Formation of regular toroids has been assumed during DNA condensation in theoretical modeling and experimental interpretations,but not vigorously tested.In this work,DNA condensation is followed by Brownian dynamics simulation and magnetic tweezers experiment.The simulations reveal that torus formed in DNA condensation does not follow a circular helical wrapping path and the complex DNA wrapping causes complicated relations between DNA extension and backbone rotation.Stretching condensed DNA under high forces discloses that the unfolding process follows different pathways at different forces due to the force-dependent deformation of torus structures.It leads to a surprising non-monotonic dependence of unfolding rate on force,resembling the catch-bond behavior as reported in protein unfolding field.These novel results are consistent with experimental results using free orbiting magnetic tweezers by simultaneously analyzing DNA extension and backbone rotation.Overall,these findings provide so far the most complete description of multivalent cation dependent DNA condensation and decondensation under tension.