Single Molecule Study Of The Torque Of DNA Braids

The torsional properties of DNA influence numerous cellular processes,especially DNA compaction,replication,transcription and protein-DNA binding.The topology and torsion of DNA entanglement are critical to understand DNA organization and the activities of a wide variety of enzymes such as the families of topoisomerases and recombinases.All these indicate that studying of DNA torsion is necessary and important.The torsional properties of DNA are investigated by study of single DNA twisting and double DNAs braiding.The behavior of a single DNA molecule under torsion has been extensively investigated experimentally and theoretically,including the study of torsional torque and the interaction between two strands.There are but a few experiments or models dealing with the braiding of two DNA molecules,especially for the study of two-DNA braiding torques.So far,only Charvin et al.has calculated the dependency between turnnumber and the related torque of two-DNA braiding using Monte Carlo simulation.Here we use single molecule force spectroscopy to obtain the extension-catenation profile of double DNAs at forces ranging from 0.2 to 8 pN.And Maxwell relations are used to deduce the effective torsional modulus of DNA.The experiments data shows that DNA extension has a sharp decrease under all forces when the magnets rotate the first turn and the reduced value increases with the distance between the two DNAs.After the first turn,extension of DNA decreases gradually with increasing turn-number,and force has little influence on the variation tendency.The relative torque of DNA braids calculated by Maxwell relations increases with the turn-number and force,which indicate that the absolute value of torque also increases with increasing force.