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Phases of meiosis I

In meiosis I, cells go through four phases: prophase I, metaphase I, anaphase I, and telophase I. During prophase I, chromosomes pair up and exchange genetic material, creating more variation. In metaphase I, chromosomes line up in the middle of the cell. Anaphase I separates homologous pairs, while telophase I forms two new cells with a haploid number of chromosomes. Created by Sal Khan.

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  • purple pi pink style avatar for user Hope Langworthy
    Which cells use Mitosis and which use Meiosis?
    (9 votes)
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  • blobby green style avatar for user AnnQ929
    At the end of the video (about to the end of video), Sal describes the cells at the end of Meiosis I as having a haploid number of chromosomes. I keep looking at the drawings from the germ cell and then to the cells at the end of Telophase I and they seem to have still the same number of Chromosomes that they started with, only attached at a centromere. Please help me understand how this is a haploid, rather than a diploid.
    (15 votes)
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    • area 52 yellow style avatar for user Ramon Yambo
      Louie Da Dawg explains it below very nicely. I’ll quote him here (I hope you don’t mind Louie, but I wouldn’t be able to answer it any better):

      “Haploid is the total number of chromosomes, not total number of DNA strands. The replicated DNA (chromosomes shaped like an X, 2 DNA strands) is one chromosome, and unreplicated DNA (the squiggle chromosome, 1 DNA strand) is one chromosome.“

      So at the beginning, the cell has four individual chromosomes (the “squiggle chromosomes”). But by the end of Meiosis I the cell has two chromosomes (the chromosomes shaped like an X).

      I know your question is 9 years old, but I had the same question and I am sure others did too.
      (6 votes)
  • piceratops seed style avatar for user Stanton
    What happens to the other organelles (i.e, the endoplasmic reticulum) during cell division? Do they fade like the nuclear envelope?
    (6 votes)
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    • leaf red style avatar for user p.m.
      They don't fade away. If they did, the daughter cells would not have any organelles. The number of organelles is increased during the gap phases (G1 and G2) in preparation for cytokinesis.
      (11 votes)
  • blobby green style avatar for user Megan Diaz
    What is it that attaches sister chromatids together? Is it the centromere or the kinetochore? Both or neither?
    (5 votes)
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    • female robot grace style avatar for user tyersome
      Neither — a protein complex known as cohesin, forms rings that hold the sister chromatids together.

      To learn more:

      ADDENDUM: Some sources claim that the kinetochores also help hold the sister centromeres together, but I believe that is inaccurate — I certainly haven't found any convincing primary literature that shows that to be true. Also, artificially cutting cohesin is sufficient to trigger chromosome separation in cells arrested in metaphase, which seems to rule out a significant role for the kinetochores in holding sister chromatids together.
      (10 votes)
  • leaf green style avatar for user Hud Saeed
    can alleles transfer in mitosis and not JUST meiosis
    (6 votes)
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  • duskpin ultimate style avatar for user Katie Huttens
    How are the cells haploid after Meiosis II?
    (5 votes)
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  • blobby green style avatar for user ayahelaboudi00
    For those who are wondering where the in-depth Prophase 1 video is, here is the link: https://www.khanacademy.org/science/ap-biology/heredity/meiosis-and-genetic-diversity/v/chromosomal-crossover-in-meiosis-i
    (7 votes)
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  • leafers tree style avatar for user Joshua Carlson
    How can these random cells create such complex beings? If random creation and random evolution were true, wouldn't we expect to find random deformed creatures? The chance of millions of predictable and extremely complex beings existing all at once is almost zero when the universe is based on chance. Even if random chance could create one, single complex being; natural selection would eliminate all but that creature. And one more question, wouldn't we expect to find that all the evolving cells would die before they could unite to make a united fully functional cell? Scientists say that the earth before life could not have supported life, and these cells would die off even before uniting.
    (3 votes)
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    • male robot hal style avatar for user Charles LaCour
      It is not correct to say that evolution is random. Only a portion of evolution is random. The changes to DNA is random but the selection of what organism survives is not random. If the change causes a decrease in ability to survive to pass on its DNA then they will likely die out but if the change is beneficial it will have a greater chance to spread.

      Evolution is not that simple. There are a number of other factors that are involved in speciation. When you have a population of organisms that are able to share DNA they have a tenancy to evolve together but is a portion of the population develop a new trait that is not shared because of some limitation like physical separation the genetic drift in these groups will diverge and they will eventually become different species.

      Also if you have an organism that uses a resource A and a portion of the population gets a mutation that allows them to use resource B in addition to resource A there is actually a benefit to both groups if the portion of the population that can use B specializes is using B only since there is less competition for B and this lessens the competition for A as well. Then if there are different concentration of A and B in different areas you will find that the groups will migrate to these locations and will share DNA less and over time will drift apart genetically becoming different species without either one needing to die off.

      Since the resources and environments around the world very so will the adaptions that occur. It would be less likely that only a single organism type would be the outcome from evolution.
      (9 votes)
  • leaf blue style avatar for user AN18
    Aren't the centrosomes he talks about at actually the centrioles. I thought that the centrosome spits up into two centrioles and then goes to opposite sides of the cell.
    (4 votes)
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  • blobby green style avatar for user Hari Shankar Karthik
    what is the difference between kinetochore and centrosome?
    (4 votes)
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    • starky ultimate style avatar for user TL The Legend
      A kinetochore is on the chromosome, and it's where the spindle attaches. A centrosome, located at the poles, produces the microtubules and the spindle. Pretty much, the spindle is made in the centrosome and then becomes attached to it. It then finds the kinetochore on the chromosome and attaches to that, pulling the chromatids apart.
      (3 votes)

Video transcript

- [Voiceover] In the last video, we had just started to get into meiosis, and to be more precise, meiosis I, and to be even more precise than that, prophase I, but we spent a good bit of time on prophase I because some interesting things happened. Some things happened just like prophase in mitosis where the nuclear envelope disappears or starts to disappear, you have the chromosomes going into their dense form that has kinda this classic shape that you could see from a microscope, but what was unique or what was interesting about meiosis I and prophase I in particular is that you have this chromosomal crossover, that is a pretty typical thing to happen in meiosis I, and it tends to happen in a fairly clean way where homologous sections of these homologous pairs crossover, so these sections of the chromosome tend to code for the same genes. They're just different variants of those same genes. They might have different alleles, and then once again, this just adds more variation as we get into sexual reproduction, so it's a kind of neat thing that happens here. But now let's continue with meiosis, and in particular meiosis I, and you could guess what the next phase is going to be called. It is metaphase I, metaphase, metaphase I, and it has some similarities with metaphase in mitosis. So in metaphase I, let me draw my cell, so this is the cellular membrane right over there. I have my centrosomes, which are now going to play more significant roles. The nuclear membrane is now gone, and just like in metaphase in mitosis, my chromosomes are going to line up along the, here I'll draw it, kind of this up, down axis. So let's do that. So you have this one right over here. This is one chromosome, two sister chromatids, and we had the chromosomal crossover, so it has a little bit of pink here. I'm gonna take a little bit of time to switch colors a little bit more frequently. And then you have the one, at least most of which you got from your mother, yeah but there's been a little bit of chromosomal crossover here as well. So let me draw that. Let me draw that. And then you have this one, and just for the sake of, so you have this one, this chromosome from your father. It has replicated, so it's now two sister chromatids. And this one from your mother, and I'm not gonna show the chromosomal crossover here. Maybe it didn't happen over here. No homologous recombination over here. So these are, I guess, shorter. Now let me draw the centromeres. The centromeres I started doing in this blue color. So the centromeres, the centromeres, and then the centrosomes, you have these microtubules that start, they can push the centrosomes away from each other. But they also attach at the kinetochores to the chromosomes, to the chromosomes, just like that. And these are, the microtubules, you'll see people talk about oh these connect, and they're able to move things around, but I find this incredible that you just have a bunch of proteins through just kind of chemical and thermodynamic processes, are able to do really interesting things like move chromosomes to different parts of the cell, so that we eventually can get these gametes that can participate in sexual reproduction. This is an amazing thing, and it's developed over billions of years of evolution, but it's just mind boggling to think about the complexity, and not all of this is completely understood exactly how all of this works. I mean you have these kind of motor proteins that help move the chromosomes along, these microtubules can elongate and shorten in interesting ways. So it's a really fascinating process. But anyway, this is what's happening in metaphase I. Now you can probably guess what happens after that. We then move to anaphase I. So let me, we now go to anaphase I. I'll write that over here. Anaphase, anaphase I, and just like anaphase in mitosis, over here, the chromosomes start getting pulled apart, except for one significant difference, and this is actually a very significant difference. In mitosis, the sister chromatids get pulled apart. The sister chromatids get pulled apart to become two daughter chromosomes. That does not happen in anaphase I. In anaphase I, the sister chromatids stay together. It's the homologous pairs that get pulled apart. So let me draw that. So this homologous pair up here gets pulled apart. The two sister chromatids do not get pulled apart here. So you have this one is getting pulled onto this side. So this one's getting pulled onto this side. It has a little bit from the original, so a little bit of that right over there. And then you have this one getting pulled on this side. So draw it the best I can, the colors, alright, so it looks like that, although it's nice to have, it's kinda easy to keep track of cause these switch colors like that. And then you have this one getting pulled on this side. This one getting pulled on this side. And finally finally this one getting pulled onto that side. And let me draw the centrosomes. So that's my, oops, centrosome, and once again, it's pulling, or I guess you could say the chromosomes are being moved and these things are pushing each other apart. The two centrosomes might be pushing apart to get to the opposite ends of the actual cell, but they're bringing, there's all sorts of interesting mechanisms that are bringing along these microtubules, bringing the chromosomes, once again splitting the homologous pairs. And how they split is random. You know, this pink one could have been on the right side, this orange one could have been on the left side, or vice versa, and once again, this adds more variation amongst the gametes, so even all of the resulting gametes that get produced, they all will have different genetic information. So this is anaphase I. You're pulling these apart, and then you could imagine what happens in telophase I. So telophase I, telophase, telophase I. Telophase I, and this is fairly analogous to what happens in mitosis in telophase. So now you have your cytokinesis is beginning, and actually, it might even begin earlier, in mitosis it happens as early as anaphase, at least the cytokinesis is starting, but you're starting to see that. The homologous pairs are fully split apart, and they're at opposite ends, and actually they can begin to unravel into their chromatin state, so this one began to unravel into its chromatin state. It has a little bit of the magenta. Oops, it has a little bit of the magenta right over here. This is unravelling as well. This is unravelling like that, once it gets into its chromatin state. The cellular, and let me do the other ones as well. So this is this one right over here. It's beginning to unravel. This one over here, beginning to unravel. It's got a bit of orange on it. It's got a little bit of orange on it. The nuclear membrane begins to form again. The nuclear membrane begins to form again. In some ways, it's reversing what happened in prophase I where the nuclear membrane disappeared, and the chromosomes condensed. And let me draw, let me draw the centrosomes, which are outside the nuclear membrane, just like that. And the microtubules are also dissolving. The microtubules are also dissolving. And you have your cytokinesis. So your cytokinesis, so these separate. These separate into two cells. So once again, when we did the overview of meiosis, we said look, the first phase of meisosis, you go from a diploid germ cell to two haploid cells. And these aren't quite our end product yet. This right over here, what we have just gone through, what we have just gone through, all of this combined that we have just gone through, this is meiosis I. And in the next video, we're gonna go through meiosis II. Whoops, I didn't mean to do that. This is, so let's see, all of this is meiosis I. Let me write that in a different color, in bold. So this is all meiosis, meiosis I here, and you can see each of these cells now have a haploid number. They now have a haploid, haploid number of two chromosomes each. Now each of those two chromosomes do have two sister chromatids, and as we'll see in meiosis II, which is very similar to mitosis, is going to split up the sister chromatids from each of these chromosomes, which gives us two daughter chromosomes. So we're gonna see that over here. So your haploid number here is two. You have two chromosomes here and you have two chromosomes there. And we'll explore meiosis II in the next video.