Organizing The Spindle Apparatus
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. In the search-and-capture model, the spindle is predominantly organized by the poleward separation of microtubule organizing centers (MTOCs). Spindle microtubules emanate from MTOCs 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. Although these may be viewed as 'alternative' models, both phenomena likely contribute to the organization of the mitotic spindle.
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