<P> Opposing the action of these microtubule - stabilizing proteins are a number of microtubule - depolymerizing factors which permit the dynamic remodeling of the mitotic spindle to promote chromosome congression and attainment of bipolarity . The kinesin - 13 superfamily of MAPs contains a class of plus - end - directed motor proteins with associated microtubule depolymerization activity including the well - studied mammalian MCAK and Xenopus XKCM1 . MCAK localizes to the growing tips of microtubules at kinetochores where it can trigger catastrophe in direct competition with stabilizing + TIP activity . These proteins harness the energy of ATP hydrolysis to induce destabilizing conformational changes in protofilament structure that cause kinesin release and microtubule depolymerization . Loss of their activity results in numerous mitotic defects . Additional microtubule destabilizing proteins include Op18 / stathmin and katanin which have roles in remodeling the mitotic spindle as well as promoting chromosome segregation during anaphase . </P> <P> The activities of these MAPs are carefully regulated to maintain proper microtubule dynamics during spindle assembly, with many of these proteins serving as Aurora and Polo - like kinase substrates . </P> <P> In a properly formed mitotic spindle, bi-oriented chromosomes are aligned along the equator of the cell with spindle microtubules oriented roughly perpendicular to the chromosomes, their plus - ends embedded in kinetochores and their minus - ends anchored at the cell poles . The precise orientation of this complex is required to ensure accurate chromosome segregation and to specify the cell division plane . However, it remains unclear how the spindle becomes organized . Two models predominate the field, which are synergistic and not mutually exclusive . In the search - and - capture model, the spindle is predominantly organized by the poleward separation of centrosomal microtubule organizing centers (MTOCs). Spindle microtubules emanate from centrosomes and' seek' out kinetochores; when they bind a kinetochore they become stabilized and exert tension on the chromosomes . In an alternative self assembly model, microtubules undergo acentrosomal nucleation among the condensed chromosomes . Constrained by cellular dimensions, lateral associations with antiparallel microtubules via motor proteins, and end - on attachments to kinetochores, microtubules naturally adopt a spindle - like structure with chromosomes aligned along the cell equator . </P> <P> In this model, microtubules are nucleated at microtubule organizing centers and undergo rapid growth and catastrophe to' search' the cytoplasm for kinetochores . Once they bind a kinetochore, they are stabilized and their dynamics are reduced . The newly mono - oriented chromosome oscillates in space near the pole to which it is attached until a microtubule from the opposite pole binds the sister kinetochore . This second attachment further stabilizes kinetochore attachment to the mitotic spindle . Gradually, the bi-oriented chromosome is pulled towards the center of the cell until microtubule tension is balanced on both sides of the centromere; the congressed chromosome then oscillates at the metaphase plate until anaphase onset releases cohesion of the sister chromatids . </P>

The spindle of mitosis is responsible for separating sister chromatids. this spindle is made up of