So, mitosis is cell division. It's a type of cell division that produces identical daughter cells. This is the type of division that the overwhelming majority of every cell in your body undergoes. So your skin cells, your kidney cells, your toe cells, literally most of the cells in your body undergo mitosis to produce more skin, more kidney, more liver, more stomach, or whatever it's producing. There are many different steps. You probably remember this from an intro class, but we're still going to go over it because you will be tested on it. The first phase is the interphase, and this is the initial stage of the cell cycle. And this is, kind of, the stage that's in between division. So it's not technically part of mitosis, but we always start with it first because it is what happens before the division actually takes place. And there's some really important steps that go on at interphase.
The first one is G1. G stands for growth. It's a growth phase, the first growth phase. And this happens before DNA replication. And this is actually when the majority of the cell grows. Right? If it's going to divide, it pretty much has to be double its size before it does, or else it would start losing cytoplasm and it would just start shrinking. The cells don't shrink when they divide, they just divide into two equal cells, so there has to be a growth phase where that cell expands. So, the majority of that growth happens here in the G1 phase. Now, it happens before DNA replication, so there's kind of a check the cell goes through right here and it says, you know, am I big enough to divide? If I'm not, what happens is the cell enters into G0, and this is a phase that the cell is not going to proliferate anytime soon. Now, it may wait until it's bigger and then enter proliferation. Sometimes, these cells never divide. They just never got big enough to begin with. But G0 just means that the cell isn't ready to replicate the DNA. Some cells stay there their whole life, some only for a short time. But if the cell checks it and it says, you're big enough, you're ready to replicate, it goes on to the S phase, and this is the DNA replication phase. And so, the sister chromatids are made here because they are the copy of the same chromosome. So you have a chromosome here, and it gets replicated. These two are now sister chromatids. And I'm mentioning this definition here because I'm going to use this term later, and I want you to make sure you understand what's going on.
Then, after the DNA replication, the cell takes another growth phase. This is a much smaller and shorter growth phase. It's really just a pause to make sure that the DNA was replicated correctly, but it does grow a little during this time. And then once it's grown enough and the DNA is replicated, it can enter into prophase, which is the next step. But let's go over this example. So here's the interphase. You have DNA replication occurring. The cell is preparing for mitosis. So here, you start with this cell. You can see there's replication happening, different things are happening in the cell, different things are forming, it's growing, etcetera, etcetera, etcetera. Then we get to the good stuff.
So the first step is prophase, the first step of cell division. And what happens in prophase, is there's this structure called the centrosomes, which if you noticed, are here in this image. These guys, these spider-looking things. And these are made of microtubules and they begin to move to opposite ends of the cell. And these are super important in division. Because microtubules that are given a special name called spindle fibers, they extend out from the centrosomes and they are what allows the cell to grab onto the chromosomes, which we'll talk about in further steps. But the first step here is that the centrosomes begin moving to the opposite ends of the cell. The nuclear envelope separating the DNA from the rest of the cell begins to break down. Chromatin, which if you don't remember, is DNA plus protein. This begins to condense, and there's an important protein here, cohesin, and it comes in and is beginning to hold the two sister chromatids together during prophase. So here we have prophase, not too much happening. Essentially, the main part here, as you can see, the centrosomes, they're now on opposite ends of the cell. And the chromatin is now condensing into chromosomes. And so these are the chromosomes that, you know, you normally see. This is what they look like. Well, they really only look like this when they've been condensed during the cell cycle. So this is what happens in prophase, and you can see them here.
Now after prophase, there's a short cycle called prometaphase. And this is where the chromosomes move to the middle. So the portion of chromosome movement is prometaphase. Once the chromosomes have arrived in the middle, we call that metaphase. So the chromosomes are no longer moving in metaphase. Right? That happens in prometaphase, so the chromosomes move. They get to the middle, that's called metaphase. Now they line up in the middle at a place we call the metaphase plate, and that's just the midline region of the cell. And an important thing that happens here is there's a protein complex called the kinetochore, and it begins attaching to the chromosome centromere. And this is important because the kinetochore is sort of that mediator between the spindle fibers that are attached to the centrosomes and the chromosomes. So that attaches the chromosomes to the cell that is going to be made. So if you have your chromosome here, you have your centromere here. Here we go. I'll just point here, centromere. And then you have the kinetochore come in. This is the kinetochore. It comes in and this is what attaches the cell to the spindle fiber microtubules that will pull it this way when the cell divides. And the same thing happens over here. Right? Not drawing the different colors, but essentially, it's the same thing that happens. And that pulls the chromosomes to the opposite end. Right now, we're in metaphase. They're just sitting in the middle. These things are attaching right now. They're not pulling anywhere, they're just attaching. And cohesin, which I mentioned before, was responsible for keeping those sister chromatids together, are now beginning to degrade, because you need to be able to pull these chromosomes to the opposite ends of the cell. And so in order to do that, they don't need to stick together. They need to be able to be pulled apart. So cohesin has to be degraded. But this is an interesting phase. So here we have chromatophase and you can see the chromosomes are moving. And when they arrive at metaphase, they're standing here in the middle and then lots of things are attaching to them. So you have the spindle fibers, you have the kinetochore, cohesin is breaking down. So the chromosomes aren't moving themselves, but all these things are happening to them or around them. And you can see that the cells, they begin to start moving again as we move into the next phase, which is anaphase.
Anaphase, the chromosomes move, they begin to separate into other daughter cells. At this point, the cohesin that was keeping them together completely degraded, and we call this disjunction if the chromosomes are separated correctly. We give it the term, nondisjunction, which you may have seen in some of the other videos, if they are separated incorrectly. Nondisjunction causes things like Down syndrome and other chromosomal abnormalities. So anaphase is there beginning to move. Telophase is the final stage of mitosis. They're still moving. They're further apart now, and a pretty much complete set of chromosomes is présent at each side of the cell, but the cells haven't completely separated yet. And cytokinesis, which happens after cell division or cell or after mitosis, cytokinesis actually divides the cytoplasm and creates those two cells. Now in plant cells, there's an extra thing made. This is called a cell plate. This forms the cell wall, and in animal cells and also in plant cells, a cleavage furrow is created and that sort of invaginates that cell membrane, and that allows that cell membrane to eventually pinch off and form two cells. So, we're starting here. Start here and go up. So here we have anaphase. You can see the sister chromatids are separating. They're getting pulled pretty far apart in telophase. And then you have cytokinesis, where the cleavage furrow was here. And you can see that there's this curve here, and where they were once connected, and then they, through that pinching off of the plasma membrane via the cleavage furrow. So, I think that's it for mitosis. So there's a lot of steps. Hopefully, it's clear what happens in them. If you just forget, go back and check. But, yeah. That's what's happening in mitosis, so let's now turn the page.
