FIG. 1: Experimental measurements of the (a) looped and (b) unlooped cumulative probability distributions for various noise conditions (right to left: T$_{α}$/T = 0 , 0 . 01 , 0 . 05 , 0 . 12) at a mean tension of 153 fN. The solid lines are exponential (biexponential) fits to the looped (unlooped) data. The insert to (a) shows a typical raw trace of the DNA extension vs. time. in the figure, the lifetime of the looped state is independent of the fluctuations. This is consistent with previous findings that the looped state is insensitive to femtonewton forces [7]. The lifetime of the unlooped state, however, is clearly seen to decrease as we increase the fluctuations. At 2 σ , approximately 95% of the noise distribution is accounted for. Since volume exclusion forces are on the order of 35 fN [9], σ = 60 fN or T$_{α}$/T = 0 . 12 are the largest fluctuations we can apply to the DNA without significantly clipping the distribution. We fit the cumulative probability distributions using the kinetic scheme detailed in [7] to extract loop dissociation and association lifetimes. In summary, the looped lifetimes are simply fit by a single exponential function parametrized by the looped lifetime τ$_{L}$ : S(t)=1 − exp( − t/τ$_{L}$ ). The unlooped kinetics, however, are more complicated, and may accurately be described by collecting all time intervals beginning with an unlooping event and ending 4