DNA becomes stiff by turning
Researchers from Delft University of Technology and FOM have developed a new instrument, magnetic torque tweezers, to measure the torsional stiffness of individual DNA molecules. They can use this to study both pure DNA and DNA covered with repair proteins. A better understanding of such material properties of biological molecules is important for the construction of DNA-based nanostructures and for understanding how molecular motors work. The results of the research, carried out with a Veni grant from Jan Lipfert, were published online yesterday by Nature Methods.
The researchers have developed magnetic torque tweezers to better measure the torsional force. Using the tweezers they affix a piece of DNA between a glass plate and a small (~ 1 micrometre) magnetic sphere. The measurement of the torsional force is based on a new algorithm with which both the position and rotation of this sphere are imaged using video microscopy. Moreover, a new magnet set up allows a force and a torsional resistance to be applied to the DNA.
Experiments with the new instrument demonstrate, for example, that the torsional stiffness of DNA is dependent on the force with which the DNA is pulled, and that the torsional stiffness increases if DNA repair proteins bind to the DNA. This observation could provide the basis for an improved understanding of how molecular motors on DNA function.
DNA can acquire a torsional resistance due to its double helix structure: DNA consists of two long strands that grab each other and are linked by base pairs. The two strands rotate around each other like a spiral staircase. When the strands unravel, this torsional force plays an important role. In polymers, unravelling takes place in a manner similar to how a braided rope is unravelled. The many cross-ties (base pairs) of DNA are responsible for the torsional stiffness that is characteristic of DNA. This has important consequences in biology: when molecular motors unravel DNA, for example during DNA copying or repair, the torsional force develops.
For further information please contact:
Dr Jan Lipfert (Department of Bionanoscience, Kavli Institute of NanoScience, Delft University of Technology); +31 (0)15 278 35 52.
Prof. Nynke Dekker (Department of Bionanoscience, Kavli Institute of NanoScience, Delft University of Technology); +31 (0)15 278 32 19.
Jan Lipfert, Jacob W.J. Kerssemakers, Tessa Jager, and Nynke Dekker, 'Magnetic Torque Tweezers: Measuring Torsional Stiffness in DNA and RecA-DNA Filaments', Nature Methods (2010) http://dx.doi.org/10.1038/nmeth.1520