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|Chapter Five: Cell Reproduction|
Mitosis is the process by which the cell gives identical copies of its DNA to each of the daughter cells. Although the process does not start and stop into different stages, biologists have separated it into four phases in which major changes occur. These phases are prophase, metaphase, anaphase, and telophase.
During prophase, the chromatin mass in the nucleus condenses into sister chromatid structures. Both the nucleolus and the nuclear membrane disappear during prophase. Also, two structures called the centrioles (which were previously adjacent to one another near the nucleus) begin to move toward opposite ends of the cell. As they do this, a network of fibers begins to form between them. These fibers are called the spindle fibers, and they become very important later on.
When prophase ends, the centrioles are at opposite ends of the cell and the chromatin has fully condensed. The cell is now said to enter metaphase, during which the spindle fibers attach to the centromeres (or rather a part of the centromere called the kinetochore) and pull the sister chromatids to the center of the cell. Metaphase ends when all of the chromatid pairs are aligned in the center.
During the next phase, called anaphase, the centromeres of all of the sister chromatid pairs break simultaneously. Once separated, each chromatid is referred to as a chromosome. The chromosomes from each pair are tugged toward opposite ends of the cell by the spindle fibers still attached to it. At the end of anaphase, the chromosomes are completely at opposite ends of the cell.
The final stage of mitosis is telophase. The spindle fibers, no longer needed to direct the motion of the chromosomes, break apart. New membranes begin to form around each of the two sets of chromosomes, which begin to disperse back into the mass of chromatin. Nucleoli reappear around each set as well, and a second centriole is formed next to each of the existing in preparation for the next division. Finally, cytokinesis (the division of the cytoplasm, which is discussed in the next section) begins to occur.