All language subtitles for 77031x_MH_Gene_Function_02_Yeast_Biology_v1-en

MICHAEL HEMANN: Saccharomyces cerevisiae are budding yeast.

So in the top left, I'm showing yeast-- yeast that are budding.

Yeast are really important for us.

Biologically, they do things--

critical things-- for our society,

like make bread and make beer.

For those of you that haven't seen yeast grow, on the right,

this is a culture dish with yeast colonies growing.

So yeast are really interesting because, among other things,

they can exist as haploids--

meaning that they can exist with only one copy of their genome.

So we would call that 1n, which is 16 chromosomes.

So they can exist in a 1n state or they

can exist in a 2n state--

essentially as a diploid.

So 1n we call haploid, and 2n, we call diploid.

So at the top here, I'm showing essentially the life cycle

of saccharomyces cerevisiae.

So in the transition from G1 phase-- so prereplication to S

phase-- they start budding.

In S phase, they replicate their DNA.

They make a copy of it.

And they essentially package half of their genome

into their daughter cell.

And this occurs in both haploids and diploids.

So in both cases, they're budding.

In both cases they undergo this normal replication cycle.

The difference between haploids and diploids

is that haploids can actually mate.

So there are two essential varieties, or mating types,

of yeast.

One is referred to as mating type a the other

is referred to as mating type alpha.

You can think of these as sort of a yeast sex,

although that description is not entirely accurate in terms

of what this actually means.

But an a mates to an alpha.

And in doing so, they can create a diploid.

So yeast grow as haploids and diploids.

So how do you think they grow better?

Like what do you think they prefer?

Do they prefer to be haploids or diploids?

It's a bit of an anthropomorphic question.

But how do you think they grow better?

So it's about 40/60 haploids to diploids.

It turns out they generally prefer to be diploids.

Life is tough as a haploid.

If you only have a single copy of your genome,

among other things, you don't tolerate mutations.

So say you have an essential gene and it's lost.

Well you're in trouble if you don't have two copies.

And you can't repair DNA.

So it's a little bit difficult to exist as a haploid.

But what happens is the haploid phase

actually confers a certain degree of protection

to yeast in certain contexts.

So a diploid yeast in a very stressful situation undergoes

meiosis.

And after meiosis, it forms a structure here.

We'll talk about this later in another lecture,

in talking about tetrad analysis.

But this is essentially a cluster

of four spores that turns out to be very durable.

So in stressful situations, it undergoes meiosis.

You sporulate, you drift to a different place

to try to find a better growth condition,

you become a haploid, and you start growing as a haploid,

but then you rapidly once again become a diploid

because it's a more robust, essentially,

type of growth condition.

But we can use this characteristics

of a haploid yeast, and the ability to mate

haploid yeast, to actually look at genetic interactions

between two different alleles.

All right.

So let's think about two yeast, or two individual yeast cells.

These are both 1n, so they're haploid.

You guys know of any other haploid organisms

or haploid eukaryotes?

Any examples?

It's pretty rare-- the ability to live as a haploid.

A bee's a cool example.

So a drone bee, or a male bee, exists as a haploid.

And the queen bee and the worker bees are diploids.

Curiously enough, a haploid bee and a haploid yeast

actually have the same chromosome number, which

is actually 16 chromosomes.

They're very different sizes.

There's not necessarily a good correlation between genome size

and chromosome number.

But a haploid yeast has 16 chromosomes.

And so we here have a mat a yeast cell.

And we have a mat alpha cell.

And although they have 16 chromosomes,

we're only going to look at one of them.

And we're going to look at one gene on one chromosome,

this gene gene, right?

So we can mate a mat alpha and a mat a strain

and get a resulting diploid or 2n cell, where

we can look at the interaction, essentially, of these two

alleles, right?

We can bring these two alleles together.

So we can look at their relative interaction

their relative contribution to the overall phenotype

of the resulting strain.

So, importantly, in the context of yeast,

these yeast haploids and diploids

are what we call isomorphic.

Meaning that the phenotype of the haploid and the diploids

are the same if essentially they have the same genotypes.

So if you have one little g, your phenotype

is pretty much the same as if you have two little gs.

So it allows us essentially to compare

haploid state and a diploid state

looking at very similar phenotypes.