Meiosis in Humans

by Georgina Cornwall, PhD

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    00:00 Let's take a look at the production of sperm versus the production of eggs.

    00:06 The production of sperm happens in the testis, specifically in the Sertoli cells in the seminiferous tubules. But that's for anatomy and physiology. Either way, you can see that we have our Sertoli cells at the top. These are going to contain germ line cells. Recall, germ line cells are diploid.

    00:26 They have all 46 chromosomes. 23 pairs. And then, they are going to be determined to become sperm at a certain point. And we call that the primary spermatocyte. It's diploid. It undergoes meiosis I and becomes haploid. So now we have haploid spermatocytes or secondary spermatocytes. After meiosis II, the sister chromatids separate and we have spermatids. Now they are not quite ready to go.

    01:00 There are a lot of modifications that have to happen to the spermatids to become actual spermatozoa and be mobilized and able to swim and go find the egg. Let's contrast this to what happens in female gametogenesis and the production of eggs. In females, this happens in the ovaries clearly. And the egg is ovulated, floats down the fallopian tube. One of the interesting things here is that in females, we have our germ line cell. And prior to birth, it begins, once they are determined to become eggs, that germ line cell begins meiosis I. But then it stalled till ovulation. So for 15, 30, I don't know 45 years maybe, you have those eggs stalled during meiosis I. Then, at the time of ovulation, meiosis I is completed and it's not until that egg becomes fertilized that it actually undergoes meiosis II. This is one of the issues with women reproducing later in life. The machinery has been in place since prior to birth.

    02:24 What if those spindle fibers don't produce themselves quite properly and don't grab on to the kinetochores properly and pulls the chromosomes apart properly. So there's opportunity during both meiosis I and meiosis II for things to go awry and chromosomes not to separate properly. We could have the homologous pairs not separating or we could have sister chromatids not separating properly. And that is where we result in something like Down's syndrome. Where we have not separated chromosome 21 usually in meiosis II I think.

    02:59 If we don't separate one of those sets of sister chromatids, we might end up with three copies in one cell giving us Down's syndrome and one copy in the other cell. So, reproducing later in life for women, is a little bit more of an issue than it is for men. Because men make fresh sperm every day.

    03:17 All the way through life. So, that's the explanation for that difference.

    About the Lecture

    The lecture Meiosis in Humans by Georgina Cornwall, PhD is from the course Cell Cycle and Cell Division.

    Included Quiz Questions

    1. Oogenesis — sperm production
    2. Fertilization — oocyte completes meiosis II
    3. Spermatozoa — mature motile male gametes
    4. Secondary spermatocyte — haploid
    5. Gametogenesis — production of haploid gametes

    Author of lecture Meiosis in Humans

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD

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