Daniels Bryan C, Forth Scott, Sheinin Maxim Y, Wang Michelle D, Sethna James P
Department of Physics, Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, New York 14853, USA.
Phys Rev E Stat Nonlin Soft Matter Phys. 2009 Oct;80(4 Pt 1):040901. doi: 10.1103/PhysRevE.80.040901. Epub 2009 Oct 15.
While slowly turning the ends of a single molecule of DNA at constant applied force, a discontinuity was recently observed at the supercoiling transition when a small plectoneme is suddenly formed. This can be understood as an abrupt transition into a state in which stretched and plectonemic DNA coexist. We argue that there should be discontinuities in both the extension and the torque at the transition and provide experimental evidence for both. To predict the sizes of these discontinuities and how they change with the overall length of DNA, we organize a phenomenological theory for the coexisting plectonemic state in terms of four parameters. We also test supercoiling theories, including our own elastic rod simulation, finding discrepancies with experiment that can be understood in terms of the four coexisting state parameters.
在恒定外力作用下缓慢转动单个DNA分子的两端时，最近观察到在超螺旋转变处出现了不连续性，此时突然形成了一个小的螺旋结构。这可以理解为突然转变到一种拉伸态和螺旋态DNA共存的状态。我们认为在转变处延伸和扭矩都应该存在不连续性，并为此提供了实验证据。为了预测这些不连续性的大小以及它们如何随DNA的总长度变化，我们根据四个参数构建了一个关于共存螺旋态的唯象理论。我们还测试了超螺旋理论，包括我们自己的弹性杆模拟，发现与实验存在差异，这些差异可以根据四个共存态参数来理解。
