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Meiosis

Meiosis. Meiosis is referred to as reduction division because meiosis results in haploid cells called gametes (sex cells). Male and female gametes fuse during fertilization (syngamy) to form a zygote. Meiosis may be thought of as a “double mitosis.”

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Meiosis

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  1. Meiosis

  2. Meiosis is referred to as reduction division because meiosis results in haploid cells called gametes (sex cells). Male and female gametes fuse duringfertilization (syngamy) to form a zygote.

  3. Meiosis may be thought of as a “double mitosis.” • Characteristics of meiotic phases are similar to the characteristics of mitotic phases. • If you know what happens during mitosis, then you know most of what happens during meiosis (with a few differences).

  4. Meiosis occurs in two parts:Meiosis I and then Meiosis II. Meiosis I –(Starts with a diploid cell-2N) Phases: prophase I, metaphase I, anaphase I, and telophase I, then cytokinesis which results in two haploid cells. Modern Biology (Holt)

  5. PROPHASE I • During Prophase I pairs of homologous chromosomes called tetradsalign themselves closely and exchanges pieces. • When chromosomes of the tetrad are near enough next to each other, it is called synapsis.

  6. During Prophase I, as chromosomes of a tetrad are in synapsis, and exchange of genes can occur. • This exchanges of genetic material is called “crossing-over.” • The point at which crossing-over occurs is referred to as a chiasmata Modern Biology (Holt)

  7. What is the advantage of crossing-over? Crossing-over increases genetic variety in organisms, by “shuffling” the genes for different combinations.

  8. Meiosis I then proceeds through metaphase I, and then anaphase I. • All phases are similar to the mitotic phases. (spindle formation, tetrads move toward center, tetrads line-up along the center, tetrads separate and chromosomes move to opposite poles) • Cytokinesis occurs and the 2N cell has now become 2 haploid daughter cells. Modern Biology (Holt)

  9. Two Haploid cells result These cells go directly into prophase II --- the first phase of Meiosis II. Modern Biology (Holt)

  10. Meiosis I leads to Meiosis II • Meiosis I leads directly to Meiosis II, with no interphase in between. • During meiosis II phases are similar to mitosis phases: (chromatid pairs move toward the center, sister chromatids separate and then cytokinesis) Modern Biology (Holt)

  11. Finally – 4 haploid cells result Modern Biology (Holt)

  12. If the gametes are to be male, then 4 individual sperm cells form. Modern Biology (Holt) The process of forming sperm cells is called spermato-genesis

  13. If the gametes are to be female, then the result is 1 egg cell and 3 polar bodies. The process of producing the egg cell is called oogenesis. The egg cell gets most of the cytoplasm and the much smaller polar bodies disintegrate. Egg cell – Ovum Egg cells - Ova Modern Biology (Holt)

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