Probing Chromosome Structure with Dynamic Force Relaxation

We report measurements of the dynamics of force relaxation in single mitotic chromosomes, following step strains applied with micropipettes of force constant $\sim 1\mathrm{nN}/\mu \mathrm{m}$. The force relaxes exponentially after an elongation $\left({l/l}_0\right)$ to less than $3\times$ native length, with a relaxation time $\sim 2\sec$. This relaxation time corresponds to an effective viscosity $\sim {10}^5$ times that of water. We experimentally rule out solvent flow into the chromosome as the mechanism for the relaxation time. Instead, the relaxation can be explained in terms of the disentanglement dynamics of $\sim 80\mathrm{kb}$ chromatin loop domains.