meiosis

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Meiosis (pronounced /maɪˈoʊsɨs/ ( listen)) is a special type of cell division necessary for sexual reproduction. In animals, meiosis produces gametes like sperm and egg cells, while in other organisms like fungi it generates spores. Meiosis begins with one diploid cell containing two copies of each chromosome—one from the organism's mother and one from its father—and produces four haploid cells containing one copy of each chromosome. Each of the resulting chromosomes in the gamete cells is a unique mixture of maternal and paternal DNA, ensuring that offspring are genetically distinct from either parent. This gives rise to genetic diversity in sexually reproducing populations, which enables them to adapt during the course of evolution. Before meiosis, the cell's chromosomes are duplicated by a round of DNA replication. This leaves the maternal and paternal versions of each chromosome, called homologs, composed of two exact copies called sister chromatids and attached at the centromere region. In the beginning of meiosis, the maternal and paternal homologs pair to each other. Then they typically exchange parts by homologous recombination, leading to crossovers of DNA from the maternal version of the chromosome to the paternal version and vice versa. Spindle fibers bind to the centromeres of each pair of homologs and arrange the pairs at the spindle equator. Then the fibers pull the recombined homologs to opposite poles of the cell. As the chromosomes move away from the ...

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