Double-stranded DNA can be unwound (denatured) by applying heat to break the hydrogen bonds holding the bases together. The denatured DNA exhibits stronger UV absorption, showing a hyperchromic shift during "melting".

• Q: Which sample of DNA in the diagram has a greater percent of GC pairs?
  • A: The green sample with the higher melting temperature (Tm). The Tm increases as GC content increases, since GC pairs have 3 hydrogen bonds while AT pairs have 2 hydrogen bonds.