DNA contains flat ring structures called bases. These bases absorb ultraviolet light very well, especially around 260 nanometers. In a single strand of DNA, these bases float freely and absorb a lot of light.
However, when two strands join to form a double helix, the bases stack neatly like coins. This tight stacking forces the electrons in neighboring bases to interact closely with each other. These close electronic interactions change how the molecules respond to incoming photons.
As a direct result, the stacked bases become worse at absorbing ultraviolet light. A folded double-stranded DNA molecule absorbs roughly 30 to 40 percent less light than separated strands. Chemists use this specific light-fading behavior to study DNA melting in the lab.
When you heat DNA, the double helix eventually breaks apart into two single strands. As the strands separate, the stacked bases unravel and face the open water. The absorbance immediately shoots back up to its original high level. This reverse process of gaining absorbance is known as the hyperchromic effect.