Unraveling DNA tori under tension.

Motivated by recent experiments, we develop a model for DNA toroids under external tension. We find that tori are the equilibrium states for our model up to a critical tension, above which they become only metastable. Above this tension, we find a cascade of transitions between discrete toroid states that successively lower the winding number, until the ground state (rod) is reached. In this process, this model predicts a nearly constant force plateau as a function of extension, in agreement with experiment.