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Fig. 1. The process of mitotic spindle formation. (A) Newt lung cells fixed and stained for microtubules (green) and DNA (blue); images reprinted from Current Biology, 1999, 9:R193 with permission from Elsevier (left image) and from Cell Motility & Cytoskeleton, 1999, 43(3) with permission from Wiley (right image). The transition from two arrays of astral microtubules organized by centrosomes (left) to typical fusiform metaphase spindle (right) requires the establishment of connections between chromosomes and spindle poles via dynamic microtubules. (B,C) Search-and-capture models of chromosome congression. In the original formulation of search-and-capture (B), the space around passive chromosomes is probed by growing and shrinking microtubules (red) nucleated from centrosomes (centrioles in green). Successive capture events (1) on sister kinetochores result in bi-orientation and congression (2). Additional mechanisms of search- and-capture (C) initiated by the chromosomes. K-fibers nucleated at kinetochores via a RanGTP gradient (blue) elongate toward the periphery and capture astral microtubules (3). They are subsequently incorporated into the pole by dynein-based transport. Mono-oriented chromosomes can also use K-fibers of other, bi-oriented, chromosomes resulting in congression before bi-orientation (4).
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