- All of the examples of cell-cell signaling
that we've looked at so far have been cells
within an organism, but what I want
to do in this video is point out that you can
also have cell-cell signaling
between different organisms,
and even between different unicellular organisms.
And so what we have here, these are pictures
of yeast cells.
Yeast are unicellular eukaryotic
organisms which means they have
a nucleus and they are officially fungi,
and what you see happening in this picture
right over here, it looks like you have
a second yeast cell budding off of the first one.
This happens fundamentally through mitosis,
but yeast can actually reproduce in two different ways.
It can reproduce asexually through mitosis like this,
but it can also reproduce sexually, and that's
where we're going to talk a little bit
about signaling between the cells, and they do
this with mating factors, and this is just
an interesting discussion of yeast generally.
So if you start with a yeast cell,
so this is a yeast cell right over here,
and this is a diploid yeast cell,
so it has its full complement
of chromosomes.
So it's going to have 16 pairs,
16 pairs of chromosomes,
or 32 total chromosomes.
That's a diploid.
So through mitosis it could split apart
and it would do so like this.
The one that buds off for most species
of yeast will be smaller, although they're some
that kind of do more even mitosis.
But through mitosis you could produce
two diploid yeast cells
just like this.
So all of these characters right over
here are a diploid.
And actually, just to stress,
that they are eukaryotes.
Let me draw a little nuclear membrane
in here.
But it can also divide
through meiosis,
so this is mitosis right over here,
mitosis right over here help produce
these two, but you can also go through
meiosis.
Meiosis,
and if we go through meiosis we're going
to produce four cells and what's interesting
about these is that these will come in one
of two varieties, and you could view these
as the sex of these yeast cells.
So they could either be
in the type a variety,
so let's make these two type a.
So that's a type a, that's a type a right there,
or they could be in the type alpha variety.
So this is a , I'm using
a different color.
So this is
a type alpha,
and this is another type alpha,
and all four of these are going
to come through meiosis
from this original diploid cell.
And since we went through meiosis,
each of these are going to be haploid.
So these are all going to be haploids.
This is going to be 16 chromosomes total,
16 chromosomes, chromosomes,
each of them.
So they're all going
to be haploid
and in fact, each of these can,
to some degree, live a life of their own
and continue to reproduce through mitosis.
That's worth pointing out,
that's interesting.
You normally wouldn't associate sex cells,
if it was anything but humans you wouldn't think
that sperm cells would be able to replicate
on their own, or egg cells would be able
to replicate on their own but in the case
of yeast, they actually can.
So this type a cell could turn into,
through mitosis, it could turn into
two type a cells.
So this is all just interesting background, mitosis.
So even the products of meiosis can
then mitose themselves.
That's all interesting, this is all the set up
to think about how they actually signal
because these, these haploid cells
that are producers of byproduct of meiosis,
these are, these right over here,
they all came from the original ancestor,
which is actually fairly typical when we're talking
about reproducing and kind of pseudo-sexually
reproducing yeast cells.
They don't have to, maybe one of the type alphas
came from another ancestor, but what happens is
they each release what's called mating factors,
or you can consider them to be mating pheromones.
So this type a right over here, it's going
to produce, it's going to produce
the type a mating factors and it's going
to release them into the space around it.
So it's releasing those type a
mating factors and then
the alphas are going to release the type alpha
mating factors.
So I'll do that in this orange-ish brown color.
So they're going to release the type alpha
mating factors.
And what happens is each type has receptors
for the other type's factors.
So the type a is going
to have receptors
for the type alpha,
for the type alpha.
It's going to be in all over the outside
of the cell, and the type alpha's going
to have type receptors for the type a.
Receptors, I'm going to do that in orange.
It's going to have receptors for the type a.
Let me just draw it like that, that's not exactly
what they look like, or actually nowhere close
to what they look like but it gives you a sense
of things.
It gives you a sense of things.
And as through these receptors it starts bonding
a lot of, say we're on the alpha cell right over here,
it starts bonding to a lot of these mating pheromones,
these mating factors.
Then the yeast cell itself, it doesn't move
but it starts growing in that direction,
it starts growing in the direction where
it says, "Hey, I'm sensing a lot of this mating factor."
So it starts growing in this direction,
it starts doing something like that.
And the corresponding thing is going
to happen for the type a cell.
It's going to have the alpha factors attached
onto it, and because of that it says, "Okay,
"I'm sensing once again, the factors."
The ligand, it attaches to the receptor,
that's the sensing part.
Then you have signal transduction and it's actually,
it's not too dissimilar
from the MAPK,
the MAPK transduction pathway
that we talk about in other videos.
And then the response the cell has is,
once again, to start growing in the direction
of where it seems that these mating factors
are coming from.
And these kind of elongation, these outgrowths
of these haploid yeast cells, they're actually
called shmoos.
So that right there, that is a shmoo.
And you might say, where did that come from?
Well, it came from this character right over here,
which was a cartoon character in the middle
20th century, and I'll, you know...
well, let's avoid any snickering over the course
of this video, but that's what it looks like.
And as they grow to each other,
that actually allows them to join together
and form another diploid,
another diploid yeast cell.
So if this process were to continue,
if this process were to continue,
these two shmoos will join together
and the genetic makeup joins together
and so we could get another diploid cell.
And now this diploid cell is going to have
kind of a new combination
of DNA, so it could benefit from some
of the variation
of sexual reproduction.
This is diploid.
Although for yeast cells,
they've like studied it because it's not clear
that these are necessarily coming from
a separate gene pool.
These actually could have the same
ancestor molecule so it is a bit
of an open question in biology
of why do they have this type of sexual
reproduction, especially when they're
coming from when these are essentially
sibling cells as a byproduct of meiosis.
But anyway, the whole point of this
is one, to appreciate these fascinating things
that are happening even at the level
of yeast cells, and that you do have
cell-cell signaling, not just within
an organism, but across organisms,
especially, or even unicellular organisms.