Phases of meiosis I | Cells | MCAT | Khan Academy
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Phases of meiosis I | Cells | MCAT | Khan Academy


– [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.

About James Carlton

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17 thoughts on “Phases of meiosis I | Cells | MCAT | Khan Academy

  1. Thanks soo much this helped me so freaking much to understand Meiosis | that I couldn't understand from our stupid Biology book -_- even the drawing in it are wrong

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