Meiosis. Meiosis is referred to as reduction division because meiosis results in haploid cells...
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Transcript of Meiosis. Meiosis is referred to as reduction division because meiosis results in haploid cells...
![Page 1: Meiosis. Meiosis is referred to as reduction division because meiosis results in haploid cells called gametes (sex cells). Male and female gametes fuse.](https://reader030.fdocuments.us/reader030/viewer/2022020417/5697c0101a28abf838ccb11d/html5/thumbnails/1.jpg)
Meiosis
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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.
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The zygote divides by mitosis to form an embryo.
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Human embryo develops into a a fetus.
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• 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).
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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.
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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.
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• 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
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What is the advantage of crossing-over?
Crossing-over increases genetic variety in organisms, by “shuffling” the genes for different combinations.
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• 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.
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Two Haploid cells result
These cells go directly into prophase II --- the first phase of Meiosis II.
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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)
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Finally – 4 haploid cells result
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Meiosis
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Meiosis
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FAST MEIOSIS
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If the gametes are to be male, then 4 individual sperm cells
form.
The process of forming sperm cells is called spermato-genesis
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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 – OvumEgg cells - Ova
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Relative Sizes of Gametes
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