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.
