperative melting transition with a midpoint (Tm) typically falling in the upper-middle Celsius range, often around 70–90°C for many practical applications (e.g., PCR denaturation, thermal melting curves).

Step-by-Step Solution:

Verification / Alternative check:Thermal denaturation profiles recorded by UV absorbance at 260 nm (hyperchromic shift) commonly show sigmoidal transitions with midpoints in this range for average GC content. Changing salt or organic solvents shifts Tm accordingly, but the baseline expectation remains 70–90°C for many samples.

Why Other Options Are Wrong:

• 50 to 60°C / 40 to 55°C: too low for stable duplex DNA at typical salt concentrations.
• 60 to 80°C: lower bound can be true for AT-rich or short duplexes, but the upper bound omits many common cases; less representative than 70–90°C.
• 80 to 100°C: upper portion includes boiling; many DNAs melt well before 100°C.

Common Pitfalls:Assuming one fixed Tm for all DNA. Tm shifts with GC content, length, salt, and additives (e.g., formamide lowers Tm). Always interpret ranges as typical, not absolute.

Final Answer:70 to 90°C.