<Tr> <Th> chromosome number (ploidy) </Th> <Td> meiosis </Td> <Td> produces four genetically unique cells, each with half the number of chromosomes as in the parent </Td> </Tr> <Tr> <Td> mitosis </Td> <Td> produces two genetically identical cells, each with the same number of chromosomes as in the parent </Td> </Tr> <P> Meiosis begins with a diploid cell, which contains two copies of each chromosome, termed homologs . First, the cell undergoes DNA replication, so each homolog now consists of two identical sister chromatids . Then each set of homologs pair with each other and exchange DNA by homologous recombination leading to physical connections (crossovers) between the homologs . In the first meiotic division, the homologs are segregated to separate daughter cells by the spindle apparatus . The cells then proceed to a second division without an intervening round of DNA replication . The sister chromatids are segregated to separate daughter cells to produce a total of four haploid cells . Female animals employ a slight variation on this pattern and produce one large ovum and two small polar bodies . Because of recombination, an individual chromatid can consist of a new combination of maternal and paternal DNA, resulting in offspring that are genetically distinct from either parent . Furthermore, an individual gamete can include an assortment of maternal, paternal, and recombinant chromatids . This genetic diversity resulting from sexual reproduction contributes to the variation in traits upon which natural selection can act . </P> <P> Meiosis uses many of the same mechanisms as mitosis, the type of cell division used by eukaryotes to divide one cell into two identical daughter cells . In some plants, fungi, and protists meiosis results in the formation of spores: haploid cells that can divide vegetatively without undergoing fertilization . Some eukaryotes, like bdelloid rotifers, do not have the ability to carry out meiosis and have acquired the ability to reproduce by parthenogenesis . </P>

When do cells undergo a round of dna replication in meiosis
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