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 then you can see that the different phases And also you should understand what's the point of mitosis. Mitosis is technically the division of the nuclei. But what we're doing is really making sure that we can make two 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 a cell doing during 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 the 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 a 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 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 they're spending most of their time in the inner phase. 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, you know, we're looking at the cells that And 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 picture, or in this time lapse, the yellow is going to be dyed to represent the chromosomes. Then the blue is actually the microtubules. It 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 will 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. here. 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. Alright.
Let's go through that movie again. This time let's look at the centrosome. Now what is a centrosome?
So what is a centrosome? A centrosome 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 would 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 centrosome.
And then quickly we're going to have two. Now let's watch what happens to those centrosomes. They form microtubules between them. That kind of pushes them apart.
And then each cell is going to have a centrosome in each one. So that would be replicated again. And then this thing goes over and over and over again. Now let's look at the most important thing 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 pack it 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 chromatid and they're attached together at the centromere in the middle. So their 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.
And we've got a little 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, or attaching to the kinetochores. What happened now is that they've all lined up right across the middle. There's this straight line along the middle of the cell. Watch what happens now to those 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 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 is 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 is starting to condense here. And then we're attaching those microtubules all the way across from the centrosomes. So the chromosomes condense.
And then we have What else happens at this point is that the mitotic spindle is starting to form. So when you were looking back at that sea urchin video that was that dyed blue in color. It's really organizing the division of the nuclei itself.
Now let's go into what's called prometaphase. And so during prometaphase what are the big things that happen? Well you 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. So what's happening here is that the mitotic spindle 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 that 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 meat 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 are moving apart. And I always think of the A standing for apart. 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 an A plant cell, they form what's called a cell plate, which is a new cell wall 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 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 would 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 ask my students to remember this is IPMAT-C.
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.
Where technically interphase is not part of mitosis, either is cytokinesis on the end. And so you should be able to point at each of these. So if I point at 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 we were to look back here, what's this going to be?
That would be prometaphase. 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.
It allows cells to replicate. Made all the cells in your body. And I hope that was helpful.