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.
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