  The chiral nature of DNA plays a crucial role in cellular processes. Here we
use magnetic tweezers to explore one of the signatures of this chirality, the
coupling between stretch and twist deformations. We show that the extension of
a stretched DNA molecule increases linearly by 0.42 nm per excess turn applied
to the double helix. This result contradicts the intuition that DNA should
lengthen as it is unwound and get shorter with overwinding. We then present
numerical results of energy minimizations of torsionally restrained DNA that
display a behaviour similar to the experimental data and shed light on the
molecular details of this surprising effect.
