Hi. It's Mr. Andersen and in this video I'm going to go through the phases of meiosis.
Meiosis is a lot like mitosis. It starts with interphase, but remember the point of mitosis
is to make two identical cells. And in meiosis what we're trying to do are make four genetically
different cells. Because they're destined to be gametes. They're destined to be sperm
and egg. And that's the whole point of sex. We want the next generation to be different
than the generation before. And so when you look at a diagram of meiosis, it's a little
bit daunting. And we're not going to go through all of it right now. We'll go through this
diagram at the end kind of as a way to review it. But I want to step through each of those
phases of meiosis. A quick mnemonic PMAT times 2 is going to remind you the different phases
that we have to go through. So it's prophase, metaphase, anaphase telophase and then we
go through that again on the second division. Before we get to it we should talk about the
major structures that you're going to see as we go through meiosis. The first one of
course are the chromosomes. And so you're going to see two of each chromosome at the
beginning. And so we get a chromosome 1 from our dad. And a chromosome 1 from our mom.
We call these homologous chromosomes. Remember each of these chromosomes has hundreds of
genes on it. And so you get two copies of all those genes. One from dad. One from mom.
This would be chromosome number two because it's shorter. And in my model I'm just going
to use 2 pair of chromosomes. Remember in a real human cell we're going to have 23 pairs
of chromosomes. But it's almost too difficult to follow what's going on if we had that many
chromosomes. You just have to multiply it times 13. Now lots of times you'll see chromosomes
not look like that, but like this. And so what's happened here is that we duplicated
that chromosome from dad. And so these two what are called sister chromatids are exact
copies of the DNA. And so you know when you see a chromosome that looks like this they've
already gone through that duplication. Another important structure is the centrosome. The
centrosome is going to organize the spindle, which is essentially dividing the nucleus
and also dividing the cell. And in animals it's made up of two things. We have the centrioles
on the middle. And then these microtubules that go around the outside. In plant cells
they're going to lack these centrioles in the middle. And the nuclear membrane is actually
organizing a lot of this division. But let's begin at the beginning, at the beginning of
interphase. So this is just as the cell has been formed. And you can see right here in
the nuclei that we have those two pair of chromosomes. Chromosome 1 and chromosome 2.
And if we look at the centrosomes we just have 1 centrosome. And so that's not usually
what a cell looks like. This is what a cell usually looks like. And so what's gone on
here, you can see that in interphase we've duplicated those centrosomes. So we have two
of those. And the DNA is all loose. It's not tightly would up into these chromosomes that
we characteristically see. As we go through interphase what eventually happens is then
we can see those chromosomes again. Now do you remember what it means when they look
like this "X"? It means that during the S phase of interphase we've copied the DNA.
So we have two complete sets of DNA. And this looks identical to mitosis. But it's just
about to change. And so what happens next is prophase I. During prophase I the chromosomes
undergo what's called synapsis. And so what's happening is chromosome 1 from dad and 1 from
mom are coming together. And they're wrapping around each other really really tightly. And
what's really going on is that they're swapping parts of their chromosomes. In other words
these are pretty much identical except for the changes in the genes. And so they undergo
what's called crossing over. So segments of chromosome from mom are switching with chromosome
from dad. Same thing over here. And same thing with chromosome 2. Now what's that giving
us? It's giving variation. If this didn't occur, synapsis didn't occur, the chromosomes
that you get from your mom and your dad you would give to your children either as a chromosome
from your mom or a chromosome from your dad. And what we're doing in crossing over is we're
combining the two chromosomes that we got from our two parents and making a brand new
chromosome that we want to deliver to our child. That's the importance of this. If we
keep watching what happens next, we then go into metaphase I. What's happening in metaphase,
you can see that they're all lining up or meeting in the middle of the cell at what's
called the metaphase plate. Now they could have organized themselves live this, with
the blue one on the left and the red one on the right. But they could have easily organized
themselves like this. So they could have been in a different position. So this would be
a totally different orientation of the chromosomes. It also could organize like this from chromosome
2 or it could organize like this. And so what do we have? We have four different ways that
these chromosomes could orient themselves independently at metaphase I. What is that
giving us? Well it's giving us variation. And so there are two ways that the number
of pairs possibilities of how they could arrange at independent orientation of metaphase I.
So how many are there? We would say 4. What if I had three here? Then there would be eight
ways that they could arrange themselves. Still doesn't see like much variation. But remember
in humans we have 23 chromosomes. And so there are over 8 million ways that all of those
chromosomes could independently orient themselves during metaphase I. And so that's where we're
getting, again that and crossing-over is giving us a huge amount of variation in meiosis.
And remember that one sperm has to find that one egg. And so it's really over 64 trillion
possibilities of an offspring just based on independent orientation itself. Let's keep
watching that. So another thing that happens in metaphase I is that spindle is going to
attach. So you could see that the centrosomes move to either side of the cell. And the spindle
attaches to the centromere of each of those homologous chromosomes. During anaphase I
it's pulling them apart. So we can see that those homologous chromosomes now are going
to either cell. And then during telophase I what's happening is we're reforming a new
nuclei at each side. We've divided the nuclei, which is meiosis, but now we have to divide
rest of the cell and that's called cytokinesis. And so we're done with meiosis I. We've gone
through prophase where we saw that synapsis. We went through metaphase where we had that
independent orientation. And now we've divided the nuclei into each of those cells. But we're
not done yet. So what's going to happen next is we're going to go through prophase II.
Now during prophase II there's no more crossing over. But what's going to happen is those
two chromosomes are going to line up again. They're going to meet right in the middle.
And the spindle it going to attach to each of those centromeres. Now if we look at what
happens. Each of those chromosomes are being pulled to another side during anaphase II.
And then finally during telophase II and cytokinesis we've created these four cells that we wanted
in the beginning. And so if we look at where did we being? Again way back in the beginning
we had 4 chromosomes or 2 pair of chromosomes. Now we have 4 cells and each of those only
have 2 chromosomes, a 1 and a 2. And so what would happen next, in the circle of life,
if these were sperm they would fertilize an egg. And then we would start over again. And
we'd have a brand new organism that's going to be created through mitosis. Now this is
how we make sperm. Each of the sperm are going to be made like this. Again there would be
way more chromosomes in us because we have 23. It's a little different with the eggs because
there's so much important in the cell that only one of these nuclei will actually be
used and the other ones won't be used genetically in that cell. And that allows us to keep all
the important parts of the cell, like the mitochondria, endoplasmic reticulum, in that one cell. And
so now let's kind of review and go over that confusing diagram at the beginning. So what
are we looking at here? This would be interphase at the beginning. You can see we just have
one copy of all those chromosomes. This would be at the end so you could see that we duplicated
the DNA. Next what do we have? This is prophase I. Remember what important thing is occurring
there? We've got crossing over. And then we've got independent orientation during metaphase
I. They then are pulled apart. And then we have two cells. And now in this second meiosis
what are we doing? We are just lining up those chromosomes and then they're splitting up
into each of the sides. And so what do we get at the end? Each of those cells. If we
were to go back to here, each of those cells, look at this one and that one and that one
and that one, are totally different than that original cell. They also have half of the
DNA that the original cell did. And so that's meiosis and I hope that was helpful.