SUPERCOILED AND BRAIDED DNA UNDER TENSION

Double-helix DNA has a twist persistence length comparable to its bend ing persistence length, and thus its entropic elasticity depends on ho w much it is twisted. For low amounts of twist (less than about one tu rn per twist persistence length) there is linear elasticity for weak f orces, while for large forces there is a threshold at which all chiral ity is expelled from the extended polymer configuration. For larger am ounts of twist, plectonemic supercoiling is stabilized and there cease s to be any extension of the polymer for small forces; large forces cr eate a mixed state of plectonemic supercoil and extended molecule. The free energy of two DNAs braided around one another is computed: to in sert more than about one braid per persistence length requires expendi ture of many k(B)T per braid. Sufficiently tight braids wrap around th emselves to form plectonemic supercoils: the force-distance behavior o f braids should thus be similar to that of twisted DNA. Finally, the r elevance of the thermodynamics of braiding to the disentanglement of l arge DNAs in living cells is discussed.