| (a) Spindle assembly depends on microtubule self-organization, whereby microtubules form in the vicinity of the sister chromatids and then become organized by motors and other proteins into a bipolar array. This process depends on numerous motor proteins (Xs), including motors that cross-link antiparallel plus ends (black), motors that focus minus ends at the poles (red), and chromatid-associated motors that help orient the array (white). (b) In most animal cells centrosomes facilitate the self-organization process shown in panel (a). Rapidly growing and shrinking microtubules that radiate from the centrosomes search the space between the poles and bind to kinetochores. Many sister-chromatid pairs are initially attached by this search and capture mechanism. Centrosomal microtubules can also capture preformed microtubule bundles and pull them into the poles. Bi-orientation eventually results when a pair of sister kinetochores is connected to both poles. |
