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- 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.