<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> <P> In this model, microtubule organizing centers are localized to the cell poles, their separation driven by microtubule polymerization and' sliding' of antiparallel spindle microtubules with respect to one another at the spindle midzone mediated by bipolar, plus - end - directed kinesins . Such sliding forces may account not only for spindle pole separation early in mitosis, but also spindle elongation during late anaphase . </P> <P> In contrast to the search - and - capture mechanism in which centrosomes largely dictate the organization of the mitotic spindle, this model proposes that microtubules are nucleated acentrosomally near chromosomes and spontaneously assemble into anti-parallel bundles and adopt a spindle - like structure . Classic experiments by Heald and Karsenti show that functional mitotic spindles and nuclei form around DNA - coated beads incubated in Xenopus egg extracts and that bipolar arrays of microtubules are formed in the absence of centrosomes and kinetochores . Indeed, it has also been shown that laser ablation of centrosomes in vertebrate cells inhibits neither spindle assembly nor chromosome segregation . Under this scheme, the shape and size of the mitotic spindle are a function of the biophysical properties of the cross-linking motor proteins . </P>

Which phase attaches spindle fibers to the chromosomes at the middle of the cell