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 similar to1 nN/mum. The force relaxes exponentially after an elongation (l/l(0)) to less than 3X native length, with a relaxation time similar to2 sec. This relaxation time corresponds to an effective viscosity similar to 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 similar to 80 kb chromatin loop domains.