Figure 264

Comparison of mitosis and meiosis in an idealized cell with two pairs of chromosomes (2n). The chromosomes of maternal and paternal origin are depicted in red and blue, respectively. The mitotic division produces daughter cells that are genetically identical to the parental cell (2n). The meiotic division, which has two components, a reductional division and an equatorial division, produces a cell that has only two chromosomes (n). In addition, during the chromosome pairing in prophase I of meiosis, chromosome segments are exchanged, leading to further genetic diversity. It should be noted that in humans the first polar body does not divide. Division of the first polar body does occur in some species.

only one member of each chromosome pair, are described as haploid (In). During gametogenesis, reduction in chromosome number to the haploid state (23 chromosomes in humans) occurs through meiosis, a process that involves two successive cell divisions, the second of which is not preceded by an S phase. This reduction is necessary to maintain a constant number of chromosomes in the species. Reduction in chromosome number in the first meiotic division is followed by reduction in DNA content to the haploid {In) amount in the second meiotic division.

During meiosis, the chromosomes pair and may exchange chromosome segments, thus altering the genetic composition of the chromosomes. This genetic exchange, called crossing-over, and the random assortment of each member of the chromosome pairs into haploid gametes give rise to infinite genetic diversity.

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