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Stretching the truth: JILA biophysicists help unravel DNA stretching mystery
Date:1/20/2011

Using a new experimental test structure, biophysicists at JILA have unraveled part of a 15-year mystery in the mechanics of DNAjust how the molecule manages to suddenly extend to almost twice its normal length. The new test structure should support research on DNA elasticity as a standard for tiny forces and help refine studies of how drugs and other substances bind to DNA.

In a new paper in the Journal of the American Chemical Society,* JILA scientists disprove a leading explanation for DNA overstretching, a curious behavior in which the molecule's double helix structure (see image) suddenly extends by 70 percent when subjected to 65 picoNewtons (pN) of force. The exact steps of the process have been controversial since overstretching was discovered in 1996. Contrary to a popular theory, the new JILA work shows that DNA's backbone does not need to have a small gap, often called a nick, or sport loose ends for the dramatic extension to occur at 65 pN.

JILA is a joint institute of the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder.

"Overstretching is a really amazing phenomenon, and even after 15 years, people are still debating it," says senior author Tom Perkins, a NIST expert in the mechanics of single molecules. "DNA stretches and stretches like a rubber band and you think it's going to break, and then all of a sudden it nearly doubles in length. Something wonderfully interesting happens at this transition."

The leading theory is that overstretching introduces so much energy that the DNA melts, with a single strand peeling off from nicks in the backbone or free ends. This model assumes that nicks or ends are essential. Scientists did not test this assumption until now because there was no platform for restraining the ends without also locking the molecule's structure and rotation.

The JILA team's key advance was a clever geometry that binds a looped end of DN
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Contact: Laura Ost
laura.ost@nist.gov
303-497-4880
National Institute of Standards and Technology (NIST)
Source:Eurekalert

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