In dividing eukaryotic cells, most microtubules spread out from regions known as centrosomes, or microtubule-organizing centers (MTOCs). When a cell is not undergoing division, a single centrosome is present. However, when a cell does begin to divide, both in mitosis and meiosis, the centrosome replicates early on in the process. The spindle apparatus then begins to form. Each centrosome serves as a center of organization for the microtubules of half the spindle.
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Dividing human cell: The centrosomes are shown as gray circles,
the chromosomes are in blue, the kinetochores in pink, and the
spindle apparatus in green (hairlike strands are microtubules).
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