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The easy answer is that is maintains the number of chromosomes that is unique to a species generation after generation.
Meiosis is the process that creates sex cells within an organism. During meiosis, DNA replicates only once. However, there are two rounds of division within meiosis. This differs from mitosis (the form of cellular division that creates somatic, or body, cells within an organism). Mitosis undergoes one replication of DNA and one division of that genetic material.
The extra round of division results in sex cells receiving only half the number of chromosomes that somatic cells for that species contain. This is important because, upon sexual reproduction and the fertilization of an egg by a sperm, the number of chromosomes for that species is restored.
For example, humans have 46 chromosomes, which double to 92 chromosomes prior to cellular division. The 92 chromosomes divide once to 46 chromosomes during Meiosis I (the first division of meiosis). The chromosome number is reduced to 23 chromosomes during Meiosis II (the second round of meiosis).
During fertilization, 23 chromosomes within a sperm combine with 23 chromosomes from an egg. This restores the 46 chromosomes generation after generation.
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