Spindle

 The mitotic spindle is a bipolar array of microtubules which forms during the prophase of mitosis. Each microtubule has a minus end, which is embedded in the centrosome and a plus end, which points out from the centrosome. During metaphase, the spindle attaches sister chromatids to opposite poles of the cell via kinetochore microtubules. Then during anaphase the kinetochore microtubules separate the sister chromatids to form a pair of daugther chromosomes, which are then pulled towards each pole of the cell. Once the chromososmes have been pulled to the opposite polesof the cell, during cytokinesis the spindle disassembles to reform the interphase configuration of microtubules.

Spindle formation begins at the G2/M checkpoint in the cell cycle, when Cdk activity increases and so increases the phosphoylation of proteins which control assembly of the spindle.