  During cell division, sister chromatids are segregated by the mitotic
spindle, a bipolar assembly of interdigitating antiparallel polar filaments
called microtubules. Establishing a stable overlap region is essential for
maintenance of bipolarity, but the underlying mechanisms are poorly understood.
Using a particle-based stochastic model, we find that the interplay of motors
and passive cross linkers can robustly generate partial overlaps between
antiparallel filaments. Our analysis shows that motors reduce the overlap in a
length-dependent manner, whereas passive cross linkers increase it
independently of the length. In addition to maintaining structural integrity,
passive cross linkers can thus also have a dynamic role for size regulation.
