Dynamics of the expanding DNA nucleoid released from a bacterial cell

A semiquantitative theoretical discussion is given of a DNA nucleoid escaping from a bacterial cell after lysis. There is a strong excluded-volume effect causing the nucleoid to expand, though it may be held together globally by various constraints. It is argued that the friction is dominated by the fairly local motion of superhelical segments. In an intermediate regime, hydrodynamic screening is strong. In a diffusion model a self-similar solution is presented of the non-linear diffusion equation. An affinely deforming model for the expanding nucleoid is also given. The predicted rate of expansion seems to be in qualitative accord with recent, preliminary experiments on lysed Escherichia coli viewed under the microscope.