Biology
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Mitosis is basically (in science layman terms)creating two whole CELLS out of one. When a new chromosome duplicates out of an existing one, it will IMMEDIATELY need somewhere to go, so it can get in charge and start coding for polypeptides(genes) in that cell, that's you need to divide exactly during mitosis. Having extra chromosomes in one cell may also lead to genetic dieases, for example Down's syndrome
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If DNA is messed up in the slightest way it would critically effect how cells make proteins.
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Because you'll either turn into Superman or a Zombie.