(a) Prophase. Duplicated centrosomes migrate around the nucleus. (Centrosomes, consisting of a pair of previously replicated centrioles surrounded by pericentriolar material, nucleate MT assembly and organize spindle poles.) (b) Prometaphase. The nuclear envelope breaks down allowing MTs to move chromosomes to the equator (e) in a process termed congression. (c) Metaphase. Sister chromatids (double arrowheads) face opposite poles (p). MTs are oriented with their plus-ends distal to the poles, and are organized into four sets, namely: astral MTs linking spindle poles to the cell cortex; chromosomal MTs linking chromosome arms to poles; kMTs linking poles to kinetochores; and ipMTs linking the two poles. (d) Anaphase A. Chromatids are moved to opposite poles (segregation). (e) Anaphase B. Pole–pole spacing increases. During late anaphase, the division plane is determined by a mechanism involving spindle–cortex interactions and the cleavage furrow containing a contractile ring assembles from actomyosin II and begins to contract. (f) Telophase/cell–cell scission. Nuclear envelopes reassemble around decondensing segregated sisters. The contractile ring contracts (furrow ingression) developing a barrier between the daughter cells and constricting the spindle mid-zone (the array of ipMTs lying between separated chromatids) into a structure called midbody (the remnant of the mid-zone). During abscission, the furrow “seals” and separates the daughter cells, apparently involving vesicle transport/exocytosis. Reprint from Scholey et al. [294] with permission from Macmillan Publishers Ltd: Nature 422, ©2003.