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Science 17 October 2008:
Vol. 322. no. 5900, pp. 446 - 449
DOI: 10.1126/science.1158881
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Reports

Remeasuring the Double Helix

Rebecca S. Mathew-Fenn,1,2* Rhiju Das,2,3*{dagger} Pehr A. B. Harbury1,2{ddagger}

DNA is thought to behave as a stiff elastic rod with respect to the ubiquitous mechanical deformations inherent to its biology. To test this model at short DNA lengths, we measured the mean and variance of end-to-end length for a series of DNA double helices in solution, using small-angle x-ray scattering interference between gold nanocrystal labels. In the absence of applied tension, DNA is at least one order of magnitude softer than measured by single-molecule stretching experiments. Further, the data rule out the conventional elastic rod model. The variance in end-to-end length follows a quadratic dependence on the number of base pairs rather than the expected linear dependence, indicating that DNA stretching is cooperative over more than two turns of the DNA double helix. Our observations support the idea of long-range allosteric communication through DNA structure.

1 Biophysics Program, Stanford University, Stanford, CA 94305, USA.
2 Department of Biochemistry, Stanford University, Stanford, CA 94305, USA.
3 Department of Physics, Stanford University, Stanford, CA 94305, USA.

* These authors contributed equally to this work.

{dagger} Present address: Department of Biochemistry, University of Washington, Seattle, WA 98195, USA.

{ddagger} To whom correspondence should be addressed. E-mail: harbury{at}cmgm.stanford.edu

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