The DNA double helix, although stabilised by hydrogen bonding, can be denatured by heat. This occurs at characteristic temperature called denaturation temperature or melting temperature T_m.

T_m is defined as the temperature at which the helical structure of DNA is lost.

Since G—C base pairs are more stable than A-T base pairs, the T_m is greater for DNAs with higher G—C content. Thus, the T_m is 650°C → 65°C for 35% G—C content, while it is 700°C → 70°C for 50% G—C content. Formamide destabilises hydrogen bonds and, therefore, lowers T_m.