The nuclear division proceeds by the formation of a spindle derived from microtubules , and the replication of the chromosomes along their length to form two chromatids , attached to each other at the centromere . The chromatids contract and become more dense, so that with suitable staining they are visible under the microscope. This constitutes prophase . During metaphase the nuclear membrane disintegrates, and the centromeres become attached to the spindle at the metaphase plate. In telophase a new nuclear membrane forms around each group of daughter chromosomes, which now become less dense and are no longer visible as discrete structures under the microscope.
Mitosis normally takes one to three hours to complete.
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