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The Big Meiosis Thread

Discussion in 'DAT Discussions' started by DRHOYA, Jun 21, 2008.

  1. DRHOYA

    2+ Year Member

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    Ok guys, let's go over some Meiosis. This can be a tricky topic covered on the DAT for some people. I'll start off the top of my head. I'll keep it simple with a diploid 2n # of 4 to keep bookkeeping of chromosomes easy. This will also be occuring in males. Ok:

    Life starts in the G1 phase, where the cell grows, organelles are present, and metabolism is occuring. Cells that do not divide are arrested here.

    Now we move onto the S-phase, where the DNA duplicates, and dividing machinery is produced (spindles and things).

    Now we go to G2-phase, where the gets ready to start divisions.

    So Meiosis begins with prophase I, which is a long phase (longer than mitosis prophase) that is characterized by the condensing of chromosomes, spindle organization, and tetrad formation. Since there are 4 chromosomes, 2 tetrads are formed. Crossing over occurs here.

    Next is metaphase I, where the the tetrades align at the equatorial plate. This line up is different than mitosis, due to the tetrads. There are two lines of kinetochores.

    Next is anaphase I, where disjunction occurs, and the tetrads are split as they move to each pole of the cell. This is the main reduction step in #, so now we are haploid n = 2.

    Telophase I is next, and is characterized by equal division of the cytoplasm called cytokinesis. Now 2 haploid cells are present.

    END OF MEIOSIS I.

    Meiosis II begins with prophase II. Both haploid cells have condensed chromosomes and dividing machinery forms. There is no tetrad formation here, as n=2. During metaphase II, chromosomes line up again at the equatorial plate, this time however there is a single line of kinetochores. Anaphase II results in movement to opposite ends, and Telophase II results in equal cleavage of cytoplasm to result in 4 haploid cells each containing n = 2.

    For reference since this was a male cell, they start is spermatogonia and grow to primary spermatocytes. After meiosis I they are secondary spermatocytes, and after meiosis II they are spermatids, and after they mature, become viable spermatozoa (acrosome at tip containing degrading enzymes to break through protective covering of egg, head containing nucleus and genetic info, midpiece consisting of tons of mito's, and a flagella tail.


    For reference, females only produce 1 viable ovum from Meiosis, and 3 non-functional polar bodies. The cycle starts as Oogonium, then grows to primary oocyte, after meiosis I becomes secondary oocyte + 1 polar body. BOTH polar body and secondary oocyte have chromosomes and undergo meiosis II and the secondary oocyte becomes ootids + 1 polar body (3 total b/c the polar body from meiosis I undergoes meiosis II as well to form 2 polar bodies). Lastly the 1 ootid matures to an ovum.

    Ok, well I hope that lays it out. Feel free to post any corrections are any other helpful information that you can add. :thumbup:
     
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  3. harrygt

    2+ Year Member

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    This is indeed a very rich description of Meiosis and the details by DRHOYA. I'm sure the readers will benefit from his post. I just added a few corrections.
     
  4. DRHOYA

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    Thanks for the corrections and wording. I think one of the most difficult things about this cycle is the bookkeeping of chromosome #. Good point about the Telophase I resulting in 2 cells......haploid...not anaphase I, b/c in anaphase I they just moved to the poles not split yet.
     

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