Mitosis

Mitosis is eukaryotic cell division. Now you got to be careful because some people will get really kind of picky about it because technically, it's about making two nuclei not necessarily two cells because there's some kinds of organisms like fungi that sometimes don't bother doing the final step where you do cytokinesis and divide off the two cells. So you wind up with two nuclei and one cell. So just to remind you mitosis is cell division from making two identical nuclei. You might have heard that i. It's to help you remember what mitosis means. I'll use that to help differentiate between mitosis which is for identical nuclei and meiosis more properly meiosis. Meiosis which is for reproductive purposes, for sexual reproduction that is.

So what happens is that during mitosis, this is at the end of the cell cycle during what's called M-phase. The rest of the time, the cell is in what's called interphase which is its normal life. You go through the g1 phase where it's expanding, growing to its adult size, maturing, et cetera. S-phase is when it's getting ready for mitosis and that's when it copies the DNA through DNA replication. It then goes through g2 where then copies off the instructions on how to build the tubular that makes the spindle fibres that they can be in so much in use during mitosis. Then you'll go through the 4 major steps of mitosis; prophase, metaphase, anaphase and telophase. There is that final other stage called cytokinesis. But again that's technically considered outside of mitosis.

So, I've got a little thing that'll help you remember what's going on during cell division. So put your hands together like this. This represents the nucleus of a cell that's in proph- sorry in interphase. So it's already it's still intac- the DNA is spread out. So then you tighten your hands together and that represents the condensing of the DNA that occurs during prophase. Then you put your hands like this so that this part and this part are touching. that's called metaphase. this is how the cell's chromosomes are aligned down the middle of the cell. Then in anaphase, those chromosomes get pulled apart, like your hands are. Then during telophase you wind up with two new dense nuclei. So I'll go through it again. Interphase, prophase, metaphase, anaphase, telophase and cytokinesis. Cha cha cha. Alright?

Let's take a look at a video that shows us a little bit more indepth. Here's a great one from YouTube. We zoom in. Now here we see the cell's nucleus undergoing prophase. We see the DNA spread out in chromatin form. Now it's condensing into the visible chromosomes. This yellow ball here is the nuclear envelope. Now once the chromosomes have condensed, we can now break down the nuclear membrane as the two centrioles if we're talking about plants or just center. Centrioles in animal cells, centrosomes in plant cells. And we'll see the spindle fibres start to go towards the chromosomes to hook on to the as well as to push each other apart. Here's a spindle fibre attaching to that part of the chromosome and to this part of the chromosome. Let's pause it right here, because this is an important thing.

This chromosome here is actually made out of two separate DNA molecules. Now they are held together this place right here called the centromere which means literally the center part. These two little things on either side, they've always looked like really big audi belly buttons. These audi belly buttons are what the spindle fibre here and here attach to. You can think of them as belly buttons with belly button rings. They got a rope tied to this belly button ring of this identical twin who's identical to that one. So we got a couple of siamese twins with pierced belly buttons that we're going to hook up ropes and then pull them apart. Alright? So that's what's going on with this chromosome. Every other chromosome in the nucleus well, the nuclear membrane is gone now, but every other chromosome in the cell is having the same thing happen.

Just as a double check. You know that this entire thing is a chromosome. What are these two identical parts called? That's correct. They're called chromatids right now while they're still attached. Once they're separated however, we no longer call them chromatids. They're now just considered separate chromosomes.

Alright, let's get this started up again and we'll see that as this chromosome is pulled by this side it's also pulled on that side. And that means all of the chromosomes are lying in the middle. Let's stop it there. When they're all in the middle you'll see a single row with all the chromosomes like this, remember that was our metaphase, then they're being pulled apart. Once we're in the pulling apart phase, that's anaphase, a part. Prophase was preparing, metaphase in the middle, anaphase the moving apart. Alright, let's get it going again. And we see during anaphase, the chromosomes moved opposite sides or pulls of the expanding self.

Now, once they reach an appropriate part, we are going to reassemble the nuclei and loosen the chromosomes back into that loosely organized DNA form called chromatin. And now we're entering into telophase.

Now that the two nuclei reformed the DNA loosens up or relaxes back in the chromatin and then the cell if it chooses to conduce cytokenesis and separate and we wind up with two identical cells to the original parental cell. That's mitosis.