Video Transcript: Phases of Mitosis

Phases of Mitosis

Hi. It's Mr. Andersen and in this video we're going to go through the phases of mitosis. If you ask anybody what they remember from biology, invariably it's the word mitosis. But they probably couldn't identify the different phases of mitosis. And you should be able to by the end of this video, look right here and say, Oh, that's going to be telophase, or that's going to be metaphase. Right there. And also, you should understand what's the point of mitosis.

Mitosis is technically the division of the nuclei, but we're doing is really making sure that we can make to exact daughter cells that are identical to that parent cell. And that's how you went from that fertilized egg. That was the zygote that was you to the billions of cells in your body. And it's how you replace cells as they die over time. And so you should know this, that mitosis is technically part of the cell cycle. And so a cell cycle is how a cell duplicates itself. And most of the time is in this orange phase, which is called interphase. Now, what's the cell doing, they're in that phase, during the G1 part of interphase, the cell is growing, so it's getting larger, it then enters into the S phase, or the synthesis phase, what's going on there, it's duplicating all of the DNA. And then finally, it goes into the G2 phase where this cell continues to grow. Now, some cells don't go through this, they don't copy themselves, like nerve and muscle cells. And so they enter into what's called the G0 phase, they never divide. But if you are going to divide after the G2 phase, then you go into what's called the M phase. And that's the mitosis where we're dividing the nuclei. What do you get when if you go through this whole cell cycle, you get two cells, and each of those independently go into their own cell cycle. And so you can think of it almost like a clock. And so cells are spending various amounts of time in each of these different phases, where are they spending most of their time in the interphase, the cells growing, copying the DNA and then growing again.

And so a classic lab that you do in biology is to look at cells that are undergoing mitosis. And so what we're looking at here are onion cells. And so we've dyed the chromosomes. And so what you see is that most of these cells not much is going on, you just have the nuclei on the inside. So they're in what's called interphase. But some of them are going through this division of the nuclei. And so this one would be in prophase, this would be anaphase. And that's metaphase. And so since they're spending less of their time there, in this one picture; we're going to be represented a smaller amount. And so the big thing that you should understand is that mitosis is not individual phases. It's not just quickly changing between them. But it's a movie. And this is a pretty amazing movie. This right here is a sea urchin undergoing cell division becoming a larva of a sea urchin and eventually an adult sea urchin. And so in this picture, the or in this time lapse, the yellow is going to be dyed to represent the chromosomes. And then the blue is actually the microtubules, that makes up the spindle that allows us to divide that nuclei in half.

And so if you think of it like a movie, it makes more sense to go through the whole movie before we get into each of the individual phases. And so what we're going to go through are all the steps of mitosis. And we're just going to play it like a movie. And we're going to concentrate on what goes on, on four different structures. So let's start with the cell membrane. So if we look through the different phases during the cell membrane, just keep an eye on that don't look at anything else. So watch what happens to the cell membrane over time, nothing really. And then boom, the whole thing divides in half. And so the cell is getting larger, and then it'll divide in half. So we're forming what's called a cleavage furrow in the middle. And now we have two cells. And so now let's look at a different structure. So we understand what's going on with the cell membrane, what happens to the nuclear envelope or that membrane around the nuclei, let's watch that. So not much is happening, it got a little bit smaller there. And then boom, that whole nuclear envelope fragments apart, and it's gone, so we don't see it anymore. And then it reforms once those new cells form, then we have the new nuclei forming on each side. All right, let's go through that movie. Again, this time, let's look at the center zone. Now what is a central zone, a central zone is going to be made up of two things, these microtubules and those are going to form a spindle and it's going to allow us to equally divide those chromosomes to either side. And then in the middle, you have what's called the centrioles. And we think that the centrioles organize those microtubules and organize that spindle. Now, if this were a plant cell, there'd be some differences like a cell wall, but they don't have centrioles. We think that the nuclear envelope is important in basically maintaining those microtubules as it goes through mitosis. And so let's watch what happens to those centrosomes. And it's going to happen right away.

So let's watch this. So the first thing that happens is it actually replicates. And so we're going to have one central zone, and then quickly, we're going to have two. Now let's watch what happens to those central zones. They form microtubules between them, kind of pushes them apart. And then each cell is going to have a centrism in each one. So that would be replicated again. And then the same goes over and over and over again.

Now let's look at the most important thing as we as we switch through these slides. And that's the chromosomes themselves. So chromosomes is the genetic information. And so it's the DNA inside the nuclei. Remember that DNA is wrapped around these histone proteins. And so when we want to condense the chromosome, we just packet more tightly and more tightly over time. And eventually, it looks like this. So this is that characteristic shape of a chromosome. Generally, when the cell is in interphase, it's just going to be loose DNA doing its job. But when it replicates, then we have to make sure that it's condensed, and we can equally share it. And so what's important to know is that in this chromosome, each of these sides is called a sister chromatids. And they're attached together at the centromere in the middle. So they're exact copies, since during that S phase, we copied all the DNA, each of the sister chromatids are exactly the same. Now one other thing that's going to show up as we go through mitosis is the kinetochore. And the kinetochore is a protein that's going to be found on the center of each of those sister chromatids. And it's going to attach to the microtubules. And I'll point that out as we switch through it. And so let's look at these chromosomes now and watch them over time. So they're real loose at this point. Now we've duplicated it. So we've copied the DNA, so you can see way more chromosomes. Now they're condensing together those chromosomes in the middle. At this point, you can see that the chromosomes so the kinetochore, right here is attaching to the microtubules of that spindle. Now, some of those microtubules go all the way across the cell. But you can see a lot of them are attaching to that are attaching to the kinetochores. What happened now is that they've all lined up right across the middle, there's a straight line along the middle of the cell. Watch what happens now to this sister chromatids, they're pulling apart. And now they're going to each of their individual nuclei, you can see that they were condensed, but now they're starting to loosen up again. And now we've just got loose DNA. And so those are the big things that are going on.

And so now let's go through the whole phases, we're going to add names to it, and then just kind of talk about the most important things that occur in each of those different phases. And so we'll start with interphase, which Nick, you know, it's technically not part of mitosis. And so let's watch what happens during interphase. And so this would be early in interphase. And now this is later. So what's one big thing that happened is that we copied the DNA, we duplicated the DNA, and then another thing that occurred during interphase, and that those centrosomes were replicated. So now we have two centrosomes. And we had one just before. So now keep an eye on that cell and see what happens as we move into mitosis or prophase. So in prophase, you can see two big things happening, you can see the DNA starting to condense here. And then we're attaching those microtubules all the way across from the centrosomes. So the chromosomes condense. What else happens at this point is that the mitotic spindle is starting to form. So when you're looking back at that sea urchin video that was that died blue in color, it's really organizing the division of the nuclei itself.

Now let's go into what's called pro-metaphase. And so during pro-metaphase, what are the big things that happen? We can see right away that that nuclear envelope is fragmenting so it's breaking it apart. And then the other big thing that's happening at this point is that those microtubules are starting to attach to the kinetochores in the middle, not all of them, some of them are reaching all the way across the cell. But you know, it's prometaphase, if you see kind of bad explosion of that nuclear envelope, what happens during metaphase? Well, you can see now that they've all lined up in the middle. And that's how I remember metaphase means meet in the middle. And so what we get is all of those lining up along what's called the metaphase plate, it's one of the straightest lines in all of nature. Let's keep an eye on those chromatids as we move into anaphase. So what's happening is that they're moving apart. And I was thinking of a standing for a part, the chromatids are separating. And so we're getting equal amounts of DNA in each of those daughter cells. And then finally, we go into telophase and cytokinesis. During telophase. What we're going to form is a new nuclei on each side, the cells are elongating and that cleavage furrow is forming. So this is that cleavage furrow right here. It's basically taking that one cell and pinching it in half. Now, if this was in a plant cell, they form what's called a cell plate, which is a new cell, well, right in the middle.

What are the other big things? Remember those microtubules I pointed to earlier that weren't attached to the chromosomes, they're actually elongating the cell and allowing it to divide. And then the other thing, the important thing that happens is that we're forming a new nuclear nuclei around each of those daughter chromosomes. Now, where did that nuclear envelope come from? Remember, when we fragmented that nuclear envelope before those fragments are coming back to make the two new daughter nuclei? Eventually we will go back into interphase. Now where are each of these cells headed? They're going back into the cell cycle again. And so the way I asked my students to remember this is it mat see, and so you can just write that on the top of the test. There's  mnemonics to remember it, but this tends to work for me. Remember, technically Interphase is not part of mitosis either is cytokinesis on the And, and so you should be able to point at each of these. So if I pointed this one right here, what phase is that going to be? That is anaphase. And so what's going on? You can see those chromatids moving apart. If I were to look back here, what's this going to be? That'd be pro metaphase. That's where you get the explosion of those nuclear envelopes. And so that's mitosis. It makes sure that we equally divide the DNA in those daughter cells and allow cells to replicate made all the cells in your body and I hope that was helpful.