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Cell Cycle and Mitosis · Acedra
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Cell Cycle and Mitosis
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0:00
All right, so we talked about DNA because it's really important. And we understand DNA and replication, and how that is important for the cell cycle. And that's just the life cycle of a cell. We call it the cell cycle. So when we look at the life cycle of a cell, most of the time, our cells are hanging out in this phase outlined in purple here. So if this whole circle represents the life cycle of a cell, most of the cell cycle is spent in interphase. "Inter" means between, so it means
0:34
between the phases of mitosis, when a cell might divide to replace itself, to reproduce, or to repair. So most of the time, our cells are just hanging out doing their things, undergoing metabolism, doing what that cell does. So we call that process, again, interphase. Then, if that cell needs to divide, there are triggers for it to begin to enter the division stage, where it's going to divide and make a new cell. And that dividing phase is the light blue portion of this pie
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chart, and that's mitosis. So that's why it says M. So we can take the cell cycle and break it up into four parts. There's the G1 phase, where the cell is growing in size just after it was produced and is a new cell, just after it left mitosis. Then there's the S phase, when it's getting ready to divide again. So DNA is replicated during the S phase, so we're going to make a new copy of DNA because the cell needs to divide again. And then the G2 phase is when it adds
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different organelles necessary for mitosis, which are the centrioles. Do you remember when we talked about the centrioles cells back in the cell unit? We'll talk about those again. And then it enters into mitosis again. But not all cells divide and replace themselves. Like in the human body, we know that nerve cells, like brain cells, don't replace themselves. They do somewhat. Like if a part of a nerve cell is damaged, it can repair itself. But it can't make brand new nerve cells.
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Once we're done growing and puberty is complete, we can't make more brain cells. And that's why it's important that we take care of our brain cells, right? And we know certain things kill brain cells. Marijuana use kills brain cells. I mean, there's other drugs, too, that do, as well, but common ones that people do more regularly. Marijuana and alcohol we know kills brain cells. And they talk about the damage that occurs during the college years, especially in young men whose
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brain development sometimes doesn't finish until age 24. The damaging effects of marijuana and heavy alcohol use in the college year interferes with learning and memory, and can do lifelong damage, as the brain is still growing, and then we're kind of introducing these other toxic substances that kills brain cells as they're growing. So when you need your memory the most is in college, right? And if you're damaging it with-- marijuana use, especially, we know
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is linked to short-term memory loss, which can be lifelong. So we've got to really take care of our brains and take care of those cells that don't divide and replace themselves. So G1, S, G2, and M. Big concept again is DNA replication occurs during the S phase of interphase, so make sure you highlight that. It's in the S phase of interface is when this DNA replication that we just talked about occurs. And again, it has to be in cells that are dividing, repairing, reproducing.
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Or cancer, heaven forbid, right? OK, so we're going to look at these steps. Prophase metaphase, anaphase telophase and then cytokinesis, and then you get your new cell. So here is the cell that we have. It's growing, getting bigger, making new DNA, so we have an extra copy for the new cell, and then out pops that second cell. So we're going to talk about that. So before the next division, during this growth and S phases, the cells are getting bigger.
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The organelles are doubling, and DNA is replicating. Because we need a new set of organelles and DNA for this new cell that's going to form. Because what we have is, you have one cell, like this, and then as mitosis continues, that cell starts to pinch in in the center, and it's eventually going to divide into two. So if this cell is going to divide into two cells, we need some DNA for this cell, this side, we need DNA for this side. So in order to get two copies of DNA,
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we have to replicate that DNA in the S phase of interphase so each new cell, when this process of cell division is done, has a copy. And it has to have a copy of essential organelles, as well, so that's going to happen in that G2 phase of interphase. So interphase, like we said-- most of the cell cycle is when our cells are just hanging out and dividing. So 3/4 of the cell cycle is in interphase. It's between the phases of mitosis--
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3/4 yeah. It performs its usual functions, like metabolism. And we said nerve cells do not really complete the cell cycle. They do not divide. So once you are done with puberty, you have all the nerve and muscle cells you'll ever have in your life. All you can do with muscle cells is make them bigger or smaller. The same thing with fat cells. All you can do is make them bigger or smaller. So that's why childhood obesity is such a problem. If kids have more fat cells they're
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bringing into adulthood, that makes it harder to maintain weight as they get older because they'd have more fat cells and a person that didn't have excess calories and a sedentary lifestyle during childhood. And with video games and phones, people young people are spending a lot more time sitting than they ever did before. Teenagers, especially teenagers are in sports. Some of them, right? And they're running around, getting good exercise. And then they go from that active lifestyle to college,
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where they're sitting and studying, and then sitting on their phones. You know, just hanging out and watching TV, right? Netflix bingeing and all the things that we do to pass the time, especially in the winter. That's just really bad for fat cells and gaining weight. So anyway, back to this. So interphase, G1, happens right after mitosis is done. So right after what mitosis is complete, we have the recovery phase. So that's when everything kind of goes back in order.
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It's really hard to write with a mouse. I need to get something better. So right after mitosis is complete, we have the G1 phase. And the S phase is if we need to enter mitosis again, we're going to make another copy of DNA so we can do that. So in the S phase, you have one chromosome, like this. It's all coiled-up DNA. And after DNA replication is complete, you have this X-shaped structure. Well, this is really bad.
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And it's combined in the center. And then you have the centromere that connects the two. So you go from a single-stranded chromosome to a double-stranded chromosome with chromatids, with a centromere that connects them. And then the G2 phase is after that, like we said, and that's when it's getting ready to start mitosis again. So this is before mitosis, just before mitosis.
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So right after mitosis is G1. Just before mitosis is G2. Again, looking at this diagram, this is G2, getting ready for mitosis, and G1 is recovering from mitosis and growing in size.
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Because we talked about-- cells aren't going to continue to grow bigger and bigger and bigger because it loses efficiency the bigger a cell gets. So instead of getting huge, the cell divides into two, so we have a greater surface area for exchange with the environment, right? That was one of our test questions. Surface-to-volume ratio is really important for cells, and that's why they will divide rather than keep growing in that G1 phase. So there's checkpoints to stop that G1 phase and enter
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into the S phase if we need to divide that cell, especially, again, during growth, you know, puberty. So mitosis is defined-- if we define the word "mitosis," it's division of the nucleus. And cytokinesis is defined as division of the cytoplasm. So mitosis divides the chromosomes, and cytokinesis pinches off that plasma membrane and divides the cell into two parts. So we need to take those replicated chromosomes and split them apart so each side of the cell is going to get a copy of DNA.
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So we have these signals that are going to stimulate this process, and you don't need to know the names of those. But just so you know that there are internal and external signals to tell cells to divide. And that's what controls this movement around the cell cycle. And again, when does mitosis occur? Like we said, growth and repair?
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When something becomes an embryo for reproduction, and any time we're healing a cut or a broken bone, those are examples of the repair , part right? So that's when mitosis occurs.
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So when we look at what is going on with the DNA, what we call DNA, in the nucleus. When it's not very well-organized, we call it chromatin. It kind of looks like spaghetti. So chromatin is just kind of loosely bound DNA. And then when it becomes chromosomes, then it has a very structured appearance, and it looks like a coiled spring.
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So a chromosome is much more organized than chromatin. Yes? So what's the difference between chromatid and a nucleoid? Nothing really. Nucleoid is kind of just the space for DNA that is that-- genetic material of a bacterial cell, yeah. It's very similar. Because chromatin is loosely coiled DNA that's not wound. Yeah. Yeah, good observation. So if we look at the number of chromosomes, we can see that it's not really-- it doesn't make you more advanced to have
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more chromosomes because, look, we have a plant with 1,320 chromosomes. That's a lot of chromosomes. So it doesn't mean it's a more advanced organism because human beings, Homo sapiens, contain 46 chromosomes. And a goldfish contains 94 chromosomes, and I hope we're more advanced than a goldfish, right? So it's not the number of chromosomes that determines complexity. So we have two sets of chromosomes. So we use the term 2n to describe two sets of chromosomes.
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And we get one set from the sperm that formed us, and the other set we got from the egg that formed us, so we have two exact copies of 23 chromosomes that define human beings. So we have 23 from sperm, 23 from egg. Together that makes 46. We have two sets of chromosomes. We'll talk more about that when we get into the genetics chapter. But just so you understand what a chromosome looks like, this is a replicated chromosome. So this chromosome has been replicated.
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This is at the end of the S phase. We went from the single coiled DNA strand to this X-like structure. So we call these identical chromosomes that were replicated, we call these sister chromatids, and they're held together in the center by a centromere.
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Kinetochore. Those are special proteins that allow the spindle from the centrioles to attach and pull those sister chromatids apart. Because we have just one side of these-- before DNA is replicated, like I said, all we have is just a single coiled DNA strand, and it looks really ugly because I'm drawing with a mouse. But that's what we have before DNA replication. After DNA replication, we have a new copy, and that new copy is bound to the old copy by the centromere, OK?
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So again, we said DNA replication is semi-conservative, right? So each strand is going to have a copy of the parent strand, each chromatid, and the new strand, which we call the daughter strand. So this is a chromosome that has been replicated that looks like an X. And each half of the X is called a sister chromatid. OK, then we have these specialized organelles called centrioles. They are essential for making the spindle. And they're going to pull apart these chromatids,
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so each half of the cell is going to get a copy of DNA, one chromatid going to each side of the cell. So the centrioles are going to produce a spindle, which will attach to the centromere, specifically the kinetochore, and pull those chromatids apart. So let's go to our worksheet. So you have this worksheet here, and I'll pull it up.
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I don't know which one it is. I just saw it-- mitosis right here. OK. So a typical cell that is getting ready to divide is going to have visible centrioles and chromosomes, as it's in the first stage of mitosis, which we call prophase. So letter A here is an example of a cell that's in prophase. And how you know it's in prophase is I can see visible chromosomes. So I'm going to see if I can add a note here. So this structure here is a chromosome. So let's label these things together
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so you know what you're looking at.
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"Chromosome." That's what the X-like structures are, right?
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This structure is the nuclear membrane. And we should start to see that breaking down.
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That should disappear. Because I can see here it's disappeared. So we want that nuclear membrane to disappear. And then these structures are what again? Yeah, those are centrioles.
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OK, so this particular phase we would call early prophase because the nuclear membrane is still present. "Early prophase."
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So if I go down and I look in these actual slides of a root tip, if I want to find early prophase, I can see here that the nuclear membrane is starting to break down, and I'm seeing these dark structures here in the center. These would both be examples of prophase. Now I can contrast that to this cell. This here would be an example of interphase because can you see visible chromosomes here, in this one? No. That's because the DNA exists as chromatin, which doesn't stain. It's very loose.
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It's not tight and coiled. It doesn't pick up a lot of stain. So this dark structure here is a nuclear membrane that's crowding out the whole nucleus. So this is interphase. So let's label that so we just know that that's what an interphase cell looks like, so we can kind of use it as a reference frame. So we're going to talk about that. We're going to talk about that. So A, then, is more along the pathway of cell division than this. This is that resting phase that nothing's going on.
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Here, we see the nuclear membrane as kind of spotty and breaking down, and we can see chromosomes showing up. So this is prophase. I'm just going to abbreviate "pro." You can finish writing it. Prophase. All right, now, if we look here, I can see the centrioles have made a spindle, and they split my X's here. See those chromosomes? It appeared as X's. Each sister chromatid, each half, is a sister chromatid. They were pulled apart during a special phase, which is called anaphase. "Ana," "apart."
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[STUDENT SNEEZES] Bless you. So anaphase is "ana," "apart."
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And again, these little lines here are called spindle fibers. They attach to the centromere of my sister chromatids. "Spindle fibers."
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And the centrioles secreted those spindle fibers, and they pull them apart. Here they're lined up in the middle of the cell. This is what happened-- before they were pulled apart, these have to line up in the middle. So we call this stage, when the chromosomes are lining up in the middle, before they're pulled apart, that's called metaphase. "Meta," "middle."
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And in this-- so let's start with anaphase. Let's go down here. Which one of these represents anaphase?
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C represents anaphase. Is there another one? E. Say it again? E. Yeah, C and E are both anaphase.
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And then which one of these represents metaphase?
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Yeah. See how they're lined up in the middle? Yeah. So that's metaphase.
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And then once all of-- so once they land-- so we start out with prophase, then they line up in the middle. Then they're pulled apart. Then you'll notice each sister chromatid is in each cell. So this is going to continue to pinch here. And this special structure right here, where it indents-- I need to be able to type. I wonder if I can somehow type. I haven't figured it out yet, though. Hm.
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Yeah. I don't know how to do that. I'm just going to-- I feel you can definitely right-click. I'm certain. There's nothing. This is called a cleavage furrow in animal cells. "Cleave furrow," F-U-R-R-O-W. That's what this indented region is here. And that's going to continue to pinch inward. The process of forming of cleavage furrow is called cytokinesis, this process of pinching inward. And that pinching inward will continue until we have two new cells. And then the nuclear membranes reform.
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The nucleolus reappears. Because this is the nuclear membrane, this is the nucleolus, and we have two new cells. So this is late telophase, this process. This is late telophase.
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So this is way late. So earlier, we would see clusters of chromosomes, but maybe no nuclear membrane forming yet. So can you see that in one of these cells down here? The exact point when it switches from anaphase, but when it switches from anaphase to telophase, is it like second that when cytokinesis happens? Yes, when cytokinesis is complete-- because cytokinesis starts in anaphase. And when it's complete is at the end of telophase. So anaphase kind of flows into telophase.
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So we'll be real clear when I show you. What I usually like to do is think of telophase as a figure 8 appearance, but we're looking at plant cells here, so you're not going to see a cleavage furrow in this case. What you'll see is a cell plate that forms in plant cells. And I know these are plant cells because look at how square and regular each cell is. Can you see the cell wall here? We know plant cells are very organized because of that cellulose in the outer cell wall
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that animal cells don't have. So which one of these is telophase, then? Obviously this one, right? If I look at this, can you see how it's still one cell yet, but the DNA is starting to uncoil and form chromatin? And then pretty soon we're going to see a cell plate form to completely divide these new cells. So this is telophase here.
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OK? So if we follow the order of things, it goes from prophase, to metaphase, to anaphase, to telophase. That's kind of the order of things. 1 2 3.
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So let's-- yes, so here's a thing. Interphase is where cells hang out. And then it enters into mitosis. So the order is I, P, M, A, T. IPMAT. If you're taking an anatomy class, I tell my students, I Passed My Anatomy Test. I, Interface; P, Prophase; Metaphase, Anaphase, Telophase. So with that information, can you match each of these to the correct description? I'll let you guys do that on your own.
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And there's an animation of mitosis in the Blackboard folders. You can see all these things in action, and I recommend you look at that.
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