Polymer loop extrusion with motor-motor bypassing can explain mitotic chromosome elasticity

During mitosis, chromosomes are rapidly folded into an array of polymer loops to form mitotic chromosomes. This process is largely governed by condensins, protein motors that collectively and dynamically structure chromosomes. To do this, each condensin performs 'loop extrusion', by binding the chromatin fiber and subsequently reeling it in and extruding it as a loop. Currently, the loop extrusion model explains mesoscale chromosome structure, but not the mechanical properties of chromosomes. We performed polymer simulations to investigate the chromosomal mechanical properties that emerge from loop extrusion. We focused on the consequences of the ability of condensins to bypass each other while extruding, as seen in vitro. As in experiments with isolated chromosomes, condensins contribute to the stiffness of chromosomes in simulations, with differential contributions depending on condensin extrusion dynamics. Simulated chromosomes that are randomly cut, as by restriction enzymes, can maintain their mechanical integrity as observed in experiments, if condensins can bypass each other while extruding loops. Thus, the condensins generate gel-like structure in simulated mitotic chromosomes. The model suggests how different extrusion dynamics can generate specific signatures in the mechanical response and structure of chromosomes.