Heat denaturation of DNA, also called melting, causes the double helix structure to unwind to form single stranded DNA. When DNA in solution is heated above its melting temperature (usually more than 80 oC), the double-stranded DNA unwinds to form single-stranded DNA. The bases become unstacked and can thus absorb more light. 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 30-40%.
Hyperchromicity can be used to track the condition of DNA as temperature changes. The transition/melting temperature (Tm) is the temperature where the absorbance of UV light is 50% between the maximum and minimum, ie where 50% of the DNA is denatured.
Campbell, Mary K. & Farrell, Shawn O. (2006). Biochemistry. Thomson Brooks/Cole.