Stretching a double-stranded DNA: Nature of the B -form to the S -form transition

The abrupt extension of the contour length and the self-unwinding of the double helix in the transition from the B-form to S-form of a double-stranded DNA under a stretching force is investigated in the framework of the model with basepair interactions and bending [Phys. Rev. Lett. 22, 4560 (1999)]. In the region where thermal fluctuations can be neglected the classical mechanical approach is employed and equations governing the detail structure of the DNA are derived with some analytical results obtained. The transition from the B-form to S-form can be understood in terms of an effective potential with a barrier separating these two states and resulting in a first-order transition. The double helix of the DNA is almost fully unwound across the transition. Detail structural configurations, such as the loci of the two strands, relative extension, linear extension coefficient, and the threshold stretching force are calculated. The mean torque release as the dsDNA untwist across the transition is also estimated. These results are in agreement with various experimental data.