Mitosis

Hi in this video we're gonna be talking about mitosis. So mitosis is cell division, it's a type of cell division and this produces identical daughter cells. So these 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 or more kidney or more liver or more stomach or whatever it's producing. So there are many different steps. You probably remember this from an intro class but we're still gonna go over it because you will be tested on it. So the first phase is the inter phase 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 an interface. So 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 growth, right? If it's gonna 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. But 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 G. One phase now. But 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 G. Zero 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 if they just never got big enough to begin with but G zero, it just means that the cell isn't ready to replicate the D. N. A. Some cells stay there the whole life. So I'm only for a short time. Um But if the cell isn't ready to replicate the D. N. A. This is what it enters into. But if the cell checks it and it says you're big enough you're ready to replicate. It goes onto the S. Phase and this is the DNA replication phase. And so the sister chroma tides 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 chroma tickets. 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. Um This is a much smaller and shorter the growth phase it's really just a pause to make sure that the D. N. A. Was replicated correctly. But it does grow a little bit during this time and then once it's grown enough and the DNA is replicated it can enter into pro face. It's just the next step. But let's go over this example. So here's interface. You have DNA replication occurring the cells preparing for mitosis. Um So here you start with the cell you can see there's replication happening. Um Different things are happening in the sell different things are forming, it's growing etcetera etcetera etcetera. Then we get to the good stuff. So the first step is pro face. First step of cell division. And what happens in pro fes is there the structure called the central's which if you noticed are here in this image these guys these spider looking things and these are made of micro tubules and they begin to move to opposite ends of the cell. And these are super important in um division. Because micro tutorials that are given a special name called spindle fibers. They extend out from the central's and they are what allows us what allows the cell to grab onto the chromosomes which we'll talk about further steps. But the first step here is that the central's begin moving to the opposite ends of the cell. The nuclear envelope separating the D. N. A. From the rest of the cell begins to break down um Chrome button which if you don't remember is DNA plus protein. This begins to condense and there's an important protein here cohesion and it comes in and is beginning to hold the two sister chroma tides that were made before that were replicated together. Because you need to keep those together to make sure that they divide properly. So this is a really important protein. So here we have pro phase. Um Not too much happening essentially the main part here as you can see the central there now on opposite ends of the cell and the chroma tin is now condensing into chromosomes. And so these are the chromosomes that 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 pro phase. And you can see them here now after pro phase there's a short cycle called pro meta phase. And this is where the chromosomes moved to the middle. So the portion of chromosome movement is pro meta phase. Once the chromosomes have arrived in the middle, we call that meta phase. So the chromosomes are no longer moving in meta phase. Right? That happens in pro meta phase. Where the chromosomes move may get to the middle, that's called meta phase. Now they line up in the middle at a place we call the meta face 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 kinetic or and it begins attaching to the chromosome centrum ear. And this is important because the kinetic core is sort of that mediator between the spindle fibers that are attached to the central's and the chromosomes. So that attaches the chromosomes to the cell that is going to be made. So the kinetic core is super important. So if you have your chromosome here you have your central meter here. Here we go. I'll just point here centrum here and then you have the kinetic or come in. This is the kinetic core. It comes in and this is what attaches the cell to the spindle fiber micro tubules 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 the opposite in right now we're meta phase, they're just sitting in the middle. These things are attaching right now. They're not pulling any where they're just attaching and um cohesion which I mentioned before was responsible for keeping those sister chroma tips 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 pull be able to be pulled apart. So cohesion has to be degraded. But this is this is kind of an interesting face. So here we have promo to face and you can see the chromosomes are moving and when they arrive at meta phase 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 kinetic or cohesion 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 an A. Phase. And phase the chromosomes move. They begin to separate into other daughter cells at this point the cohesion that was keeping them together completely degraded. And we call this disjunction. If the chromosomes are separated correctly we give it the term non disjunction which you may have seen in some of the other videos if they're separated incorrectly. And non disjunction causes things like down syndrome and other chromosomal abnormalities. So anna phases, they're beginning to move tele phase is the final stage of mitosis. They're they're still moving. They're further apart now and pretty much a complete set of chromosomes is present on each side of the cell but the cells haven't completely separated yet. And psychokinesis which happens after cell division or sell or after mitosis psychokinesis 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 um and animal cells and also in plant cells. The cleavage bureau is created. And that sort of imagine it's that cell membrane and that allows um that cell membrane to eventually pinch off and form two cells. So um we're starting here, I start here and go up. Um So here we have an A phase. You can see the sister chromosomes are separating. They're getting pulled pretty far apart in tele phase. And then you have psychokinesis where the cleavage bureau was here. And you can see that there's this like curve here and where they were eventually they were once connected. And then they separate through that pinching off of the plasma membrane via the cleavage furrow. So I think that's yeah 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. Um But yeah that's mitosis. So let's now turn the page

Okay. So now let's talk about cell cycle regulation. Now the cell cycle controls the growth of us, it controls the growth of offspring, it controls death and all sorts of different things. So the cell cycle must be intricately regulated because if it's not if you have unregulated cell growth you get diseases like cancer. That's exactly what cancer is. It's unregulated cell growth and that creates a tumor. And that's obviously bad. I mean everyone knows cancer is bad. So um the cell really wants to make sure that it's dividing in a regulated fashion. And so it has a lot of checkpoints that have been set up to make sure that the cells replicating or dividing correctly. So uh these checkpoints, there's three main ones that you need to know. There's more than this. There's a lot more than this. But the three ones that you need to know for this course are the G. One S. The G. Two. M. And the M. Now the G. One S ensures cell size is appropriate. And this happens before the S. Face. So this happens in G. One, right? G. One before S. Essentially. So this one make sure that the cell size is appropriate for the D. N. A. To replicate. You have the G. Two. M. Meaning that it happens after G. Two. But before um before mitosis. And this ensures that DNA has been replicated correctly. And then you have the M. Checkpoints. And this actually happens during or before during a meta phase. And this ensures that the spindle fibers have been correctly attached to the chromosomes to make sure that when they're pulled apart the correct disjunction happens and not non disjunction. So here's super important checkpoints. Now these checkpoints um you know it's not always think of checkpoints is like I don't know police checkpoint or something where people are standing out there and they have like the barricade and they're saying oh you can't pass like let me see your I. D. And license. And this is kind of the same for the sale cycle except for instead of police what you have are proteins and these proteins. The two important ones are called cycle independent kindnesses. And these are the proteins that um add phosphates to molecules, kindnesses are responsible for adding phosphates. That's their purpose. And phosphates have an interesting ability on proteins because phosphates can activate or deactivate the sale cycle. So these are kind of like phosphates are kind of like the police, right? The police can pull you over and stop you or they can check your I. D. And say you're good to go. And so phosphates say okay you're activated, you're good to go or no we're gonna stop here. And so the cycle independent kindnesses go and they add those faucets onto proteins. They say here the police are going to be here and checking all of these people coming by or all these other proteins coming by now the cyclone dependent penises. So these policemen in the cell, they are actually controlled by Cyclones. Cyclones you can kind of think of as the police chief or the mayor or something. Now. Cyclones, these are the master control proteins and these ensure proper regulation. And so after something has happened correctly in the cell certain cyclones are released and that releases those cycle independent highnesses, if something incorrect happens if D. N. A. Isn't replicated properly then different cyclones are released and that activates other cycling dependent kindnesses telling them okay, shut off the cell cycle. So I'm not giving you a lot of details. There's a ton of different cyclones and cycle independent penises and we don't we don't necessarily need to know, you know, C. D. One or cycling one interacts with you know, psycho independent six. And this does this at this point in sales cycle, that's way too much, you don't need to know that. But just know that there's a bunch of different proteins and different combinations of them either activate or deactivate the cell cycle. So here's an example. You commonly see the cell cycle written in a circle. Right? So here we have G one S. Phase. That's DNA replication G two. And then mitosis mitosis is generally a very short portion of the cell cycle interface is much longer. And throughout here you can see I've written some different checkpoints. Here's a G. One check point. So this is the one happening before DNA is replicated. You have the s checkpoint, making sure that DNA has been replicated correctly. You have the G. Two checkpoint making sure that everything's good to go before um mitosis happens. There's another one here called Start I didn't talk about and then you have the spindle assembly checkpoint making sure that. So this is what's called the M checkpoint above. And let's make sure that the spindle fibers are attached correctly. And so every time the sale goes through the cell cycle it goes past the past each one of these checkpoints plus other ones that I'm not even talking about um to make sure that the cell is being replicated appropriately. So with that let's not move on.