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Cells divide bcoz of three reasons: Cells divide for the purposes of reproduction, replacement of lost or dead cells and to promote growth of the structure in which they are located. Only the first of these reasons applies to single-celled organisms, but the others are essential to the efficient functioning of a multicellular body. 
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Cells divide as a way of reproducing or growing or to create sex cells.Cells can only reach a certain size due to a lack of balanced growth between their parts, so they have to divide once they reach a certain point. Sex cells, such as eggs and sperm, are only created through specialized cell division. 
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