Meiosis I: Metaphase I

by Georgina Cornwall, PhD

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    00:00 And we skip the pro-metaphase piece and move directly into metaphase. I'm not sure really why but that's just some general convention. And we talk about what's happening on the metaphase plate.

    00:11 Now in meiosis it's particularly important to talk about this metaphase plate as it is in mitosis.

    00:18 It is a fantastic place for you to compare chromosome number and such as you begin diagramming these processes. So first of all, we still have kinetochore microtubules. And you're still going to see that we have the polar microtubules. Remember the kinetochore microtubules have grabbed on to the kinetochores of the chromosomes, but this time they are grabbing on to kinetochores of non-sister sets of chromatids and their jostling them around and moving them to align on the metaphase plate.

    00:51 The key language point here is homologous pairs or replicated homologous pairs are lining up on the metaphase plate. Think back to mitosis for a moment. What was lining up on the metaphase plate during mitosis? During mitosis, we had a single file line so that each of the 46 chromosomes were lined up single file down the center. We only had to undergo one division and sister chromatids would separate.

    01:26 In meiosis, the big distinction is we are separating homologous chromosomes. So that's where the language really comes into play. Note that down before we move on.

    01:39 So, here's a diagram to depict that. Metaphase, we've got the synaptonemal complexes. We are going to pull them apart in metaphase I. Metaphase of mitosis, sister chromatids, kinetochores on each sister chromatid are being pulled by the microtubules. Just a quick note. When you're reading something like an exam question and it just says metaphase, you don't need to ask yourself "Well is it I or II or mitosis or meiosis?" Metaphase alone without a I or a II, that means mitosis. Otherwise we would say metaphase I or metaphase II. So, just a little hint there. This is the second point where we could acquire some genetic variation. The way that chromosomes align on the metaphase plate, whether it's the paternal one on this side and the maternal one on this side or vice versa, is completely random.

    02:38 So it's the shuffling of chromosomes as they align on the metaphase plate that is the second place in meiosis that we will see genetic variation resulting in the gametes that are formed.

    02:52 So where was the first place? Yeap. The first place was when we had crossing over of the non-sister chromatids during prophase.

    03:02 So the alignment of homologous chromosomes on the metaphase plate during meiosis I is our second place to acquire genetic variation in gametes. You could potentially have all 23 of your paternal chromosomes, and all 23 of your maternal chromosomes line up on opposite sides. That would result in production of a sperm or egg that had all of your grandmothers information and all of your grandfathers information.

    03:34 So this sort of explains; It's generally not what happens but it sort of explains how sometimes a child will look very much like one side of the family and not so much like the other.

    03:44 Generally, with that many chromosomes, we'll see a little bit more shuffling and some features are there from one parent and some features are there from the other parents family.

    03:54 But again, metaphase, genetic shuffling. Right? Genetic recombination during prophase I.

    About the Lecture

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

    Included Quiz Questions

    1. Replicated homologous chromosomes
    2. Sister chromatids
    3. Single chromatids
    4. Unreplicated chromosomes
    5. Unwound chromosomes
    1. Crossing over during prophase I and alignment of chromosomes on the metaphase plate during metaphase I
    2. Prophase II and metaphase II
    3. Metaphase I and metaphase II are responsible for genetic recombinations in the offspring
    4. Anaphase I and telophase I result in recombinations
    5. DNA replication during the S phase of the cell cycle and cytokinesis during meiosis I

    Author of lecture Meiosis I: Metaphase I

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD

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