TY - JOUR T1 - Evidence for a Solenoid Phase of Supercoiled DNA JF - bioRxiv DO - 10.1101/302661 SP - 302661 AU - Andrew Dittmore AU - Keir C. Neuman Y1 - 2018/01/01 UR - http://biorxiv.org/content/early/2018/04/16/302661.abstract N2 - In mechanical manipulation experiments, a single DNA molecule overwound at constant force undergoes a discontinuous drop in extension as it buckles and forms a superhelical loop (a plectoneme). Further overwinding the DNA, we observe an unanticipated cascade of highly regular discontinuous extension changes associated with stepwise plectoneme lengthening. This phenomenon is consistent with a model in which the force-extended DNA forms barriers to plectoneme lengthening caused by topological writhe. Furthermore, accounting for writhe in a fluctuating solenoid gives an improved description of the measured force-dependent effective torsional modulus of DNA, providing a reliable formula to estimate DNA torque. Our data and model thus provide context for further measurements and theories that capture the structures and mechanics of supercoiled biopolymers. ER -