Phil Nelson
Department o f Physics
University of Pennsylvania
 

The DNA in living cells is often described as a passive database of pure information, the genome. In fact, however, the DNA molecule itself actively collaborates in its own packaging and regulation. Unraveling the underlying mechanisms of these crucial processes requires an understanding of the basic mechanical properties of the DNA duplex. But how are we to measure them? Who has a nanometer-size torque wrench? I'll review the status of mechanical experiments on single molecules of DNA. Understanding these experiments requires us to invent and solve a new kind of random walk. I'll show how new experiments give a clean incarnation of this fundamental mathematical problem. Remarkably, the solution to the problem uses a mathematical analogy to a classic problem in quantum mechanics. The resulting theory fits the measured curves of DNA extension versus stretching force and applied overtwist for two different experiments, with a wide range of parameters, and yields a numerical value for the twist stiffness.