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.” 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).

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)

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

This exchanges of genetic material is called “crossing-over.” 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)

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

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)

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

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)

Finally – 4 haploid cells result Modern Biology (Holt)

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

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)