Cell Biology | Cell Cycle| Interphase & Mitosis

All right, engineers. In this video, we are going to talk about the cell cycle. The cell cycle is so important. Why? Because the cell cycle, which we're going to talk about interphase and mitosis, is the series of phases and steps that a cell goes through to replicate itself. So we're going to turn one cell into two cells. And this is an important, important process. Not only is the cell cycle just important for being able to replicate cells, but it's also important to be able to control cell growth. We'll talk in another video about the regulation of the cell cycle. We'll talk about proto-oncogenes. We'll talk about tumor suppressor genes, and we'll talk about DNA repair enzymes and genes. Okay? But in this video, we're going to discuss the cell cycle. So we're going to go through the various stages of interphase. Then we're going to go through mitosis. And then another thing for you guys is during the mitosis part, I'm going to show you what's going on the board, but just to get a different view, we're going to take models that are going to show you guys a little bit more of what it would look like in the cell during prophase, metaphase, anaphase, telophase. Okay? So let's go ahead and get started on the cell cycle. Before we do that, how would you describe a cell? What is a cell? A cell is basically, it's the basic unit of all living things. And a cell is classified by technically having three different things. So this is important to remember. A cell is classified by having three different things. What are these three things? Generally, since we're talking about eukaryotic cells, because there's eukaryotic and prokaryotic cells, right? We're going to talk about eukaryotic and specifically human cells. They have to have what's called a cell membrane. So they have to have a cell membrane. And remember that the cell membrane is a phospholipid bilayer, right? That is actually surrounding the entire structure. It also has to have a nucleus where it houses its genetic material. Okay. In the form of chromatin, which is the DNA wrapped around different types of histone proteins. And the last thing is you want it to have cytoplasm. This is the three basic units that are needed for an actual cell. So a cell is made up of three different things, a cell membrane, a nucleus and a cytoplasm. What we're going to do is we're going to take this and make another one, an identical cell. In the nucleus, we have a structure though. We briefly described it here and we said it's DNA, right? So we're going to take the DNA during this process of the cell cycle. We want to duplicate the DNA. We want to replicate it. We want to synthesize a new double stranded DNA. And we're going to talk about that in this video. So let's go ahead and get started here. So the first part of the cell cycle, let's say we take a normal cell, right? A normal cell, that cell is going to get ready to go into the cell cycle. What's the first point that it'll go into in the cell cycle? The first phase is called the G1 phase. So it's called G1 phase. Sometimes you might even hear it referred to as gap one. It's the gap one phase. So it's either G1 or gap one phase. Now in this phase, what is the cell going to be doing? So now let's pretend we take a cell, right? So here we're going to have a cell and the cells entering into this phase here, right? It's entering into the G1 phase. Now, a cell we said has a cell membrane, right? It has a nucleus, which houses this genetic material and around it has the cytoplasm. Well, the first thing we're going to want to do is is we have to be able to get this cell ready so it can replicate, right? We want to take one cell and turn this one cell into two cells. That's the whole goal. And we want them to be identical, not only just identical, but house the same amount of genetic material. So in general, we know that this is a diploid. You know, in all of our cells we have our chromosomes, right? And there's a total of 46 chromosomes. 23 of them are maternal and 23 of them are paternal. We want to be able to pass the chromosomes down. So we have to duplicate it in order to duplicate it. We have to have both of these cells also be diploid. So we have referred to 2N as diploid, meaning that as a total of 46 chromosomes. So 2N is representing 46 chromosomes. The N is basically representing the number of chromosomes. And again, we have 23 maternal and 23 paternal. So if we take that 23 times two is going to give us 46 total chromosomes. And then what we want to do is we want to replicate this into two identical cells with the same number of genetic material, same number of chromosomes. That is mitosis. All right. But in order for us to go into mitosis, we have to have this first part here called interphase. We'll talk about this. All right. So now first thing for the gap one phase, if we need to be able to replicate these cells, what should I do? You know, another thing that these cells have, our eukaryotic cells have is they have different organelles like ribosomes. They might have mitochondria, you know, they can have different types of organelles. So the first thing we should do is we should increase the number of organelles. Let's make more organelles. So the first thing here that we're going to want to do here is make more organelles. Okay, cool. What else are you going to want to do? Well, you know, inside, we said inside of this actual nucleus, what do you have? You have your genetic material, your DNA. Well, you know, there's a process. We'll talk about it. It's called DNA replication. In order for DNA replication to occur, we need to have certain types of enzymes, certain types of proteins, right? And an order and even transcription factors. So if that's the case, then what do we need to start doing? We need to start preparing the cell by making tons and tons of different types of enzymes. So we need to start synthesizing proteins and enzymes. Now because we're going to start making a lot of proteins and enzymes to help to aid in this actual DNA replication process, we have to say one more thing. Sometimes our cells, most of our cells, hey, this is another important point. You know, most of our cells usually exist in the G1 phase. Most of the cells stay in the G1 phase. So out of the cell cycle, if you, if you were to ask, if you were asked which out of the whole cell cycle, which phase is the cell most likely in most of the time it's in the G1 phase because it's variable for certain types of cells. What do I mean? For certain types of cells, they might only be eight hours that exists in this phase. Other cells, it might be years. You know, there's different types of cells. We should actually talk about that. Let's come over here for a second. We'll deviate for a second, but we'll come back. There's three different types of cells that I want to talk about. One are called labile cells or another way. I like to think of them as proliferative cells. I like to think about them as proliferative cells. So what are labile cells or proliferative cells? Think about it simply. Out of your whole body, where are your cells constantly proliferating? They're constantly going through the cell cycle all of the time, right here. We're constantly shedding skin cells. So all the stratified squamous epithelial tissue on your epidermis and where else in the GI tract, in the urethra, the vagina, many different places that's constantly undergoing replication. So for these labile cells, what can I say? We could say the epithelium of skin. Where else? The GI tract and maybe even the urinary tract. So even the urinary tract. Okay. In other places, this is the coolest one. I like this one. If you think about it, we have to constantly be making red blood cells and white blood cells and platelets all the time. So because of that, you have to have some type of stem cell that's constantly replicating and producing more of these blood cells. What is that cell called? It's called a hematopoietic stem cell. So you know our hematopoietic stem cells that are located within your red bone marrow. They're also labile cells. So what are they called? They're called your hematopoietic stem cells that are in the red bone marrow, the red bone marrow. These two types, these basic types of cells, these labile or proliferative cells, they're constantly going through the cell cycle. Now there's some cells that they don't want to go through the cell cycle all the time. They're kind of stable. I'm just resting, staying in a kind of like a just not really do anything, a kind of like a resting area. Those types of cells are called stable cells. So what are they called? They're called stable cells. Now stable cells, stable cells, if we think about these guys, they're okay with not having to replicate that often. They replicate when the stimulus is strong enough, when there's a strong enough stimulus. So these guys don't necessarily replicate a lot, but they can if the stimulus is strong enough, like different types of growth factors to push them into the cell cycle. So what are these different types of cells? The liver. Oh my goodness. The liver is such an amazing organ. You want to know why? Because if you can know, you can take a good portion of liver, almost 40% of the liver. And what happens is let's say I cut 40% of my liver off. My liver can regrow itself. That's one of the beautiful things about the liver. There's different types of growth factors that the actual liver cells will release to make more liver cells. So your liver is really good, right? The hepatocytes within the liver are really stable cells. So let's put here, hepatocytes, right? So your hepatocytes within the liver. What else? Other ones is like your kidney tubules, you know, with the epithelial cells within the kidney tubules. Those are also stable cells, but if we have a stimulus necessary to push them into going into the cell cycle, they can. So the epithelium of the kidney tubules, okay? Like your proximal convoluted tubule, loop of Henle, all those different types of things, right? And then if you want even the alveolar cells of the lungs, right? Now there's the last one. And these are the ones that pretty much everybody usually knows. You have a last one and these are called permanent cells. So these cells, once they go through the cell cycle, they don't ever go into it again. What are these? Again, we said that these are your permanent, permanent cells. And these ones are the, usually the ones that people usually remember. And in other words, we call these, they're a mitotic. In other words, they don't undergo through, they don't undergo mitosis. These are your neurons. So your nervous tissue, right? So your neurons, what else? You know, your skeletal muscle, that's another one, your skeletal muscle cells. So your skeletal muscle and another one, your cardiac muscle, the one that's responsible for the heart. So the myocardium, right? So the cardiac muscle. So it's really important to understand the three different types of cells because some cells are going through the actual cell cycle very often, labile. Some will go through the cell cycle if they have a proper and strong enough stimulus, stable. And then some of them will not go into the cell cycle. And that is the permanent cells. Okay. Now that we understand that one thing we need to else to talk about for the G1, we make more organelles, we synthesize proteins and enzymes, but we got to do one more thing. Sometimes these cells can have certain types of damage. They might have certain types of problems. Sometimes they call them thymidine dimers. So in the G1 phase, you want to be able to prevent or repair these things called thymidine dimers. So you have different types of enzymes that can actually scan the DNA because you want to make sure that before you start replicating the DNA, there's no mistakes within the DNA. So sometimes people can get thymidine dimers. And what you want to do is you want to repair those thymidine dimers before you get ready to replicate the DNA. So in the GAP1 phase or G1 phase, we make more organelles in the cell. We make more proteins and enzymes to help to replicate the DNA. And we repair any thymidine dimers that will go into DNA replication. There's no mistakes previously. Okay. And the reason why you're making more organelles, why? Because you only have right now organelles for one cell. You need to make organelles for two cells. That's the whole purpose there. Okay. From the G1 phase, where does it go into? It's going to go into this next phase, the S phase. The S phase stands for synthesis. So this is the synthetic phase, the synthetic phase or the S phase. Now what happens in the S phase? We've kind of already talked about it, right? What we're doing here is we're taking a cell, right? Let's say I take this cell and I take the genetic material. You know, there's the genetic material right here. Let's say I'm taking this genetic material. Here's the DNA. What am I trying to do with this DNA? I'm trying to, we'll talk about this in a separate video, but what I want to do is I want to take this DNA and I want to open it up. So I want to open the DNA up and form what's called a replication bubble, right? You get this thing called a replication bubble. And then what happens is I want to be able to synthesize new DNA based upon whatever nucleotides I have here. So I'll make a whole new strand. And this is going to be what's by what's called the semi-conservative model. So what I want to do in this phase is I want to take and replicate that DNA. So in this phase, the primary thing that is occurring is going to be DNA replication. What's really cool about this DNA replication is it's maintained by specific types of enzymes. There's what's called DNA polymerases. And there's two types, type one and type three. Now these enzymes are so good at their job. So, so good that generally they're replicating the DNA so fast, but very faithfully, they don't make that many mistakes. You know, sometimes they can make a mistake every million or billion base pairs. That's insane. So they don't make very many mistakes that often. But we still want during this, this synthesis phase, we want to make sure that there was no errors in replication. So sometimes there are certain genes. We'll talk about that and they're called tumor suppressor genes and also DNA repair genes. And we have other genes that can read the DNA. We'll talk about all these things, but we want to make sure that whenever we replicated the DNA, that there's no errors. So we're going to want to fix that. We'll talk about different checkpoints. All right. So synthetic phase or S phase, we know what it's doing. It's replicating the DNA, right? So we're replicating the DNA. And technically, if you want to remember for replicating the DNA, we're going from two in into four in, right? Because we're taking it from a total of 46 chromosomes in one cell and doubling it. And if we're going from 46 and doubling it, you'll have 92 chromosomes, right? And 46 will go to one cell, 46 will go to the other cell. That's the whole purpose here. Another thing is how long does this phase take? We said that that one can vary from eight hours to years. It depends on the type of cells, but this one is usually constant in duration. Usually it's about six hours. This phase usually is approximately about six hours. Okay. So now we know the gap one phase and we know the S phase. Remember I told you though, that before we go into the S phase, we want to make sure that the DNA is okay because we don't want to waste energy and time on replicating DNA if it's not even good. So what we'll talk about in another video and the regulation is there's a little checkpoint right here, right here. There's like a little checkpoint where we're going to stop this cell and just check it to make sure everything is okay. That checkpoint is called the G1 S phase checkpoint. And again, we'll talk about the regulation through tumor suppressor genes and proto-oncogenes and stuff like that. But I just want you to get an idea of what's happening within the cell cycle. Okay? So G1, in order for it to go to S phase, it has to have this checkpoint where we kind of check the DNA, make sure that there's no issues, make sure that there's enough proteins and enzymes organelles for it to go and replicate. After it replicates though, now we have two, we have a cell here, right? We have a cell at this point in time now, who not only he's actually going to be what he's no longer going to be 2N, the cell is going to be 4N. Total of 96, uh, 92 chromosomes. Now one of the things we need to do here, this next phase, this G2 phase, what color should we do? Let's do this one. This is the G2 phase or gap 2 phase. Okay? Now in the G2 phase, this one's kind of a simpler phase. We've already done what to this cell? We've already replicated the DNA. We've already made more organelles, right? So let's just assume that those are ribosomes. We had one, we went to two. We had a mitochondria right here. And what did we do? We went to two. So we already made more enzymes. We made more organelles. We replicated the DNA. We checked for any types of damage. Now what do we got to do? Is this enough cytoplasm? Is this cell big enough to split into two equal cells? It has to be perfect, right? Our cells are very particular, right? So because of that, we want the cell to grow in size. So in this phase, the main function of this phase is primarily focused on cell growth. That is its primary function. The primary function of this phase is to regulate cell growth by doing what? Increasing the cytoplasm and the different types of components within the cell to make it big enough that whenever we pinch these, this one cell into two cells, it's equal. We want it to be perfect. Okay, so what are we up to now? We did gap one. We did S phase. We've done gap two phase or G two phase. These three make up a whole phase if you will. And that whole phase is called interphase. So again, I want you to remember interphase is made up of two, I'm sorry, three components. What are those three components? One is G one. The next one is S and the last one is G two and in order it goes G one to S S to G two. Okay. Now, and remember, remember that one point right here. Before we go to from G one to S you have to have a G one S checkpoint. Okay. And we'll talk about that in the regulation of cell cycle. Now we finished the interface. We have to talk about something else now. Now we have to go into what's called mitosis, the M phase. So again, let's come up here and right up here. Mitosis mitosis or sometimes they refer to it as the M phase. In mitosis you have to remember that there's specifically four parts and there's technically a fifth part in there. We'll discuss it, but you're going to have P mat. Okay. P mat. There's another one here which is going to be cytokinesis. It's kind of a part of telophase. We'll talk about it. But P is for prophase. M is for metaphase. A is for anaphase and T is for telophase. And there's a part here which we'll discuss, which is like the end of telophase, which is called cytokinesis where we'll separate the cytoplasms equally. So let's go through the first part here. Prophase. Okay. So here's what you have to remember. When we were going through this replication, this whole interface, the genetic material inside of the cell. So what is this inside of the cell? What should you have inside of it? You should have a nucleus, right? But inside of the nucleus, it was, has a bunch of different, it has a lot of DNA. The thing is though, the DNA originally was really loose. It was loose DNA. We also call this loose DNA, call it U chromatin. But here's the thing. In order for us to be able to separate the DNA properly, the chromosomes, we don't want it to be loose. We want to condense that chromatin. So what is this first phase here? This first phase here is called prophase. Okay. So prophase. And again, what do we say? We said that the actual chromatin, what is chromatin? How would you define chromatin? Chromatin is actually two basic things. One is it's your DNA. And the other one is it's your histone proteins. We'll talk about these when we talk about how DNA is organized into what's called nucleosomes. But there's many different types of histone proteins, but all chromatin is, is we're taking DNA and wrapping it around these histone proteins, like octamers on them. So what I want to do is I want to condense that chromatin. So let's condense that chromatin now. And when I condense it, you're going to get something which is going to look kind of like this. It's the easiest way to represent it. You're going to see what's called these chromosomes. So you're going to see these chromosomes and they're going to be nice and condensed. So there's my chromosomes. Now what did I tell you a cell has to have? It has to have a nucleus, but here's the thing. If I want to, I've already duplicated the DNA, right? Cause before it would look like this. Pretend here was the cell before it was going in. It would look like this. It would have, um, before it would only have one chromosome, right? Before it went to the S phase. Then after the S phase, it would actually replicate and make two chromosomes. Now from here, we want to be able to separate these chromosomes into opposite ends into two cells. So should we have a nucleus blocking it? No, because if I have the nucleus blocking this, there's no way I'm going to be able to separate these into two ends of the cell. So guess what? The nuclear envelope is actually going to get dissolved. There are special types of, um, cyclin dependent kinases and things like that that will phosphorylate different proteins of the nuclear envelope. Like for example, uh, they'll phosphorylate like lamins. They'll phosphorylate some of the histone proteins like H3A. There's even other proteins here too, that they can phosphorylate that are a part of the nuclear envelope, right? So different parts of the nuclear envelope, it's going to phosphorylate these guys. And when you phosphorylate them, it sets up specific enzymes to break them down and activate certain proteases. So there'll be some specific enzymes that will phosphorylate different proteins of the nuclear envelope, like lamins and histone proteins and other different types of proteins and cause them to get degraded by proteases. So the nuclear envelope is going to start dissolving. What else is going to happen? You know, you have these other things here, right? You start seeing these, these structures that are part of the cytoskeleton. Um, and these are here. You're going to start forming these things called your microtubule organization center. You know, I have these things called centrioles. So you have these things called centrioles and these centrioles are going to be important for forming what's called the microtubule organization center. So what is these things right here called? These are my microtubule organization center, MTOC microtubule organization center. So three things have happened. One thing, I condense the chromatin. Second thing, I start dissolving the nuclear envelope. Third thing, I start seeing the appearance of these things called centrioles or centrosomes. And it's going to be, we're going to call them the microtubule organization center because from these, you actually going to have these things called polar and astral microtubules. Guess what they do? They connect to the chromosomes to help to separate them. Okay. So we got prophase. That's the first part. Now we go to the second part. The second part is going to be metaphase. Now in metaphase, what happens here? You're going to have the nuclear envelope should now be dissolved, right? But what's going to happen is remember that microtubule organization center. It's going to start going towards during this process of where we get to metaphase, the microtubule organization center start taking up residents in the opposite ends of the cell, the different poles of the cell. So one we'll see right here and the other one we'll see at the opposite pole of the cell. So here's the one pole of the cell. Here's the other one. What did I say comes from these organization centers, these microtubule organization centers, these different microtubules, you know, there's microtubules that go to where the actual chromosomes are and there's ones that actually come off like this. Those are called your astral microtubules and these are your polar microtubules. Now, what do we say should be in here? We should have the chromosomes. So let's actually show here. Here's our chromosome. We're here. We'll have another one here, right? So here's our chromosomes. Now, since we have the chromosomes, what should be connecting the chromosomes to these actual microtubule organization center? We should have these microtubules connecting here. Now we need to come up with a little definition here cause sometimes people get confused. All right, so a chromosome, when we talk about a chromosome, let's actually do it right here. Here's a chromosome, right? So chromosome, how would you define a chromosome? A chromosome again is actually going to put chromatin DNA and histone protein. So like in this, I'm going to have DNA moving in and throughout, right? But a chromosome has a short arm and a long arm, right? So usually the short arm is up on top, long arm on the bottom, right? But more important part, the ends of it, the ends of the chromosome is called your telomeres. This is a telomere and this is a telomere and the center of it is what's called your centromere. The centromere determines the number of chromosomes you have. So for example, let's pretend I'm getting, I'm just going out there with this. How many chromosomes do I have? One, even though this thing's a fricking freak of nature, it's still one chromosome because we determine the number of chromosomes by how many centromeres we have. But a better way of describing this is we take that replicated part here, right? So pretend here, here was the old DNA. Well, generally it's actually, actually that's wrong because if it's, if we actually replicated it, it should be by the semi conservative model, right? So we should have old and new mixed in. So here I have one strand. That's the old strand. Here's another old strand. And then what should you have here? You should have a new strand. And a new strand. This is one chromosome, but the two individual components of that chromosome, what do you call these two little things here? What is this guy and what is this guy? These are called sister chromatids. Okay. Sister chromatids. But this whole thing is a chromosome, right? The whole thing is a chromosome, but these two individual entities is the sister chromatids. But this whole thing is a chromosome. All right. So just so we understand it, I wanted to make sure that we really get an idea of that. Okay, so now we're going back to metaphase. So from here, these polar microtubules, what are these guys right here? These are called your polar micro tubules. Here's your chromosomes and here's your microtubule organization center. The microtubules are now connected to the chromosome. We got to actually be specific at what part of the chromosome. Well, we said we had the centromere, right? So if we said here, we had chromosome, chromosome like this, there's a protein, a protein structure that's right on the outsides of it right here. You know what that structure is called? That, that purple structure. They call that the Conito core. Conito core. It's a protein structure. And guess what connects to the Conito core? The microtubules, those polar microtubules, they connect to the Conito core. Imagine them like a hook, right? Because what they're going to do is they're going to hook one sister chromatid, hook the other sister chromatid and separate the suckers. Right? So what we need to do is, is we have to have these polar microtubules connecting to what structure again? What's this purple structure? The Conito core. Okay. Now, once they're connected at the Conito core, you're going to notice something. I only drawn two here, but imagine there was tons of these bad boys, all of them lined up in a row and they're lined up kind of like along this midline, if you will. They're kind of lined up along this midline, or another way of saying it is on the metaphase plate. So they're aligned very, very perfectly. All of them are aligned perfectly. What are we going to do now? Okay. Now we've set up the stage to start separating them. Okay. So metaphase, we aligned them up on the metaphase plate. We have the polar microtubules are connecting to the Conito core of the chromosomes, and we're going to separate those sister chromatids. Okay. So now let's go into the next step. The next stage is anaphase. You can remember away. So sometimes how they remember this is metaphase in the middle or metaphase plate. Anaphase is they're going away from one another, right? So what should I have over here? Again, I should have my microtubule organization center, right? Microtubule organization center. And then what should I have coming over here and connecting? I should have, we'll draw three this time since we only did two last time. What should I have it connecting to? Let's say right here, I'm going to have my chromatids because what am I going to do? Remember that centromere there? I'm going to split the two. I'm going to split the two. There's a protein that's connecting them together called cohesin. I'm going to split the cohesin and there's a special regulation point of that. I'm going to split the cohesin so I can take this sister chromatid, go to this pole, this sister chromatid, go to that pole. So now look here, that chromatid is going to be coming over here. This chromatid is going to be coming over here, but really this is a chromosome. The sister chromatids were separated, but now how many centromeres do I have? One. So that's a chromosome. Then what do I have over here? Another chromosome. What do I have over here? Another chromosome, another chromosome. So now what am I doing? I'm separating the chromosomes from one another because eventually I want all these chromosomes to go to this end. I want all these chromosomes to go to this end because originally what was this whole thing? Four end. There's a total of 92 chromosomes. I need 46 of them to go to one end, 46 of them to go to the other end. So that's what we're doing here. It's just so darn cool, right? So we're separating these two opposite ends of the pole. So where will these guys be going? They'll be going this way. Now an important concept here, we're not going to go into super depth on them, but how the heck do they get there? That's how important thing with science is you have to ask yourself the question sometimes, why are these things happening? So, you know, there's different types of proteins here. We call them motor proteins. So there's special types of motor proteins. We're not, like I said, we're not going to go into super depth, but I just want you to get the idea. There's motor proteins and these motor proteins can literally walk along the microtubules, carrying whatever structure they have with them towards a specific direction. Isn't that cool? So there's different motor proteins that can move these microtubules towards the actual microtubule organization center to the opposite ends of the poles. What are these things called again? They're called motor proteins. There's particularly two in this situation. One is called dynein and the other one is called kinesin. Technically this is a minus indirected motor protein and this is a plus indirected motor protein. I'm just throwing out there. You don't necessarily have to know this. I just want you to get the idea that there is two motor proteins, dynein and kinesin. And what are they doing? They're helping to move these actual chromatids towards the microtubule. That's important. So now we've done anaphase. We've separated the actual chromosomes. Now, once we've done that, what do I need to do? I need to equally distribute this into two cells. So what this cell starts doing, you have different types of actin and myosin proteins here. Let's put here, you have these different types of actin. I'm going to represent this with like red. Here's some myosin proteins or contractile proteins. Here's some myosin proteins, which are contractile proteins. And then let's say near it, we have some actin molecules. So here's some actin molecules, which are contractile proteins. These guys start contracting the cell and they produce this little constriction ring. So we're going to try to take this cell and just squeeze it. When I try to squeeze it to push the stuff in the amount of stuff and equally into both cells, I produce this little constriction ring, but they don't like that name. They call it a cleavage furrow. They call this right here a cleavage furrow. Okay. And it produces this thing called the constriction ring. Now it looks like I'm getting ready to have two cells, right? So now what am I going to do? Remember what we had before? We didn't have a nuclear envelope. Guess what? We start forming again, guys. I don't know why I get so excited about this stuff. I just think it's so cool. But you start actually beginning to reform a nuclear envelope. So now you want to get ready for this cell to be complete. So you start reforming your nuclear envelope. You start pinching and forming this constriction ring called the cleavage furrow through myosin and actin proteins. Then what, what should you have over here? You should have your chromosomes. How many should be over here? There should be a total of 46 here, right? Or we say 2N. How many should be over here? A total of 46. We say 2N. Isn't that cool? What else should you have over here? You should have an equal amount of ribosomes. I'm only going to do a couple of things, but I want you guys to just get the idea. And then what else? You should have equal amount of mitochondria. We're separating these cells just perfectly. Our body's amazing. Now, before we end this off, what else do you have in this cell? What's pretty much the fluid in the cell? We already talked about, remember there's three parts of the cell, cell membrane, nucleus, cytoplasm. The cytoplasm is all the fluid, all the fluid of this cell. So now we want to be able to distribute the cytoplasm evenly between the two cells. So whenever we do, and we finish this process, we're going to squeeze that constriction ring completely together, cause these actual membranes to fuse and equally distribute the actual cytoplasm. Here's one more thing, right? So we said how we've squeezed the cytoplasm equally into both cells, which is the cytokinesis process, right? We produce that constriction ring. And we said that the nuclear envelope starts reforming. Well, you see how we said that we have these chromosomes here, right? We equally distributed the chromosomes. Something else happens before they were condensed, but guess what? They need to become loose again. So the chromatin starts actually becoming a little bit more loose again. So now we can see it like this in the telophase, right? So now we're going to have this loose chromatin. All right. Now, after we've pinched these actual cells off, right, we've equally distributed the cytoplasm. What does that call it again? Whenever we pinch the cells and we actually form that constriction ring, eventually separate the cytoplasm equally, it's called cyto kinesis, right? That's an important part. Now we've pinched this cell. So really we should have two cells here. We should have two cells and these two cells should have an equal amount. Let's assume that they're actual nuclear envelope, completely reformed. So here's a nuclear envelope. Here's the nuclear envelope on this one. And what should you have in there? You should have the chromatin, right? You should have the chromatin. And this should be a total of how many chromosomes? 46 chromosomes, which means it's 2N. 46 chromosomes here is what should be 2N. Now, even though these cells aren't perfectly identical in size, they should have the exact same amount of cytoplasm and the same amount of organelles. All right, guys. So we said that we're going to take a look at the phases of the cell cycle, just models, right? Getting a different look at it. So if you look here, the first one we said is interphase and interphase was consisting of the three parts, right? G1, S, G2. Easiest way to identify it again is if you remember what was happening here, you see how the chromatin is really loose within the nucleus, right? It's really, really loose. And again, what should have happened by now within at the end of interphase, at least you should have actually replicated the DNA. So now it's no longer 2N, but it should be 4N in the cell. Now another thing is actually after we get done with this interphase, we're going to go into the next phase, which is prophase. Now in prophase, what's going to be really different with this one? Look here. You see how the chromatin is still really kind of loose here. Well, another thing that should happen is that the nuclear envelope should actually start breaking down the lamins and condensin proteins, all the things that are making the nuclear envelope up. Remember we're going to phosphorylate those proteins. Other proteins will phosphorylate as like the histone proteins. And then what did we say again? What are these guys right here? These are the microtubule organization center. Remember we have the centrosome and then we have the microtubules that are beginning to form here. Then from the prophase, we can distinguish it different from metaphase. How? Remember what we said as we go from prophase to metaphase, the mitotic spindles, right? Those microtubule organization centers start taking residents up in opposite poles of the cell. And then those microtubules, the polar microtubules start connecting to the chromosomes along this midline of the cell, which is called the metaphase plate, right? Then after that, if everything is successful at that checkpoint, the M checkpoint, there's a protein. We'll talk about them in the regulation video. It's called APC and he'll help to initiate this segregation or the separation of these chromatids from one another. When they start separating from one another, let's go over here because now we're in the next phase. Anaphase. Anaphase, remember, here's those mitotics or the microtubule organization centers and the microtubules are connected to those chromatids and they're pulling the chromatids to opposite poles of the cell. This one's pulling it up. This one's pulling it down. This is how you can distinguish anaphase for the last and final phase. We're assuming that the chromatids and all the organelles and all the cytoplasm is getting equally distributed into the two different cells, right? But then you produce this little contractile ring or this constriction ring, which produces this thing called a cleavage burrow, right? We want to equally distribute all the different cytoplasmic contents into both cells, which is the cytokinesis process, right? So what do we have here? Again, you can notice the two cells that we're trying to form T2 telophase, right? We're trying to form two cells. Another thing is what do you notice here? What's happening with the chromosomes, right? The chromatin is a little bit more loose again, where here it was condensed. Now it's a little bit loose. Also, the nuclear envelope should be reforming. And again, look for that cleavage burrow. And that's how you can identify telophase. All right. So again, real super quick recap. What are these phases of the cell cycle? Again, it's interphase, prophase, metaphase, anaphase and telophase. These cells that we just replicated, what can they do? Well, some of them, guess what? They can go right back into the cell cycle, right back into G1. Some of these cells, which type of cells, the proliferative cells, the labile cells, the epithelium of the skin, the GI tract, the urinary tract, the hematopoietic stem cells, they can go right back into the cell cycle. But some of the cells, they don't really go back into the cell cycle. They go into another area. So they kind of go into this other area where they wait a little bit. What is this area called? This area is called the quiescent. They call this G0, right? So it's called G0 phase or we also call it the quiescent phase. And this is where the cells go to rest. So they can rest in this phase. They don't have to go into any type of replication. They can remain dormant, if you will. But then let's say that there's a stimulus, some type of stimulus, whatever it might be. There's a stimulus to this cell and the stimulus is strong enough to put it back into the cell cycle, to go back into G1 and start undergoing the cell cycle. Those could be some of those stable cells, but there's other cells that no matter what, once they're done, they're amyotic. Those are your neurons, your skeletal, your cardiac muscle. They're not going to proliferate anymore. Another thing that can happen with this cell cycle is, you know, as you get older, as we get older, remember, we had that chromosome right here, right? Here's our chromosome. And as we get older, remember these were the telomeres, these ends up here. As there's consistent DNA replication after DNA replication, after DNA replication, the telomeres start getting shorter over time. So as you age, as we get older, so with age, that's a terrible marker. As we get older with age, during the aging process, what happens to the telomeres? This causes the telomeres to shorten. And sometimes because of that, these cells can go into what's called cell senescence, where they are irreversibly out of the cell cycle. They can't enter into the cell cycle no matter what. So sometimes in situations, as people get older, their telomeres shorten and shorten and shorten. As a result, some of these cells with their telomeres are shorter and shorter and shorter. We put those cells into an irreversible state to where they can't enter into the cell cycle. And that's called cell senescence. Okay. So we've covered these cycles and we said that there's a G1S checkpoint. I should also say that there's one other checkpoint, two other checkpoints. So we said that we had the G2 phase and we said the times this phase is approximately about two hours, about two hours, just to throw that out there. And M phase is probably about the time that you guys have almost watched this video about an hour. So by the time this video is over, you guys have almost undergone mitosis. That's kind of cool. But anyway, there's an actual another checkpoint. This next checkpoint is right here as you're going from the G2 phase into the M phase. So about right here, there's another checkpoint. This is called the G2M checkpoint. We need to make sure that there was no mistakes in the DNA replication process. Because again, even though these DNA polymerases are very, very faithful and they're very good and they only make mistakes every one to out of a hundred thousand million base pairs, we still need to make sure that there was no damage. And there's special genes that do that called ATM genes. And we'll talk that, they produce proteins that read the DNA, but we have to regulate it at that checkpoint. Where's another one? You know, right here at metaphase right here, before we get ready to go into anaphase, there's another checkpoint. Before we get ready to separate these chromosomes, we have to make sure that these guys are aligned at the metaphase plate. Perfect. We need to make sure that there's no mistakes here. And this checkpoint is called the M checkpoint. And we'll talk about the proteins, like the APC proteins secure in and all those different proteins that help to ensure that from that point on everything has occurred successfully and properly. All right, so if you guys have watched this video, I really hope that you guys now understand the cell cycle. I truly do. It's our goal here in engineering science to help this stuff make sense for you guys. So if you guys did, please hit that like button, comment down in the comment section, please subscribe. Also, if you guys get a chance, go check out our Facebook account, interact there, leave some comments, check out our Instagram and even our Patreon account. If you guys have the opportunity, the ability to donate, we would appreciate it. All right, engineers as always until next time.