Homologous Chromosome Pairs – Meiosis

by Georgina Cornwall, PhD

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    00:00 Before we move in to examining the specifics of meiosis I and meiosis II, let's take a closer look at these homologous pairs or the tetrad formation. We're going to see that these homologous pairs not only come together but they actually stick to each other in this synaptic pairing.

    00:21 So, we form a synaptonemal complex between the two homologous chromosomes that have been replicated.

    00:30 And we're going to see that they actually cross over and exchange pieces of information.

    00:37 Now that we have this synaptonemal complex, sometimes pieces of information might change.

    00:45 And this is one of the places that we acquire genetic variation in the process of meiosis.

    00:52 There is one other place that we'll explore later on in this lecture. So, we have a paternal chromosome and a maternal chromosome. And they have paired with the synaptonemal complex. The synaptonemal complex sometimes the non-sister chromatids will cross over and much like if you were hooking arms with someone, and you swapped forearms. It would be kind of strange but this is what happens in the homologous pairing during prophase of meiosis I. So the paternal and the maternal chromosome with their replicated sister chromatid, we have a tetrad here, are crossing over and literally switching pieces.

    01:34 The point of crossing over is called the chiasmata. The whole piece where they're stuck together is the synaptonemal complex. And so the resulting chromosomes are distinct from one another.

    01:49 So this crossing over happens during prophase and here we see the distinct chromosomes.

    01:55 There is genetic recombination where the maternal chromosome has actually switched pieces with the paternal chromosome. This could happen at multiple locations or loci along the chromosome.

    02:07 So they could actually cross over twice. Again, this whole thing is called genetic recombination.

    02:14 It's one of the places that we see we get genetic variation in offspring, because the sperm and the egg are distinctly different from any of the parent cells. So, moving forward into meiosis I. Prior to meiosis I recall we were in the cell cycle. We had germ line cells that are going to become gametes after meiosis. The germ line cells have 46 chromosomes and they are going to go through S phase.

    02:48 And they are going to replicate those. And so at the end of S phase, before we go into meiosis I, we have replicated homologous chromosomes.

    About the Lecture

    The lecture Homologous Chromosome Pairs – Meiosis by Georgina Cornwall, PhD is from the course Cell Cycle and Cell Division.

    Included Quiz Questions

    1. All 4 of these cells are genetically different from each other.
    2. All 4 of these cells are genetically different from the original parental cell.
    3. Each of these cells will have five chromosomes.
    4. Each of these cells will have ten chromosomes.
    5. All 4 of these cells are diploid.
    1. It is a network of proteins that holds homologs together.
    2. It allows for synapsis between non-sister chromatids.
    3. It allows for genetic recombination.
    4. The area where microtubules attach to the centromere of each chromosome.
    5. It allows for the replication of chromosomes to homologous pairs.
    1. Chiasmata
    2. Synapsis
    3. Origin for the chromosomal division
    4. Synaptonemal complex
    5. Centromere

    Author of lecture Homologous Chromosome Pairs – Meiosis

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD

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    Explanation of concepts
    By Emmanuel E. on 10. December 2017 for Homologous Chromosome Pairs – Meiosis

    The lecturer uses great analogies to make concepts easier to understand.