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".
<LI> 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.