All language subtitles for 87031x_MH_Gene_Function_03_Histidine_Synth_Auxotrop-en

MICHAEL HEMANN: Yeast can essentially

grow on minimal media.

So all they need to survive are salts, minerals, and a sugar

source like glucose.

So they can make all of their biological molecules

or their amino acids, their fats, their nucleic acids,

their proteins just starting with these really core building

blocks.

So say you start with minimal media.

From minimal media, you can make some precursor molecule.

And then through a set of steps, make a number

of subsequent molecules all the way

to generating some biological molecule of interest.

So say we're thinking about the synthesis of histidine,

a critical amino acid that's present in most proteins.

You have essentially here, a biochemical pathway

which via a number of enzymatic steps

allows you to synthesize from really

these core simple materials a lot of precursor molecules

that eventually involve or result

in the generation of histidine through a set

of distinct enzymatic steps.

So here, we can think of each of these steps being

catalyzed by an enzyme, some enzyme 1, 2, 3, 4.

And so if you're defective for any of these enzymes

or any of the genes necessary for encoding these enzymes,

you're essentially unable to produce histidine.

And so this strain we'll refer to as his minus.

So his minus is a phenotype, generally

will draw these as having lowercase first letters so

lowercase h, right, and minus.

So they don't make histidine.

They can't essentially grow in the absence

of exogenous histidine.

So the phenotype is his minus.

Now, the genotypes are going to depend on the particular gene

that's defective here.

So if you are defective for gene 1, your his 1 minus, 2,

your his 2 minus, 3, or his 3 minus, and 4, your his 4 minus.

These are each genotypes.

The result of each of these genotypes in a haploid

or a diploid that has two copies of these defective genes

is that you're his minus.

And we refer to these strains as his auxotrophs.

So they essentially can't grow in the absence of histidine.

So an auxotroph essentially just means

you can't grow in the presence of something.

So here, they can't grow in the absence of something.

So here, they can't grow in the absence of added histidine.

So if you are a his plus strain, you grow on minimal media.

If you're a his minus strain, you

don't grow on minimal media.

But if you add histidine to the minimal media, here of course,

the his plus grows.

And now the his minus gross.

So you have to grow them in the presence of added histidine.

So let's add a few definitions here.

So we talked last time about this idea of a phenotype.

And a phenotype is essentially all

of the traits of an organism.

But it has a particular focus on the trait

that we're talking about.

So in this case, they're his minus.

That's the phenotype.

If we refer to a strain as his minus,

generally infer that all of the other characteristics

are wild type.

So it's just inferred that his deficiency

is the only relevant phenotype in what we're discussing.

So a homozygote it is a strain that has

two of the same alleles, right?

And this always refers to a diploid

for that allele, so a diploid with two of the same alleles.

And a heterozygote is a diploid with two different alleles.

And so we'll introduce this idea of recessive.

Recessive is a trait that is present in a homozygote but not

a heterozygote.

So we can think about a cross between two strains, two

haploid strains.

And so let's do this cross.

We'll start with a genotype.

And our first genotype will be mat a his 3 minus.

So these strains are deficient for that third enzyme that's

necessary to produce histidine.

The phenotype is just his minus, not

his 3 minus, just his minus.

And we're going to make this to another strain.

So in yeast in that alpha strain,

that is also his 3 minus.

The diploid genotype here is going

to be his 3 minus over his 3 minus.

And you draw the two alleles that way sort of in opposition

of one another.

So his 3 minus over his 3 minus, and the phenotype is his minus.

Again, these are isomorphic creatures.

The haploid phenotypes and the diploid phenotypes

are the same if they have essentially

the same genetic content.

And so if you put two strains that are defective

for histidine biosynthesis together

with defects in the same gene, they're

going to produce a strain that is unable to actually make

histidine.

So let's do another cross.

Here, we have the same starting strain, mat a his 3 minus.

This phenotype of course, is his minus

will mate to a mat alpha that is his 3 plus, so essentially

a wild type strain.

The resulting diploid genotype is his 3 minus over his 3 plus.

And what is the resulting phenotype?

Yeah, so it's his plus.

Now, we really don't know the answer to that question

until we do the experiment.

So these, and I'll come to this idea in a minute.

These are all essentially experimental results.

When you're talking about something

being recessive or dominant, you actually

have to do the experiment.

But in general, if one strain is deficient for an enzyme

and the other strain has that enzyme,

and you put them together, having this 50% amount that's

present in the diploid relative to his 3

plus his 3 plus is enough.

And that's generally true of most traits and most genes

that half as much is enough to restore function, right?

So we refer to this trait as a recessive trait

because importantly, it is a trait

that is present only in the homozygote here.

But it's not present in the heterozygote.