Before mitosis begins, cells are in the
interphase stage,
which is the rest period between mitotic divisions. At this time, the chromosomes
are not identifiable but are an undifferentiated loose mass of stretched out DNA molecules
within the nucleus. Chromosome replication and other cell processes are occurring in
preparation for mitosis. In addition, cells carry out normal metabolic activities at
this time. Most mammal somatic cells, including those of humans, spend
at least 90% of their time in interphase.
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At the beginning of the first mitotic stage, prophase, the thread-like doubled chromosomes contract and become visible. The two centrioles move to opposite sides of the nucleus. At the same time, the nuclear membrane begins to break down.
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During metaphase, the nuclear membrane disappears and the chromosomes become aligned half way between the centrioles. The centromere of each doubled chromosome becomes attached by thread-like spindle fibers to the centrioles which are at polar opposite sides of the cell.
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During the following anaphase stage, the chromatids of each chromosome separate at their centromeres and then migrate to the opposite poles of the cell.
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During telophase, two distinct nuclear membranes develop encompassing the two identical sets of chromosomes. At the same time, the cytoplasm divides between the two new cells, and the cell membrane begins to pinch off the cell contents into two daughter cells.
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Finally, the new identical cells separate and once again enter interphase in preparation for a later mitotic division. While the chromosomes are no longer visible, they will be replicated just before mitosis begins.
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