Meiosis
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Introduction
Meiosis is a specialized type of cell division that occurs during gametogenesis.
Meiosis consists of two rounds of nuclear division but only one DNA replication.
Meiosis differs from mitosis in that homologous chromosomes pair at prophase of the first division.
Exchanges between these chromosomes (crossing over) happens regularly, producing genetic recombination and resulting in chromosomes that contain both maternal and paternal DNA.
Meiosis I
Meiosis begins with DNA replication, which occirs by the beginning of Prophase I. Recombination occurs at this time at chiasma, the point of chromatid exchange. After homologous chromosomes migrated to opposite poles, Anaphase I occurs, followed by telophase and cytokinesis.
Meiosis II
Meiosis II consisits of the division of the duplicated chromosomes, resulting in haploid daughter cells. Recombinant and nonrecombinant chromosomes can be identified.
During meiosis I or II, on occasion chromosomes can remain joined prior to cytokinesis, leading to nondisjunction.
If this affects X or Y chromosomes, Turner's (XO) or Klinefelter's (XXY) syndromes occurs.
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