DNA Melting & Base-Pair Hydrogen Bonds

Nucleic Acid Chemistry

Temperature 25 °C

GC content 50 %

T_m (Wallace rule)

–

Denatured

–

H-bonds intact

–

Base pairs

–

How it works

A=T pairs form 2 hydrogen bonds; G≡C pairs form 3 — GC-rich DNA needs more thermal energy to separate.
T_m (melting temperature) is estimated with the Wallace rule: T_m = 2×(A+T) + 4×(G+C).
Each base pair melts when local temperature exceeds its own threshold (GC pairs run hotter than AT pairs), so the whole-strand transition is a gradual sigmoid, not a single switch.
Cooling below a pair's threshold re-forms its hydrogen bonds — renaturation/annealing, shown by the strands closing back up.

Adenine (A) Thymine (T) Guanine (G) Cytosine (C)

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