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DNA overwinds when stretched

Nature volume 442, pages 836–839 (2006)Cite this article

Abstract

DNA is often modelled as an isotropic rod1,2,3,4, but its chiral structure suggests the possible importance of anisotropic mechanical properties, including coupling between twisting and stretching degrees of freedom. Simple physical intuition predicts that DNA should unwind under tension, as it is pulled towards a denatured structure4,5,6,7,8. We used rotor bead tracking to directly measure twist–stretch coupling in single DNA molecules. Here we show that for small distortions, contrary to intuition, DNA overwinds under tension, reaching a maximum twist at a tension of ∼30 pN. As tension is increased above this critical value, the DNA begins to unwind. The observed twist–stretch coupling predicts that DNA should also lengthen when overwound under constant tension, an effect that we quantitatively confirm. We present a simple model that explains these unusual mechanical properties, and also suggests a possible origin for the anomalously large torsional rigidity of DNA. Our results have implications for the action of DNA-binding proteins that must stretch and twist DNA to compensate for variability in the lengths of their binding sites9,10,11. The requisite coupled DNA distortions are favoured by the intrinsic mechanical properties of the double helix reported here.

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Figure 1: DNA overwinds when stretched.
Figure 2: DNA extends when overwound under constant tension.
Figure 3: Toy mechanical model of DNA.

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Acknowledgements

We dedicate this work to our friend and colleague N.R. Cozzarelli, who passed away during completion of this research. We thank S. Hong, D. Humphries and M. D. Stone for help with the experiments described in Fig. 2, and S. Smith and A. Spakowitz for discussions. This work was supported by an NIH grant to C.B., the Fannie and John Hertz Foundation (J.G.), and the US Department of Energy.

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Author notes

  1. Jeff Gore

    Present address: Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts, 02139, USA

  2. Zev Bryant

    Present address: Department of Biochemistry, Stanford University School of Medicine, Stanford, California, 94305, USA

  3. Nicholas R. Cozzarelli: ‡Deceased

Authors and Affiliations

  1. Department of Physics,

    Jeff Gore & Carlos Bustamante

  2. Department of Molecular and Cell Biology,

    Zev Bryant, Marcelo Nöllmann, Mai U. Le, Nicholas R. Cozzarelli & Carlos Bustamante

  3. Howard Hughes Medical Institute, University of California, Berkeley, 94720, California, USA

    Carlos Bustamante

  4. Physical Biosciences Division, Lawrence Berkeley National Laboratory, California, 94720, Berkeley, USA

    Zev Bryant & Carlos Bustamante

Authors
  1. Jeff Gore
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Correspondence to Carlos Bustamante.

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Supplementary information

Supplementary Information

This file contains a description of the mechanical properties of the toy DNA model, and four references. (DOC 59 kb)

Supplementary Figure

Toy mechanical model of DNA. (PDF 79 kb)

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Gore, J., Bryant, Z., Nöllmann, M. et al. DNA overwinds when stretched. Nature 442, 836–839 (2006). https://doi.org/10.1038/nature04974

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