 So, today we're going to go over the process of mitosis and how it fits into the cell cycle. One of the things that you have previously learned is that reproduction is a characteristic of life that distinguishes living from non-living things. And typically we've discussed this in a macro scale where plants make more plants and people make more people. But we also have to think about it on a micro scale, where cells make more cells. The micro scale is important because this is going to function with growth, becoming bigger, making larger structures, wound repair, and asexual reproduction. Today we're going to focus on the micro version, mitosis. Later on we'll come back and talk about meiosis, which is the sexual reproduction for offspring. Before we get started with mitosis, there's a couple of terms that we need to go over and make sure that we understand very well because they're going to come back not only throughout this lecture, but then in the future lecture about meiosis. The first one is chromatin. It's a loose configuration of DNA. It's going to be this illustration right here where you see the DNA is wrapped around the proteins. This is how the DNA is stored in everyday life, where the DNA needs to be accessible so that you can make new proteins, you can repair structures, it needs to be available. But before the cell divides, the chromatin is going to be packaged down into what are known as chromosomes. You can see chromosome over here is much different from the chromatin. The chromosome is a very condensed version of the chromatin and the reason why we do this is so that none of that genetic material gets lost during the division process. You can think about it, if you're getting ready to move, you're going to pack all of your belonging into boxes, and that way all of your boxes are going to make it to their new destination. If you don't pack your belongings and you just throw them in the back of someone's truck, when you get there, you're going to be missing half of your stuff. That's not something that we want to see happen with DNA. Some more terms, sister chromatids? Sister chromatids are what make up your chromosome. Earlier we talked about the chromosome being a condensed version of chromatin. You can see that the chromatids, one on each side, so one's in black and one's in red. Those are going to join up at a central area, which is called the centromere. You have two pairs of these chromosomes in your cells. Those pairs are called homologous chromosomes. These contain the same type of genetic information, and you get one from each parent. Each cell has one chromosome from one parent and an identical from another that contains just slightly varying genetic information. Now that we understand those important terms, it's time to move through and kind of identify the process of mitosis. The first thing to notice is that it's a cell division process, or a cell reproduction process, where you start off with one cell that's diploid in nature, and it divides to create two identical daughter cells that are also diploid in nature. It's important to notice when you talk about daughter cells that this is just a term that was given. It is not imply any type of gender. These cells are not female in any way. Just one cell starts off and creates two identical daughter cells. When you talk about a diploid cell, it's one set or one pair of homologous chromosomes that we've previously seen. Remember earlier, we said that your chromosome looks like a big X, where you have one of each sister chromatids joined together at a centromere. When you talk about something being diploid, you have a pair of these homologous chromosomes. This means that cell that contains two is going to be diploid. Diploid is going to be abbreviated a lot of times as two n, so anytime you see that, just remember it means one set or one pair of homologous chromosomes. Now let's talk about how this fits into the cell cycle. The cell cycle, remember, is going to be the series of events from the time the cell is born until the time the cell dies. When you talk about all things being made out of cells, any living thing that's made out of cells, remember that these cells have an expiration date, that not all cells live forever, that there has to be a process to replace these cells that the organism will continue to be healthy and happy. The way that we're going to replace these cells is through the process of mitosis. Let's get started on the first part of mitosis. It's actually the life as usual phase. This is how the cells spend most of their existence. So they're going to spend their lives in what's known as interphase. Interphase is where the cell is going through, contributing to the organism's wellness by performing its daily functions, whether it is cleaning blood, making new proteins, storing compounds. It's going to be performing its duty that keeps the organism alive. The other thing that it's doing is it's performing metabolism itself. It's creating energy and it's using those to make new proteins for itself to keep itself alive. The interphase has three sub phases within it. You've got G1. G1, you can see, is the largest of the phases. This is when the cell is growing. It's performing its activities. It's happy. It's just happy to be there. The S phase is where you have synthesis of DNA, so S for synthesis. This is when all the DNA that we've already talked about in the nucleus is going to be replicated. It's important that you replicate the DNA because one cell becomes two daughter cells. So if you don't replicate it, the remaining daughter cells will have half of what they originally started off with. And then the last is going to be G2. G2 is another time where the cell grows and prepares for division. So most of the cell's life is spent in interphase, growing, dividing, performing its daily functions. And only a very small sliver of life is actually spent in mitosis, which I've coded and read as the M phase. So now that you understand interphase, we have to go look at what happens during cell division because it's very important to keep in mind that the one cell becomes two. There has to be a way to divide everything up equally. So there are four main phases of mitosis. Once the cell completes G1, S, and G2 of interphase, it's going to move on into prophase, metaphase, anaphase, and telophase. You can remember these in order with two sentences. The first one is please make another try, prophase, metaphase, anaphase, telophase. You can use that one. If you remember that sentence, then you won't need to make another try because you'll remember. The other one, and this relates more for me, is I passed my anatomy test, with I being interphase, and then you have prophase, metaphase, anaphase, and telophase. So if you remember these sentences, you'll remember them in order. So you start off with interphase, which is cell life, and then mitosis. So beginning with mitosis, you have prophase. During prophase, the chromatin is going to condense down to from chromosomes. Remember that chromatin is loosely configured DNA. We need to make sure that we pack it into our little chromosome boxes so that we don't misplace any during the process of division. So the chromatin is going to condense down and get packed away into chromosomes. The nuclear envelope that surrounds your nucleus is going to start to disintegrate and break into little pieces. Spindle fibers, which are protein fibers, extend from your centrioles. Remember that you have a pair of them in every cell. And those protein fibers begin to extend out to the center of your chromosomes. Remember that's known as the centromere. They also start to extend out to the end so that the centrioles can migrate toward opposite poles of the cell, opposite sides. So the second part of mitosis is known as metaphase. During metaphase, the chromosomes, which have condensed down and prophase, are going to align at the equator of the cell, which is equidistant between both poles. You have sister chromatids, which are either side of the X, facing opposite poles. The spindle fibers, which were originally attached to your centrioles, are going to be extending out and attached to a central area of your sister chromatids. The next part is going to be anaphase. During anaphase, your spindle fibers begin with work as double duty. So the first part is that the piece of spindle fiber attached to the center of your sister chromatid is going to begin to shorten, which separates your chromosome into two individual chromatids moving to opposite poles. The second part of these spindle fibers is that they extend outward at the poles, which is going to elongate the cell. This elongation is going to allow you to create two separate areas for nucleic material. So the last part of mitosis is going to be telephase. During this time, we have events that occur, which are the complete opposite of prophase. So once the chromosomes reach their designated pole, they begin to unwind back to chromatin. The nuclear envelope is going to start reforming around those to create two separate nuclei. And the spindle fibers continue to elongate the cells while retracting from the genetic material. This elongation is going to create an area in between the two nuclei, which is known as the cleavage furrow. This cleavage furrow is where the plasma membrane becomes closer and closer together until eventually it touches and two cells will be pinched off and formed. The process of dividing up your cytoplasm, which is all the fluid and organelles inside of your cell, is known as cytokinesis. And this runs hand in hand with telephase. So overall, the process of cell division with mitosis is quite simple. You start off with one cell and it's life as usual performing its daily habits. The genetic information is going to be replicated during interphase. Once it's replicated and the cell is ready for division, it enters into prophase. During prophase, the genetic information condenses down into chromosomes. You have the centrioles begin to send out their spindle fibers. The nuclear envelope is going to disintegrate. Then you have metaphase, in which all of your chromosomes line up at that equator. Once the spindle fibers attach, you now have anaphase, where the chromosomes separate. And sister chromatids are pulled to either pole. Once they reach that pole, you're going to enclose them in a nuclear envelope, which is telephase, and divide up the cells at the cleavage furrow into two individual daughter cells. So hopefully, you can see that this process really just flows from one step to the next. One becomes two genetically identical cells. These cells will then enter into interphase, where they begin life as usual. They perform their daily activities. They grow. They will eventually replicate their DNA and go through this process. So you can see one becomes two, and two become four, and so on and so forth. So that really kind of brings home how this functions in growth and wound repair. There's a really good animation that you guys can watch at home on your leisure that kind of details this process over again. It's from YouTube. It's the best probably video I've seen out there for this particular thing. All you have to do is click on it. It'll take you right to the animation. So after reviewing the process of mitosis, it's pretty easy to see that one cell becomes two, and two becomes four, and so on and so forth. They're all genetically identical, and it's vital so that you can create more of the cells that have reached their expiration date or have been damaged or lost. But if all these cells are continually dividing, there's gotta be a question of what happens when things go wrong? What happens when the cells divide up into a certain point where they start becoming a burden on the organism? And typically this doesn't happen in our bodies because there is a default setting in which the cell cycle pauses at checkpoints. Checkpoints are regulated by protein markers that come from the cell. There is a checkpoint right at the beginning of G1, which is part of Interphase. At this point, you're gonna check to make sure that the information that you have received before you get ready to start entering into that growth and development phase is accurate. All of the chromosomes are intact, everything is properly matched, and you have received enough of the genetic information to become identical to everything else. Once you have assumed that and checked off to make sure that all your chromosomes are in place, you're gonna go through the growth phase, and you'll stop once again right before synthesis to make sure that you have grown enough to enter into a replication phase. Once you receive the signal to continue on, you move through synthesis, growth and development, and then you're gonna pause once again at Metaphase in the middle of mitosis. You pause in the middle of mitosis to make sure that all the chromosomes are lined up on the equator so that you have an equal division of genetic material. Each of these is a checkpoint regulated by a protein marker, and the cell will default, pause at each checkpoint to make sure that it is identical, to make sure that it has enough of the genetic material to make sure that it will be viable on its own. Okay. There are cells in your body that do not listen to these checkpoints. They don't pause, they don't go by, stop, and they don't collect $200. They don't listen, they're cancer cells. Cancer cells do not pause at the checkpoints. They move straight through and continually divide and divide. So a lot of the times you see these cells stacking upon each other because they've run out of room. They also start to run out of resources and have to invade neighboring tissues, which is metastasis. You can see these are your cancer cells. They've got the pseudomonia sticking out, the little feet, which is really characteristic when they start to invade neighboring tissues. So it's very important that this process of mitosis, while it's something that is predictable, is also gonna be heavily regulated because without it, cancer will erupt. Okay. So as we've discussed, mitosis is a very vital part of your organism, not only in its wellness, but its sustainability. It happens really quickly, but in a predictable fashion. It's also gonna be very well regulated because of those several checkpoints. Okay. My challenge to you at home is to see if you can go through the process of mitosis on your own, without any resources, and diagram what happens from start to finish. Start off with interphase and go all the way through to the formation of the two daughter cells. If you can do this with the correct terminology, you are set. You are ready to go and you will do well. So best of luck. Thanks for visiting. Bye.