  Minuscule molecular forces can transform DNA into a structure that is
elongated by more than half its original length. We demonstrate that this
pronounced conformational transition is of relevance to ongoing experimental
and theoretical efforts to characterize the conducting properties of DNA wires.
We present quantum mechanical calculations for acidic, dry, poly(CG).poly(CG)
DNA which has undergone elongation of up to 90 % relative to its natural
length, along with a method for visualizing the effects of stretching on the
electronic eigenstates. We find that overstretching leads to a drastic drop of
the hopping matrix elements between localized occupied electronic states
suggesting a dramatic decrease in the conductivity through holes.
