ochores on each replicated chromosome (one on each chromatid), and when the replicated chromosome splits apart at its centromere at the onset of anaphase, each daughter chromosome possesses one centromere and one kinetochore. The linkages between kinetochores and microtubules are thought to be central in controlling both the positioning of the replicated chromosome at the central portion of the spindle during the alignment phase, and in moving the daughter chromosomes apart after they split at their centromeres. The separation of daughter cells from each other is a process known as cytokinesis, and is separate from mitosis. In cytokinesis, animal and plant cells differ considerably from each other. These differences are the consequence of having or not having a cell wall. During prophase, the replicated chromosomes undergo extensive condensation. The chromosomes are greatly thickened and shortened but are still contained within the nuclear envelope. Late in prophase, within some minutes of the nuclear envelope breakdown, the mitotic spindle begins to grow, and two zones become visible, with one on each side of the nucleus. With continued spindle expansion, the nucleus quickly becomes compressed and appears circular. Prophase ends with the sudden dispersion of the nuclear envelope, and the chromozone mass is no longer occupying a discrete, circular shape zone in the cell. Once the nuclear envelope has broken down, the spindle microtubules and the chromosomes are no longer separated by a membrane limit. The microtubules begin to interact with the chromosomes, and the chromosomes undergo a progressional movement, where they end up with their centromeres all situated in middle of the spindle, at a site known as the metaphase plate. Each kinetochore of the replicated chromosome is pointed toward one side of the spindle; later, in anaphase, each kinetochore moves to one of the two spindle pole regions as the daughter chromosome. The progress...
