All language subtitles for 27031x_MH_Tetrad_Analysis_01_Yeast_Tetrad_Formation-en

MICHAEL HEMANN: Basically, what could

happen if you could really see all of the meiotic outputs

of a single meiotic event.

So is it possible to actually see all

of the meiotic products?

And this is actually a real challenge or impossibility

looking at humans.

And it's really interesting, I think,

from a conceptual standpoint to think

about what would happen if you could see

all of these meiotic products.

Now you can, in fact, do this in haploid organisms,

and specifically in our old friend Saccharomyces

cerevisiae.

So budding yeast where we can actually

see all of the essential-- essentially

the four haploid products of a meiotic event.

So we've talked about yeast before,

budding yeast, which buds as a haploid and diploid,

it exists as a haploid and can mate.

But in very stressful situations,

particularly situations where there's difficult growth

conditions or a lack of nutrients,

a diploid yeast will undergo meiosis.

And this meiosis will result in this structure.

So this is a yeast tetrad, and tetrad means four,

so it's the four products of meiosis.

So four haploid cells that are each 1n cells.

So this tetrad is really tightly lumped together.

It's surrounded by this structure called

an ascus, which is really just a protective structure that

allows this tetrad to be blown away into the distance

and actually be preserved for a really long period of time.

This is a really robust set of spores

that can survive really harsh conditions until it actually

will start growing again, and then these

haploids very rapidly, if they're

in a good situation, will mate to form diploids again,

which is the preferred state.

So we have this structure this tetrad

that's found in an ascus.

And we can actually use this to examine

the four products of meiosis.

And so this is done with this kind

of tetrad-dissecting microscope.

And the tetrad-dissecting microscope

has a little glass needle that you can actually pick up

each of these haploid cells.

So basically, you use an enzyme to dissolve

outside of this ascus.

So you dissolve away this really protective coating.

And then you have a little glass needle that you go in

and you actually pick up each of these haploid cells

and you can plate them on a dish in a set of four.

So these are the four haploid cells

that grow into colonies that are derived from a single tetrad.

So in each of these cases, we can examine the four outcomes

or the four haploid cells that are

a result of a single meiosis, and we can examine them

in different conditions.

So how are we going to use this to tell us

something about meiosis and recombination?

Well, let's think about what's happening here again.

So we have two kinds of cells.

In yeast, we have MATa and we have MATalpha.

And so these are 1n cells.

And here, they'll have some gene a

that we've talked about before.

We can cross them together.

We'll get a 2n cell that has two alleles of this gene

that we can see in opposition, which

is what allows us to do our complementation studies.

And then prior to meiosis I where we have things like

recombination occurring between homologues,

this guy will then undergo meiosis I, and after meiosis I,

you'll have basically the two alleles that have segregated.

And if there are some recombination,

we'll see some mixing up of alleles

on the different chromosomes.

Then we have a meiosis II event, and following meiosis II,

we now have these four meiotic products

that are stuck together in a tetrad, but in a way

that we can actually keep them together

and we can grow them out.

So, again, all four meiotic products are together.

Which is really cool.

Because again, this is really one of the only situations

where we can actually examine all of the four

products of a meiosis.