Condensin I but not Condensin II is crucial for mitotic chromosome mechanics

During mitotic cell division, pliable interphase chromatin is transformed into stiff mitotic chromosomes able to withstand the pushing and pulling forces of the mitotic spindle. How the cell establishes this chromosome stiffness and the cellular consequences if this stiffness is disrupted, is unclear. Condensin complexes drive many of the structural changes in mitotic chromosomes. Here, we combine rapid protein depletion of Condensins I and II with live cell imaging and mechanical characterization of purified mitotic chromosomes to probe their role in mitotic chromosome mechanics. We show that Condensin I, but not Condensin II, is required to establish chromosome stiffness and chromatin elasticity, and yet is not required for maintaining these properties after chromosome formation. Nevertheless, metaphase depletion of Condensin I still leads to severe sister centromere cohesion defects. We propose that the chromatin loop network established by Condensin I is locked in place by an additional ‘crosslinking’ factor.