All language subtitles for 47062x_Meiosis_events-en

So during meiosis, because this is a modified cell cycle,

instead of just the G1-S-M cycle, we're going to go G1,

we're going to replicate the DNA.

We need all of those copies.

But then we're going to do two meiotic divisions.

We're going to do M1, and then we're

going to follow this up by M2, and that

was this two rounds of cohesin cleavage that I showed you.

OK?

So we're going to need to reduce the content by half,

and the way that we're going to achieve this is not just

one segregation but two segregations.

So to be able to achieve that, during M1, we're

going to segregate the homologs from each other, so both copies

of chromosome 1.

And then during M2, this is going to look a little bit more

like a mitotic division.

We're going to segregate the sisters.

So the things that we need to do are

we need to pair the homologs, make sure

that they can find each other.

And there's going to be a structure,

this really beautiful structure called

the synaptonemal complex.

And it's going to assemble and basically zipper up

along the chromosomes.

So number one, we're going to pair them in this way.

Two, we're going to have this process of cutting the DNA

and putting it back together and that process

is called recombination.

And we're going to do this between the homologs

and it's going to swap genetic material around.

We can make, in many organisms, more than one of these breaks

and putting them back together.

But we're going to use this to essentially also lock

the homologs together so we get both genetic diversity

and correct segregation all at the same time.

We're going to use cohesin to hold these together.

But because of these unique cycles,

we're going to break at the arms, cut at arms and then

at the centromeres, then at these middle regions,

and that cycle is going to allow us

to achieve what we need to do.

So I just want to draw what this looks

like in molecular terms in case it

wasn't as clear with my pool noodles

as I had hoped it would be.

And in this case, you imagine--

I'm going to draw two replicated sisters.

Here is the maternal chromosome, for example.

And then I'm going to draw the other one sort of this way.

And so each of these is going to have cohesin surrounding it

throughout the entire chromosome loaded during replication.

But I'm going to do something where

I'm going to create a break, so I'm going to come in here

and I'm going to actually really, literally,

cut that DNA.

There's a DNA molecule called Spo11 that's going to cut this.

And when I do that, I can fix it in a lot of different ways.

So I can re-fix it, just be OK, have no crossover event.

But I can actually recombine this

so that this strand fixes itself with the homolog

and this strand fixes itself there,

and you can see that linear connectivity.

Now we went from the father strand up to the mother strand.

And what that's going to do is it's going to do two things.

Now, you cannot get them apart unless you cut cohesin.

So the only way that we could separate these,

because we have this long strand of DNA here,

is to eliminate those molecules up on the front.

This is really, fundamentally, the strategy that achieves,

again, these two things, genetic diversity

and proper segregation.

So physically, we need to achieve

these events of cutting of the DNA and the recombination

and the swapping across of the genetic material.

There are other things that we also need to achieve.

So for example, these two sisters

have to go in the same direction when we segregate them,

and so this is very different.

So at M1, instead of them being bioriented,

we need to co-orient the sisters.

But otherwise, there's a lot of other similarities

between all of the processes that we've talked about.

Cell division is amazingly beautiful and complex.

We have this fundamental mitotic division

where we're going to drive these cycles again over again

to make millions, trillions of cells in our body,

and we have this distinct division,

meiosis, to create the gametes, the sperm and the egg,

for example.

We have a very similar process of building a spindle

and segregating, but we have this variant

where we have two meiotic divisions instead with the need

to start with this recombination.

So in the next lecture, we're going to come back.

We're going to discuss these events again.

But instead of just thinking about these physical events--

how do you pair DNA, how do you segregate it,

how do you distribute it--

we're going to talk about that regulatory component, how

we time that.

How do we know that we're doing things correctly

and that we're not moving forward

in the presence of errors?

So I'll see you all for the next lecture.