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Meiosis I: Prophase I

by Georgina Cornwall, PhD
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    00:00 And when we go into prophase I, the chromosomes have already begun to wind up. So the chromatin has been synthesized, duplicated and we are wrapping it up into chromosomes. Those chromosomes are going to continue being wound into prophase I as they pair with each other. So as they pair and begin this crossing over process, the homologous pairs are becoming much more condensed. They are forming their chiasmata. They are switching pieces or maybe many pieces. And you can see in this figure that the sister chromatids are sort of switching pieces. So let's talk about for a moment sister chromatids, right. We have one homolog and the other homolog. This is a set of sister chromatids and this is a set of sister chromatids. The chiasmata, the crossing over actually happens between non-sister chromatids. So again, a little bit more of language. The non-sister chromatids, so the paternal, one of the paternal chromatids and one of the maternal chromatids are the ones that are exchanging information. So we'll have two that are the original parental type, and then we'll have crossing over. And this will become particularly important when you start to explore genetics.

    01:22 So, here is the crossing over image. And chromosomes are condensing as they are crossing over.

    01:28 So it's not like they are really tightly wound and crossing over whole chunks of chromosome.

    01:33 They are actually slightly unwound while they are exchanging pieces. And so after they have exchanged they continue their coiling and super coiling and super super coiling in order to get these condensed chromosomes. And we are ready to move forward. So, spindle fibers begin to form.

    01:49 You are going to see very many familiarities between mitosis and meiosis. Same thing goes on.

    01:55 Those spindle fibers form. The asters microtubules are projecting out. And they are starting to move towards the poles of the cell.


    About the Lecture

    The lecture Meiosis I: Prophase I by Georgina Cornwall, PhD is from the course Cell Cycle and Cell Division.


    Included Quiz Questions

    1. …two non-sister chromatids of the replicated homologous chromosomes.
    2. …two sister chromatids of the replicated homologous chromosomes.
    3. …two sister chromatids of the replicated non-homologous chromosomes.
    4. ...two non-sister chromatids of the replicated heterologous chromosomes.
    5. …two non-sister chromatids of the replicated non-homologous chromosomes.

    Author of lecture Meiosis I: Prophase I

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD


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