A striking increase in absorbance of DNA upon denaturation is known as the hyperchromic effect.
The two strands of DNA are bound together mainly by the stacking interactions, hydrogen bonds and hydrophobic effect between the complementary bases.

In their native state, the bases of DNA absorb light in the 260-nm wavelength region.

When the bases become unstacked, the wavelength of maximum absorbance does not change, but the amount absorbed increases by 37%.
A double stranded DNA strand dissociating to two single strands produces a sharp cooperative transition.