All language subtitles for 37062x_Meiosis_overview-en

So we've talked about chromosome segregation and cell division

so far.

And we've done a very reproducible process--

the process that really allowed us to go from one cell

to all 30 trillion cells in our body, mostly.

And that's the process of mitosis.

But we're going to finish here just by briefly

talking about the process of meiosis

and how that's different.

So this is also a cell-division event,

but it's one that we want to use-- instead of making two

equivalent daughter cells, we're going

to use this to make the gametes.

So in our case, for example, we're

going to use this to make sperm and egg.

In the case of yeast, you're going to make spores--

things that can survive.

And the key thing between these and our-- most of the cells

in our body--

is that these gametes are going to be haploid.

So most of the cells in our body are diploid.

There's two copies of each chromosome.

And here, these haploid cells are

going to have one copy of each chromosome.

So we have 46 chromosomes in our cells

because we got one pair from our mothers, one pair

from our fathers.

And in this case, we want to be able to generate cells

that can reconstitute that.

So an oocyte is going to have 23 chromosomes in it.

A sperm is going to have 23 chromosomes.

And when they come back together we'll be able to fix this, OK?

So I want to take our chromosomes again.

Now, I have two.

These are the two homologues, OK?

This is chromosome 1, but it's from your mom.

This is chromosome 1, but it's from your dad, OK?

And so we have these chromosomes here.

And ultimately, our goal is to make sure

that every cell gets just one of these, OK?

And so you could imagine doing that where

I do a first division exactly like we've been talking about.

I pull these two apart.

I end up with two different ones over here.

I separately do this for those.

But when I try to do it again, when I

try to create a division that allows me to fully distinguish

them--

just one over here, one over here,

one over here, one over there--

you could ultimately end up with four different cells,

which you can in the case of yeast, for example.

All four chromosome products will end up in a spore,

and it's called a tetrad, OK?

And so each of those is a haploid cell

with one chromosome of each that you began with, OK?

So we still need to replicate the DNA,

but we need to come up with some way to distribute

it a little bit better.

And so we have this wonderful trick that we have-- cohesion.

I've solved that problem for this chromosome during mitosis.

I can do that.

I've solved that for this one.

I need these chromosomes to work together.

I need them to know that they exist

and that they're going to work together.

And that way, they're going to distribute appropriately, OK?

And so in addition to the key cohesion,

I need a way to make sure that I can glue these together

in some way, OK?

So as I'll diagram on the board in a second,

we're going to begin by really actually just pairing them--

having them find and recognize each other and line up.

And so we're going to build a structure

called the synaptonemal complex, which is

going to hold these together.

But then I'm still back in-- where

I am in that I can't actually figure out

how to separate these in any sort

of logical and coherent way, OK?

And so then we're going to actually use a second trick.

So these chromosomes are really nicely intact.

But I'm going to have a way to put them together in some way.

And so actually, I'm going to do something that you would never

want to do to a chromosome, which

is, I'm going to break it, OK?

So I've broken this chromosome here.

It's got a piece.

I'm going to break this chromosome 2.

And I'm going to create a situation

where I can allow these chromosomes to mix and match,

OK?

So normally, I wouldn't want to be able to do this.

But here, I'm going to break them in such a way

that I'm going to allow them to refuse and recombine

with each other in different ways, OK?

And so now I've created a different kind of structure.

These are going to be these recombined chromosomes.

Let's see if I can do this well.

And so now they're actually all held together.

So even when I get rid of that synaptonemal complex structure,

all of these chromosomes are held.

And it looks like a mess.

But now, we're going to do some different things to be

able to separate stuff, OK?

So I still want to do a cell-division event, OK?

I have the recombined chromosomes here.

And they're held on to each other.

So I replicated them.

I paired them.

I held it together in exactly the same way we wanted.

But then I swapped the DNA across just at one place, OK?

So we're going to divide these chromosomes.

But instead of doing that the way

that we've been doing thus far, we're

going to release the cohesion.

But we're going to hold on to it at centromeres.

So any cohesion near the centromeres,

I'm not going to cut.

But in this first meiotic division,

I'm going to come through.

I'm going to release all the cohesion along the arms.

But I'm going to protect it exclusively at centromeres.

And I'm going to do that up here, too.

And so in this case, again, that cohesion goes away.

That cohesion goes away.

And in doing that, I'm now going to release

these two from each other.

And so during this first meiotic division,

these are the things that are going to segregate, OK?

This one is going to go this way.

This one is going to go that way.

So I've lined them up and paired them.

But just by cutting the arm cohesion,

now I've released these from each other.

Then I'm going to come back through a second division.

Now I actually need to be able to distribute these.

And I'm going to go through, and I'm

going to cut that centromere cohesion.

And I'm going to end up with these two new chromatids.

But these chromatids are going to look--

still chromosome 1, still in exactly the same way.

But what you should see here is that at least one

of these chromosomes has a part from the dad chromosome

instead.

So if these were the two mother chromosomes, all of this region

here came from the father, OK?

And so this process of recombination

will help resort our genomes.

And so if you imagine-- your siblings

are going to look different even though you

came from the same source material,

if you both have the same parents.

And this way, we can actually resort the material

for evolution, and through--

within a population.

But what you really need here is this ability

to actually have this distinct cohesion and recombination

to really enable the meiotic chromosome segregation.