This video is for D2.1 on cell and nuclear division. This is standard level content, and we'll be focusing on cell division and mitosis. Cell division is the process of creating new cells, or you can think of it almost as copying cells if we're talking about mitosis.
So why do I need more cells? Well, maybe because I need to grow. Multi-cellular organisms grow by adding new cells, not by enlarging their existing cells, but by adding new cells.
We also need to repair tissue, whether that's from an injury or just normal cell death. And then reproduction also requires cell division. It's important to remember that new cells come from existing cells. That's part of the cell theory. So this process of cell division allows us to create new cells.
and allows for the continuity of life. Remember, theme D is all about continuity and change, and we'll be talking about cell division processes that illustrate both of those. So when I think about cell division, I kind of like to start from the beginning, right?
So a cell is doing its cell things. We'll talk about that later. And then it might go through nuclear division, dividing everything in the nucleus, and then I have to actually split those new cells apart.
And that's going to happen here in a phase called cytokinesis. So cytokinesis is not the division of the nuclear material like the chromosomes. This is the division of the cytoplasm. So everything outside of the nucleus, including all of the organelles. Now, in animal cells and plant cells, that will look a little bit different.
We'll start with animal cells. So when animal cells are ready to split their cytoplasm, they are encircled by a ring called a contractile ring. That contractile ring is made up of two proteins called actin and myosin, and that contractile ring starts to contract and get smaller and smaller, and it forms something called a cleavage furrow.
So that's right here. Eventually that ring will get so small that the cells will pinch apart and those new daughter cells can separate. In plant cells, this involves building a new cell wall between the two. daughter cells. So first we'll have a new cell membrane and that will come through the fusing of vesicles.
So vesicles will form in the center between where the two daughter cells will split and that will create the new cell membranes. Don't forget plant cells have cell walls so we also need to create that and that will happen with a microtubule scaffold. Each new cell is going to build their own new cell wall.
You may see that referred to as a cell plate. That's that beginning of that cell wall between those two plant cells. And that eventually will lead to two entirely separate daughter cells.
So here I have an example of a parent cell, and it's got nuclear material like chromosomes we can think of. And when that cell is getting ready to divide, it's going to need to make a copy of its genetic material. So we'll symbolize that with this.
So it has kind of like... two batches of nuclear material. Usually the parent cell will divide equally into two daughter cells. So that will look something like this. Okay, this is an even division of the cytoplasm or equal division of the cytoplasm.
So I end up with two cells, they're equal in size, they each have nuclear materials, and they would have about the same amount of cytoplasm, including the organelles. Unequal division is Not as common, but it is possible, and we'll talk about some examples of that if you get into like gamete production and things like that. So again, that nuclear material will make a copy of itself, and when this parent cell undergoes cytokinesis, if it is unequal division of the cytoplasm, then we end up with something like this, where one cell is not only bigger, but contains more of the organelles than the other one. This is possible as long as two things are true. One is that each cell receives a nucleus and that each cell has at least one mitochondria.
That is important because cells don't synthesize their own mitochondria. Mitochondria replicate independently of the cell. So if this cell did not have its own mitochondria, it will never have any.
So that's why that is important. Budding in yeast is a great example of unequal division in cytokinesis. Budding is a form of asexual reproduction, and we can see here there's some larger cells with little tiny baby cells coming off of them.
Those are the buds. And so the way that this is working is that this parent cell has replicated the nucleus, and this small bud will receive the nucleus and just... enough cytoplasm to have like maybe one copy of each organelle, and then a new cell wall will grow between them.
That small bud can then grow into a bigger cell. But again, a great example of unequal division in cytokinesis. Now, oogenesis is a great example of unequal division of the cytoplasm.
So oogenesis is the process of producing eggs. Eggs are sometimes called oocytes, or when they are mature, they're called oocytes. So we can kind of link those terms together. And they are made through a process called meiosis.
We'll discuss meiosis in a different video, but for now, it's important to understand that meiosis has two rounds of cell division. Every time there is a split or a round of cell division, there is an unequal division of the cytoplasm, and one daughter cell... receives more cytoplasm than the other.
They both receive a nucleus, but one has many more of the organelles and much more of the cytoplasm. Then during the second round, the same happens again. This one that didn't receive much in the very beginning just splits again.
They're still really small. But this one that was getting more of the cytoplasm, when it undergoes another round, it is another round of unequal cell division. So in meiosis two, there's another...
unequal division of the cytoplasm. And this produces one viable oocyte. It's got a lot more than its fair share of that cytoplasm. And then three cells called polar bodies.
And these polar bodies contain a nucleus, but they won't be able to develop into mature oocytes because they lack the organelles that are necessary. Before cells can divide their cytoplasm, they must replicate their genetic material, okay, that inside the nucleus. So the reason that they do that is so that when they split into two cells, each cell may receive one nucleus. That means that we won't end up with any anucleate cells. A means not, so or without.
So we're talking about cells without a nucleus. There are some cells that don't have nuclei, for example, your red blood cells. Red blood cells do not have a nucleus, and that means if they don't have a nucleus that they can't synthesize proteins. There's nothing there to transcribe and then translate. If they don't have proteins, they're going to have a very limited lifespan.
So this is why red blood cells typically only live like a few months in our body and must continually be regenerated. But for the most part, in order for each daughter cell To end up with a nucleus, we need to have that replication process happening before the division of the cytoplasm. Theme D is all about continuity and change, right? And mitosis and meiosis are great examples of both of those processes. So continuity is really well represented by mitosis, and that is because the goal or the end product of mitosis is two genetically identical daughter cells.
Thank you. So that genetic information has been conserved and passed along to the two daughter cells. Each of the cells produced by mitosis are diploid, and that means that their chromosomes come in pairs. And we denote that using this symbol 2N.
That's for diploid. Again, all cells are identical and have a complete genome. If an asexual reproducer is using a...
a cell division process like this, then all of their genome would be passed on to their offspring. When we get into meiosis, you'll find that that is very different. Theme D is all about continuity and change. Mitosis is an example of a cell division that really helps us to make sure that there is continuity in passing along genetic information from parent to offspring or from the original cell to the daughter's cells. Mitosis produces two identical daughter cells, and they are diploid.
Diploid means that their chromosomes come in pairs, and we use this shorthand, this abbreviation 2N, to talk about diploid cells. Again, all the daughter cells produced by mitosis are genetically identical, and in asexual reproduction, that means the full genome has been passed down to the offspring. Meiosis, on the other hand, is a great representative of change. And that's because the daughter cells produced by meiosis are not identical. Okay?
So meiosis is a process used for creating gametes, and it produces not... two but four cells at the end of meiosis, and these four cells are all haploid. That means the chromosomes do not come in pairs, and so we use this shorthand abbreviation, little n. Meiosis halves the chromosome number, which we'll go through in a later video.
We're also going to find that there's a random assortment of genes, and so this means that all of my gametes, not only are they haploid, but they are genetically unique. And this is one of the things that gives sexually reproducing organisms a lot of variation. Now, again, before cells can divide, they must replicate their genetic material. And that is a prerequisite for both mitosis and meiosis. Prior to cell division, DNA is elongated.
It's usually in a form called chromatin, a little loosey-goosey, and there's a lot of reasons for that that involve transcription and translation and a cell's ability to function. But during cell division, we really need that DNA and that genetic material to be much more organized. So two things are going to happen.
Not only is DNA replicated, but it also condenses into chromosomes. And those are the shapes that you typically associate with our genetic material. So before replication, we might see a structure like this, where we have a chromatid that's in blue and a centromere in the middle. We'll talk more about those a little bit later on.
We get this classic structure called a chromosome when that replication process has taken place. So because each... arm of this chromosome is a replicate.
We call them sister chromatids. Okay, and maybe I'll erase this here so you can see that a little bit better. Each arm is a sister chromatid, an exact replica, and they are held together by these loops made out of a protein called cohesin.
So cohesin is a protein that kind of makes sure that those sister chromatids stick together. During a phase called anaphase, which we'll talk about later, these chromatids are going to be pulled apart. So they're going to be pulled in opposite directions, and that's going to require the breaking of those cohesin loops. Now we said two things have to happen.
DNA replication has to occur. That's how we get those sister chromatids. And this must condense into that chromosome shape.
So the condensation of chromosomes means we're going from that loosey-goosey chromatin form and condensing it and organizing it. in a way that allows this immensely large molecule to move around through the cell. Keep in mind that in one teeny, teeny, tiny little cell, you have almost two meters of DNA.
It's very long and very skinny. And so in order to get it to move around efficiently during mitosis, we need to condense that into chromosomes. That is going to take place through a process called supercoiling.
So this is when the DNA is wrapped around histone proteins, which it always is, but those histone proteins then start to link together and they condense on themselves and condense further and condense further in that process called supercoiling that ends with this classic chromosome shape. Once that genetic material has been condensed, we need to start moving it to the poles, the ends of the cells, so that it can eventually become the two new daughter nuclei. that is going to be the job of something called the spindle microtubule. If you've already studied cell structure, you know that we have a structure inside of the cell called the cytoskeleton. The cytoskeleton is made up of protein filaments called microtubules.
So we're going to recycle them. We're going to disassemble that cytoskeleton and then reassemble it into this structure called the spindle microtubule. And I can kind of outline that here in black.
So the spindle microtubules kind of look like these strings, almost like puppet strings. And they are going to link with a structure on the centromere called the kinetochore. The kinetochore is going to act as a microtubule motor. So you can think of it almost like as a crank moving these chromatids to the opposite poles. And the way that it does that is pretty clever.
It's actually going to remove these little microtubule filaments, like one at a time. And when it's doing that, that's going to cause those microtubules to shorten. As these spindle microtubules shorten, and you can see that here, those chromatids are then pulled apart into opposite ends of the cell.
Mitosis is strictly the division of the nuclear material. So in another part of the cell's life, it's already replicated. that genetic material. And then it goes through mitosis, which has four distinct stages, followed by cytokinesis, the division of the cytoplasm.
So we'll talk about these four phases separately. For each phase, you need to know how to draw it, how to describe it, and how to recognize it. And when I say recognize, I mean in these nice, cute, neat little like cartoon pictures, but also in micrographs of real cells. Now we'll start with a part of a cell's life that's actually not part of mitosis. Mitosis is strictly prophase, metaphase, anaphase, telophase.
This would happen prior to that. So that's something called interphase. During interphase, DNA is in the form of chromatin. It has not condensed into chromosomes yet.
So you'll see in this micrograph, it's pretty evenly spread out. This stain is attached to DNA. So this dark material here is chromatin, and it's pretty spread out in this nucleus. So that's chromatin.
During interphase, I would expect the cell to grow. I would expect it to do normal cell functions like transcription and translation. If it's a muscle cell, it's doing muscle things. If it's a liver cell, it's doing liver things, all that good stuff.
And then at some point, right before this cell is ready to start mitosis, all of that DNA will have to be replicated. You'll notice that as we move into prophase, there are some pretty pronounced changes that are happening inside of the cell. So first of all, this chromatin has condensed into chromosomes, and I can see each chromatin here attached. The spindle microtubules are starting to assemble, and the nucleus is breaking down. So this nucleus needs to disintegrate so that the chromosomes can move into daughter cells.
In a micrograph, it's going to look something like this. So I want to look for these clumps, okay, these black things here. These are chromosomes, and I can see some white space in between them. Classic sign of prophase.
In metaphase, those chromosomes are going to line up in the middle, and you will almost never see that phrase that way on an exam because that would be too easy. You may hear things like, they line up in the center or on the... equatorial plate, something like that. But these chromosomes have moved to the middle.
So that means that in prophase, that nucleus has fully dissolved. And the microtubules, those spindle microtubules, have attached to the centromere on each of these chromosomes. Okay, so in a micrograph, it's going to look something like this.
So this cell is kind of oriented to where the equator is here. Here it's turned sideways. You need to learn how to recognize cells in both ways, but I don't have a clear defined nucleus and they're all aligned in the center. As we move into anaphase, those sister chromatids need to separate. Now, a funny bit of naming here, as soon as they, or when they're connected, they are called sister chromatids.
When they are separated, each one is its own chromosome. So the whole goal of anaphase, or the main takeaway from anaphase, is that those sister chromatids have been pulled apart, and now each pole has a chromosome. So if we think of this center area here as being the equator, the ends are called the poles, and you can see in this picture that the sister chromatids have been separated.
And now these chromosomes are moving towards the poles. Well, when they are in metaphase, those sister chromatids are connected by cohesin loops. So first those cohesin loops need to be cut.
And then those spindle microtubules, as they shorten, are going to pull these chromatids apart. We can also see that here in the micrograph, right? All these chromatids or chromosomes were together as a sister chromatid.
chromatids start to pull apart. Now I can see where each end of the cell is going to have a complete set of chromosomes. And then we'll move into the last phase of mitosis, which is telophase.
I remember this T for telophase, T for two. By the end of telophase, I should have two new nuclei. So in anaphase, all the genetic material moves towards the poles. Once it is in those poles and it doesn't need to move around anymore, those chromosomes can decondense back into chromatin and new nuclear membranes form around them. This happens simultaneously with cytokinesis.
So you can see that here, this is an animal cell and this cleavage furrow is forming. So cytokinesis and telophase happen at the same time. Here's a great picture of early telophase and then very late telophase.
Okay, so each pole has a full set of chromosomes, and I can see the very beginnings of a cell plate here. Here it is kind of at the very end, and I can see that this cytokinesis process is quite far along. So from start to finish, here we are.
Okay, interphase, normal cell, live growth, transcription, translation, okay, and then prophase, condensation of chromosomes, right? nuclear membrane dissolves, spindle microtubules form, and then we move into metaphase lining up in the middle. The spindle microtubules are going to attach.
Separation of those sister chromatids occurring in anaphase, and then telophase and cytokinesis happening simultaneously. One of the skills that you're supposed to have is preparation of microscope slides. So I usually use an onion root tip. At that root tip, especially if you're a higher level student, you should know that there's a meristem down there, a zone of cell division. So there's a lot of mitosis going on.
This is where growth is happening for onion plants, so they're really great to use. You want to cut them in a really thin layer and then stain them. So this stain is going to stick to the DNA. and really allow you to see this under a microscope. You squash it under a slide and then you look at them.
And the goal here is to identify them. I don't know how much you'll be assessed on microscope preparation on exams, but it is a very good skill to have. However, I would expect to be assessed on our ability to identify micrographs.
So I can help you here find some cells in different stages. So We have a lot of cells in interphase. They look like this.
Genetic material spread out evenly out. There's a lot that are in interphase. I won't circle all of them. So a good example of prophase, I think, is this cell right here. You can start to see those clumps happening.
Let's see, metaphase right next door, all lined up in the center there. Nice and neat for you to see. Aren't you lucky?
And then let's look for... anaphase. Here's one in anaphase.
Those sister chromatids are being pulled apart. And then telophase. Let's see. Here's a pretty good example of telophase.
Okay. This is not a separate cell just yet. I can barely see that cell plate starting to form. So again, other examples in this photo as well, but I hope that helps you here identifying mitosis stages.