Because in prophase 1 of meiosis, the chromosomes synapse which results in chromosomes having each other genes. This is called cross-over. Also in metaphase 1 of meiosis, the chromosomes line up randomly at the equator of the cell, making them all different from one another. This is called independent assortment. Mitosis however, do not do this and once it replicates the DNA in S phase of interphase, it remains the same through out the whole process of mitosis.

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