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Mitosis, a key part of the cell cycle, involves a series of stages (prophase, metaphase, anaphase, and telophase) that facilitate cell division and genetic information transmission. Centrosomes and microtubules play pivotal roles in orchestrating this complex process, ensuring the successful replication of cells. Created by Sal Khan.

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Video transcript

- In the previous video, we talked about interphase which is the bulk of a cell's life cycle as it grows and its DNA replicates, and it grows some more. And now, we're gonna talk about the actual cell division. We're gonna talk about mitosis. And if you wanna be precise, mitosis is the process by which this one nucleus will turn into two nuclei that each have the original genetic information. Now, as we exit mitosis, we get into cytokineses which will then split each of the nuclei into a separate cell when we split the cytoplasm right over here. We split, or the cell essentially turns into two cells. Well, let's see how all of this happens. So the first phase, and I'll leave the end of interphase right over here. We have this big cell, our DNA has been replicated. We have two centrosomes right over here. The first phase of mitosis involves the cell and I might draw a little bit smaller just so I have enough space here. So, involve. So, this is the cell right over here. So we're gonna go to this phase right over here. And a few things start happening. One, the DNA, the chromosomes go from being in their chromatin form where they're all spread out to kind of a more condensed form that you can actually see from a light microscope. So for example, that magenta chromosome which is now made up of two sister chromatids after replication, we talked about that in the interphase video. It might look something like this in a ... It might look something like this if you were to look in a microscope. It's unlikely to be magenta but it's gonna have kind of that classic chromosome shape that you're used to seeing in textbooks. And it has the centromere that connects these two sister chromatids. Right now, both of these two sister chromatids combined are considered to be one chromosome. Even though before replication it was still considered, the magenta stuff was still considered to be one chromosome. And we can draw the blue chromosome. Once again, it's now in the condensed form. That's one sister chromatid right over there. That's another sister chromatid. They are connected at the centromere. So they're condensing now as we enter into mitosis. And the nuclear membrane starts to go away. So the nuclear membrane is starting to go away. And these two centrosomes are starting to migrate to opposite sides of the cell. So one of them is going over here and one of them is maybe going to go over here. So they're migrating to opposite sides of the cell. And it's pretty incredible. It's easy to say, "Oh, this happens and then that happens." Remember, this cell doesn't have a brain. This is all happening through chemical and thermodynamic reactions and the way, just based on certain triggers of where the cell is and its life cycle. It's amazing that this is happening. It's amazing that the structures, and what sometimes we consider to be a simple thing but it's actually incredibly complex thing. It kind of knows what to do even though it has no intelligence here. And a lot of what I'm talking about, the general overview of the process is well understood but scientists are still understanding exactly how do the different things happen when they should happen and by what mechanism and what's actually happening. Sometimes, molecular or atomic basis. But anyway, this first phase of mitosis, the nuclear envelope, the nuclear membrane starts to disappear. The centrosomes migrate to the opposite ends of the cell. And our DNA condenses into kind of the condensed form of the chromosomes. And we call this prophase. Prophase. Prophase of mitosis. Now, in the next phase. In the next phase, let me draw my cell again. Drawing that same green color. In the next phase, your nuclear membrane is now gone. And the chromosomes start lining up in the middle of the cells. So you have the blue one right over here, the blue one that's one sister chromatid. Here's another sister chromatid and they're connected at the centromere, not to be confused with a centrosome. And then, here's the magenta, one of the magenta sister chromatids and another one. And once again, it's not magenta in real life, I'm just making it a magenta because it looks nice. That's the centromere right over there. Our centrosomes are at opposite ends of the cell at this point. At opposite ends of the cell. And you might have heard of the word, let me label this again. I labeled it in a previous video. That's centrosomes. Centrosomes where the two sister chromatids are connected. That's a centromere. Centromere. Now, you might heard of the word centrioles. Centrioles are actually, they help, they are exist inside the centrosomes. They're these two kind of cylindrical looking structures. Each of the centrosomes have two centrioles. But for the sake of this video, you can say, "Well, the centrioles are just part of the centrosomes." But I'm just listing you all the words that involves centri something. Centrioles right over there and you have two of them per centrosome. So hopefully, that helps clarify some confusion. But these things line up. And a lot of what you're about to see, this orchestration, these things moving around in the cell, things lining up and soon things pulling apart, these are all coordinated with actually a fairly sophisticated mechanism of kind of a scaffold of this kind of ropes, these microtubules. And the centrosomes' role, until now I've just been kind of drawing. I mean, what do they do? Well, the centrosomes' role is these microtubules extend from them to each other and to the centromeres of these chromosomes and to a large degree, they're not the only actors here. They help pull and push things in the right way. So these help keep make sure that the two centrosomes push away from each other. And then as we'll see in the next phase, that they're able to pull one of each of the sister chromatids from each of the pairs towards each of them. So this right over here where you see where the nuclear envelope is now gone, the chromosomes have been lined up just like this and your centrosomes are on opposite side of your cell. We call this the metaphase. We call this the metaphase of mitosis. And then you can imagine what's going to happen next. What's going to happen next? What's going to happen next is ... Let me, I don't wanna draw it too big 'cause I wanna be able to fit it all in one page. What's going to happen next is that those microtubules are going to start pulling on each of the sister chromatids. Let me draw that. So you have this centrosome right over here. You have all these microtubules in your cell. You have the centrosome right over here, all those microtubules. And this one is going to be pulling, is going to be getting one of the blue chromatids to pull towards it or to move towards it. So let me draw that. So this is one blue chromatid right over here. aAd this one is going to be pulling the other blue chromatid towards it. And same thing for the magenta. And same thing for the magenta. So that one's being pulled to that way. And this one is being pulled that way. And just in case you're concerned about the, some of more of the words of vocabulary involved, the point it was this microtubules connect to what used to be sister chromatids, but now that they're apart, we now call them chromosomes. When they emerge, this is one chromosome and they have two sister chromatids. But now they're apart, we would actually now consider this each an independent chromosome. So now, you actually have four chromosomes over here. This point right over here, we call it kinetochore. And even exactly how that interphase works and exactly how things move is not fully understood. Some of this stuff is understood but some of this is still an area of research. So something even as basic as cell division, not so basic after all. So this right over here where you can start to see the DNA kind of migrating to their respective sides of the cell. We call this the anaphase. We call this the anaphase. And that takes us to the last formal phase of mitosis and that is called telophase. Telophase. And in telophase, I'm gonna do my best shot to draw it. And you could already see I have started to draw that the cellular membrane is starting to pinch in kind of in preparation for cytokineses, in preparation for the cell splitting into two cells. So I'll do it a little bit more. Cytokineses is usually described as kinda being a separate process in mitosis although it's obviously they are, they kind of together help the cell fully turn into two cells. So now in telophase, so you have this, what used to be a sister chromatid, now we can call it a chromosome on its own. And we have this chromosome. And we have this chromosome right over there. We have to do it on both sides. So you're there and you have this right over here. And actually, let me draw it a little bit different. Because at this phase, you're really starting to unwind what happened at prophase. So prophase, you have the disappearance of the nuclear membrane and you have the condensation of the chromosomes into this form here. Telophase, that's unwinding a little bit. So actually, let me draw the two centrosomes. So you have one centrosome with that same color that I had before. So you have one centrosome right here. You have another centrosome right over there. And now, the DNA, this blue DNA, this chromosome is now here but it's starting to get a little under wind a little bit, same thing on this side right over here. Magenta chromosome is on here but once again, it's starting to get a little bit unwound. Same thing over here. And you start to have nuclear membranes forming around the DNA. So once again, it's kind of redoing what was undone in prophase. Undone in prophase. And so, when you're done, essentially, you're gonna have these nuclear membranes. The DNA is gonna go back to its chromatin form. And then you have cytokineses. And cytokineses is the process by which this gets fully pinched together and you have two separate cells. And some folks will say, "Oh, it kind of begins in anaphase "and finishes after telophase." But it's kind of happening near the end of mitosis in parallel with it. So this is cytokineses. Cytokineses. Cytokineses right over here. This essentially is how this larger cell that had two nuclei, how this divides, fully divides into two cells. And at that point, you're back to this phase of the cell cycle. Now, each of these two cells are going to go through interphase, G1, S-phase replicate their DNA, and G2 phase goes some more and then it go through mitosis again. And then these two will turn into four cells. Anyway, hopefully, you enjoyed that.