Mitosis is what makes the cell divide. If the cells in your body couldn't divide they would just die with nothing to replace them, which would lead to the person dying. The genetic material must be copied because the genetic material is what tells the cell 'what to do'. If a cell had no genetic material it wouldn't do anything.
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Most tissue replication occurs through the process of mitosis. Since two daughter cells are to be formed as the result, all the genetic material must be equally divided among them, so DNA replicates itself.
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Mitosis is for cell reproduction. It is done because more cells are needed for more complicated beings and for multiple beings.