Understanding the Cell Cycle and Its Phases

Hey everyone, Dr. D here, and in this video we are covering Chapter 12 from our Biology 12th Edition Campbell textbook. This chapter covers the cell cycle, so let's go ahead and get started. Dr. D, Dr. D, Dr. D, Dr. D, explain stuff. All right, let's delve into chapter 12, the cell cycle. So what is the cell cycle? The cell cycle is based on the reproduction of cells or cell division. This is when one cell divides into two. And there are numerous reasons for cell division. These include asexual reproduction, which we will talk about later in this chapter, growth, development, tissue renewal, and healing. All of these are requiring cell division. Now, before we delve too deep into cell division, we need to understand how our DNA works. What is our genome? What are chromosomes? And how does our DNA look? The reason we need to understand this is because it's not until we totally understand what our DNA consists of that we can totally understand how cell replication works. So first of all, all of the DNA in a cell constitutes the cell's genome. Okay, so your genome consists of all of your DNA and each of your nucleated cells in your body, you know how you have billions upon billions of cells in your body, each of those cells with a nucleus contains your entire genome, all of your different genes. A genome can consist of a single DNA molecule or chromosome in prokaryotic cells or a number of DNA molecules or chromosomes in eukaryotic cells. DNA molecules in a cell are packaged into chromosomes. So do you remember when we talked about chromosomes, when we discussed cell structure? A chromosome is a continuous piece of double-stranded DNA in the genome and Remember chromosomes? The DNA is wrapped around those histone proteins. Remember the histone core that's made up of eight histone proteins? And together, the DNA plus the histone proteins were called chromatin. Do you recall that from the chapter on cells? So you can think of each continuous piece of double-stranded DNA plus histone protein is called a chromosome. And Some organisms only have one chromosome. Remember we discussed this before, how prokaryotes, you know, bacteria, have a single circularized chromosome with no beginning and no end? Kind of like a rubber band, right? However, eukaryotes typically have numerous chromosomes. So, for example, how many chromosomes does a human have? Can you beat Wicket? Right, as always, Wicket. 46 chromosomes. A typical human has 46 chromosomes. And so we have 46 pieces of DNA inside of our nucleus, right? And so a DNA molecule of a chromosome carries several hundred to a few thousand genes. So a gene, do you recall what we said a gene was? A gene is just a segment of a chromosome that codes usually for a protein. Now there are between, you know, depending on the textbook you read, between 23,000 and 25,000 different genes. Well, if there are only 46 chromosomes, then that means that each chromosome contains hundreds or thousands of genes. Think about it. The human genome has between 23,000 and 25,000 genes. and only 46 chromosomes, this means that each chromosome houses hundreds or thousands of genes. Okay, and again, humans have 46 chromosomes, whereas prokaryotes like bacteria and archaea, they typically only have one chromosome. Here you can see a eukaryotic cell that where the DNA is visible. And you can see the chromosomes. You can see those chromosomes inside of the nucleus. This would be the nucleus and inside of the chromosomes. All right, so let me share with you in this image what's known as the lab procedure karyotyping. Here a scientist would lay out every chromosome side by side. These are the 46 chromosomes of a human being. I want you to notice something. I want you to notice that although there are 46 chromosomes, two of your chromosomes are called chromosome 1, two of your chromosomes are called chromosome 2, two of your chromosomes are called chromosome 3, and so on and so forth. There are only 23 different chromosomes, and you have two of each type. So here's some terminology I'd like you to know. A chromosome 1 through 23 is called a set of chromosomes. So if chromosome 1 through 23 is called a set, then how many sets of chromosomes do humans possess typically? That's right, Wicket, two sets. Two sets of 23. Now let me ask you this, why do you think you have two chromosome 1s, two chromosome 2s? two chromosome threes. In a typical cell, why would you have two of each chromosome? Well, if you guessed that you got, that you inherited one set from mom, your mother, and one set from dad, your father, and that's why you have two chromosome ones, well then you'd be right. One set of chromosomes, one through 23, was present in the sperm And one set of 23, a chromosome 1 through 23, was present in the egg. So when the sperm fertilized the egg, that's how you inherited your two sets of chromosomes. You essentially inherited one set of 23, you know, a number 1 through 23 from your father. and one set of 23, a number 1 through 23 from your mother. So this is the reason why you have two sets of chromosomes. Now let me ask you this, why do you think you have two chromosome 1s, two chromosome 2s? Do you think these two chromosome 1s are identical? That's right, Wicket, these are not identical chromosomes. You inherited one chromosome 1 from your mom and one chromosome 1 from your dad. You inherited one chromosome 2 from your mom and one chromosome 2 from your dad. So again, let me ask you, do you think these are identical? Do you think these two chromosome 1s are identical? No, they are not identical. Okay, they are not identical. So let me ask you this. Why do you think they're both called chromosome 1? Why do you think they are both called chromosome 1? Well, it's because of the types of genes present on chromosome 1. So, for example, I'm just making things up right now, but just to make a point. For example, if eye color gene is found on my chromosome 1, you know, it's on everyone's chromosome 1. So eye color gene, if it's on chromosome 1, in my genes, in my chromosomes, then it's on everyone's chromosome 1. So eye color gene might be right here on the chromosome 1 from your dad, and it's right here on the chromosome 1 from your mom. But does that mean that both of those eye color genes would say the same information, would dictate the same eye color? No, you might inherit blue eye color gene from your mom, and you might inherit... brown eye color gene from your dad. These are known as alleles or versions of a gene, okay? So just because it's the same types of genes that appear on the chromosomes doesn't mean it's the exact same information that appears on the chromosomes. So you can think of chromosome one, chromosome one, these two chromosomes would have the same types of genes, hair color, eye color. But that doesn't mean it's going to be the exact same information. It's not going to be the exact same hair color because you inherited one of those genes from your mom and one of those genes from your dad. And by the way, these have a name. When you have these two chromosome ones that are not identical, okay, they are not identical. One was inherited from your mom and one was inherited from your dad. What they do. do share in common are the types of genes, but not the exact same genes, not the exact same information, right? These are known as homologous chromosomes. Does that make sense? Homologous chromosomes. So what do I need you to know about homologous chromosomes? One homologous chromosome is inherited from your mom. The other homologous chromosome is inherited from your dad. One was from the sperm. One was from the egg. They have the same chromosome number because they share the same types of genes, but they are not identical because they don't, you know, one's from your mother and one's from your father. So even though they both may share, I don't know, hair color gene, one might say brown hair color while the other says blonde hair color, if that makes any sense. So are homologous chromosomes identical? No, that's right, Wicket. They are not because one's inherited from your mom and one's inherited from your dad. So now let me ask you this. If 46 chromosomes, which you see right here before your eyes, if 46 chromosomes are present in each and every one of your cells, you know, that has a nucleus, then what do you think needs to happen to these chromosomes? Before the cell can divide, what do you think we need to do with these chromosomes before cell division? And remember, the purpose of cell division, like let's say one of your skin cells divides to become two skin cells, you know, the purpose of that is to make two genetically identical cells, right? So let's say the cell just wants to copy itself. You want to make two copies of the cell. That's that. typical cell has 46 chromosomes, what do you think we need to do first? Can you beat Wicket? That's right, Wicket. We need to copy the chromosomes. We need to make a carbon copy of every chromosome. Does that make sense? Because think about it. If one of my cells wants to divide to form two cells, well, I want to make sure that those two cells also have the 46 chromosomes of my genome, right? If I don't copy all my DNA and the cell just divides, well then I'm going to dilute my chromosomes. I'm not going to have two genetically identical cells, am I? I'm going to have two cells with only 23 chromosomes, which doesn't make any sense. So before a cell can divide, right, for cell division, we need to go through and look at this. We need to make a copy of this chromosome right here. And we need to make a copy. of this chromosome. So we need to make a copy of mom's chromosome 1, and we need to make a copy of dad's chromosome 1, and we need to make a copy of mom's chromosome 2, and we need to make a copy of dad's chromosome 2. We need to go through and copy each and every chromosome, make a carbon copy. And do you know what that's called? Those are called sister chromatids. Here we go, sister chromatids. So again, for example, let me show you something. Let's just take this example right here. See this example right here? This is the homologous chromosome pair, chromosome one. One's from mom, one's from dad. They're both called chromosome one because they share the same types of genes, but they're not identical because obviously you inherited different information from mom than dad. So again, let's just look at chromosome one. Now remember chromosome one. Here's chromosome one. Okay, let's pretend this is chromosome one. This one's dad's chromosome one, right? This one's mom's chromosome one. They're both chromosome one, but you inherit one from the paternal lineage, you know, your dad. And one is maternal from mom. These are known as what? With respect to each other, what are these? That's right, they are homologous chromosomes. And again, what did I say we need to do with these chromosomes before the cell can divide? That's right, Wicket. We need to make a copy. So see this dad's chromosome one? I am going to make an exact copy of this dad's chromosome one. I'm going to make a copy of it and watch that. So now I have two copies of dad's chromosome one right here. These are identical now. In every way, they are identical copies of each other. And I'm going to do the same thing with mom's chromosome one here. Look. I'm going to take mom's chromosome 1 and I'm going to make an identical carbon copy of it as well. Here we go. On the right, you can see I've made two copies, two identical copies of mom's chromosome 1. By the way, what are these two copies called? The copies are called sister chromatids. Sister chromatids. So I need you to really, really understand this. I need you to understand this. I want you to understand the difference between homologous chromosomes and sister chromatids. So I just want to take you to the board to really drive this home and then we'll be right back here to talk a little bit more about important features of this process. What are homologous chromosomes? As you know, humans have 46 total chromosomes, right? Humans have 46 chromosomes total. But you inherited 23 of those from your dad and you inherited 23 from your mom. All right, I'm going to use blue for dad, red for mom. Okay, the egg from your mom's egg, the egg housed 23 chromosomes. One chromosome one. one chromosome two, one chromosome three, one chromosome four, you get the idea. 23 different chromosomes. The sperm from the dad housed 23 chromosomes as well. This included a chromosome one, a chromosome two, a chromosome three. So every person then has 46 total chromosomes but they actually have two sets of chromosomes. So what's a set? A set means 1 through 23. Having chromosomes 1, chromosome 2, chromosome 3, chromosome 4, a set of chromosomes 1 through 23, that's a set. So you inherited a set from your dad and you inherited a set from your mom and so you have two sets of chromosomes. You know what that means? That means, let me draw chromosome 1 for example. Let's say this is chromosome 1. from your dad. You also have a chromosome 1 from your mom. Now both of these are called chromosome 1. Okay both are chromosome 1 but are they identical? If you were to sequence the DNA would it have the exact same sequence? No. That one's from your dad and that one's from your mom. Your mom and dad are not related. So they're not going to have a similar DNA. It's not going to be the exact same. So these are not the exact same chromosome. They don't have the exact same sequence. But they're both called chromosome 1. Why are they called chromosome 1? They're both chromosome 1. and you have two chromosome twos and you have two chromosome threes and you have two chromosome fours so what constitutes the chromosome number well here's the thing these two do have something in common but it's not the exact same sequence what they have in common is that the same types of genes that are on this chromosome are found on that chromosome does that make sense You remember what chromosomes are? Chromosomes are one continuous piece of chromatin or DNA. One continuous piece. And remember the chromosomes house the genes, right? There are genes along the chromosomes. Each chromosome contains thousands or at least hundreds, I should say, hundreds of genes on each chromosome. All right, so whatever genes appear on this chromosome one, also appear on that chromosome 1. For example, if eye color, the gene for eye color was on this chromosome, it would also be on the other chromosome 1. was on this chromosome 1 from mom, it would also be on that chromosome 1 from your dad. So what we could say is the same types of genes appear on these chromosomes, but not the exact same sequence. So again, you have two chromosome 1s, two chromosome 2s, two chromosome 3s, but they're not identical. One's from your mom, one's from your dad. But what they do share in common is types of information, the same types of genes. Okay. And, and there's a term for this. You see how there's the paternally inherited from your dad and the maternally inherited chromosome one, they are called homologous chromosomes. Okay, homologous chromosomes. Homologous chromosomes are not identical. Homologous chromosomes share the same chromosome number, but remember one was inherited from your dad, one from your mom, so they have the same types of genes on there, the same types of information, but they're not identical. Okay, now again you've inherited 23 chromosomes from your dad. 23 chromosomes from your mom. You have two chromosome 1s, two chromosome 2s, two chromosome 3s. Okay, now what happens if the cell wanted to divide, right? A normal cell has 46 chromosomes, right? Well, what if that cell wants to divide into two cells? Well, it can't just go ahead and divide because if a cell went ahead and just went ahead and divided, Each new daughter cell would have 23 chromosomes. Does that make sense? That's not good. You don't want your daughter cells to have 23 chromosomes each if one cell divides into two. You want it to divide so that it retains its 46 chromosomes. So in that case, you know what you need to do? Before the cell divides, Before the cell divides you need to copy all the DNA don't you need to copy all the chromosomes So for example, if this is dad's chromosome 1 you need to make a copy of it You need to have two dad's chromosome ones right see you make a copy of dad's chromosome one So these are now identical copies of dad's chromosome one and then you also need to copy mom's chromosome one You see that you need to make a copy of that as well. So this is an exact carbon copy of mom's chromosome 1. Now you have 2 of mom's chromosome 1, 2 of dad's chromosome 1, and you're going to have a copy of all your 46 different chromosomes. So how many total chromosomes do you have at this point? You have 92 total chromosomes, right? 92. That way, that way, if you were to divide at this point, you would have 46, 46. You'd be back where you started, which is what you want. Okay, there's a term for this. You need to understand this. You see how Now you have a copy of every chromosome. Those copies have a very specific name. Those copies are called sister chromatids. Okay? Sister chromatids are identical in every way. Sister chromatids are identical in every way. Sister chromatids. And here's the thing, a normal skin cell doesn't have sister chromatids, right? It just has 46 chromosomes, one set from mom, one set from dad, okay? The only time a cell has sister chromatids is when the cell is about to divide, the cell has considered it is about time to divide the cell into two. At that point it makes a comp- of all the 46 chromosomes and only at that point do you have sister chromatids. Okay so hopefully this was helpful. You should definitely, definitely understand the difference between homologous chromosomes which are not identical. One was inherited from mom, one was inherited from dad. They share the same chromosome number because they have the same types of genes and then the difference between that and sister chromatids which only exist after the DNA has been replicated by the way foreshadowing this is going to happen during S phase of interphase okay only at that point do you have a sister chromatids All right, welcome back from the board. I just wanted to give you a little more terminology here when it comes to these sister chromatid pairs. This is a pair of sister chromatids in blue. Next to it is a pair of sister chromatids in red. Now, it is proper to say this pair of sister chromatids on the right, this pair of sister chromatids is homologous to this other pair of sister chromatids. You see, so that is proper terminology. The two blue ones are still homologous to the two red ones, as long as they all have the same shared chromosome number. So for example, if all four of these blue and red chromosomes are chromosome one, if all four are chromosome one, then yes, these two sister chromatids are homologous to these two sister chromatids, if that makes sense. Also, you see how chromosomes have this narrow waist? Every chromosome has a narrow waist, and that narrow waist is called the centromere, centromere. And the centromeres are where the two sister chromatids join together. See, so this sister chromatid and this sister chromatid, they're connected at the the hip, you could say, at the centromere. And the proteins that hold the two sisters together are called cohesin proteins. So you see cohesin proteins at the centromere hold sister chromatids together at the centromere. Okay, and there are other proteins on the outside of the centromere called kinetochore, and I'll explain what these do later on. So these X structures are probably what pops into your mind when you think of chromosomes. However, I want to tell you something really important. These chromosomes, they only look like this during cell division, when the cell is actively dividing. In fact, that's called mitosis, right? Mitosis is what we're going to be going over in just a few slides here. And mitosis is where the chromosomes segregate to result in two genetically identical cells. But what I want you to understand is this, the chromosomes only look like this during mitosis when the cell is actively dividing. Okay, the rest of the time, the DNA looks nothing like this. The DNA is very, very, very condensed here in this mitotic chromosome. But normally, when the cell is not actively dividing, when the cell is not undergoing mitosis, cell division, the DNA is very loose. It's nowhere near this tightly packed up. The DNA unravels and becomes loose. And when it does, I call it a bowl of spaghetti, right? The DNA just unravels into a bowl of spaghetti. And at that point, you can't see these X structures. You can't see the individual chromosomes. When a cell is not in cell division, when the cell is not undergoing mitosis, the chromosomes look nothing like this. In fact, you can't see the chromosomes at all because they're just kind of like a diffuse blob of DNA. You can't see how many chromosomes there are. You can't see individual chromosomes. It's just a bowl of spaghetti, right? It's just a bowl of chromatin. You can think of it that way, okay? So just realize that. And for that reason, these chromosomes here and these chromosomes here and these chromosomes here, you see how they all look like little, you know, individual Xs or individual structures. These are known as mitotic. chromosomes, okay? Mitotic chromosomes, because they only appear like this during mitosis. When the cell is not actively in mitosis, you do not see these structures because the DNA is a bowl of spaghetti. All right, so what happens during the cell cycle? Well, you need to know that there are two major phases to the cell cycle. There is what's known as the mitotic phase or M phase where the cell is actively dividing. And then there's interphase where the cell is not actively dividing. In fact, that's when the cell grows, the cell copies its chromosomes, the cell prepares for cell division, right? All of that occurs during interphase. And by the way, interphase takes up most of the cell cycle timeline. On average, interphase takes up 90% of the timeline of the cell cycle, whereas the mitotic phase, M-phase, where the cell is actually dividing, this takes, you know, around 10% of the cell cycle timeline. M-phase includes what's known as mitosis. Mitosis is where the chromosomes separate or sister chromatids separate into new daughter cells. And then cytokinesis, meaning where the cell itself separates, the cytoplasm separates to form two distinct cells. Again, mitosis is the separation of the chromosomes to result in two genetically identical cells, and cytokinesis is the separation of the cells themselves, right? And this is together they are known as M-phase. So let me tell you more about what goes on during interphase. During interphase, 90% of the cell cycle, there are three sub-phases. G1, called first gap, S-phase, or synthesis phase, and G2-phase, second gap. These are three sub-phases of interphase. So let me tell you a little more about what's happening during these sub-phases. Look, if this is the cell cycle, then the big, the big teal arrow is interphase. And remember that takes up about 90% of the cell cycle. At first, you have G1 phase or the first gap. During G1 phase, you need to know that the cell is growing. There's metabolic activity. The cell is growing. Then the cell will enter what's known as S phase or synthesis phase. This cell continues to grow, but what's most important is that DNA is synthesized, or the DNA is copied. Remember when I said the 46 chromosomes in human, they need to be copied to form sister chromatids? This is occurring during S phase, DNA synthesis. So here is when the sister chromatids form, because we copy all 46 chromosomes. In fact, at this point, the cell has 92 total chromosomes. And next you have G2 phase, or the second gap phase. There's more metabolic activity and growth, and there's preparation for cell division. Specifically, I want you to know that not only is there preparation for cell division, the centriole pairs, or the centrosomes they're called, are replicated during G2 phase. Alright, so these are the subphases of interphase. Remember, interphase is made up of subphases G1, S, and G2. During G1, the cell undergoes growth. There's growth. During S phase, the DNA is replicated. And at G2, this is where the chromosomes start. to coil more tightly. Remember what I was saying is that the DNA is usually loose, but then the DNA can coil up before mitosis, right during mitosis. And the centrioles are also replicated. So turn your attention to this image on the right here. In the nucleus, you don't see individual chromosomes, do you? Remember, this is the bowl of spaghetti. Right here you have a bowl of spaghetti. The DNA is all loose and strewn about. This chromatin is very loose. And so because the DNA is not compact, the DNA is really indistinguishable. The individual chromosomes, I should say, are indistinguishable. And look at this. Remember these centriole bundles where animal cell usually has one centriole bundle and this forms the centrosome. Well, at this point during G2, the centrioles replicated. So you have two centrosomes. Okay. Again, that's 90% of the cell cycle called interphase. And what's left? That's right. Mitotic phase. This is where the cell actually divides. During mitosis, the chromosomes segregate. During cytokinesis, the actual cell separates into two. So let's go into that in more detail. Speaking of mitotic phase or M phase, let's go ahead and start with mitosis and see what are the steps that make up mitosis. Mitosis is conventionally broken down into the following five stages. Prophase. Prometaphase, metaphase, anaphase, and telophase. So let's start with prophase. Remember, the cell has just completed interphase with G1, S, and G2, and the DNA is now fully condensed. So let's talk about what's happening during prophase, which is the first subphase of mitosis. Look inside of the chromosomes. Do you see individual chromosomes now? Yes, you do, right? You see those. X structures. Why do you see the X structures and not just a bowl of spaghetti? Well, because at this point, the DNA has condensed into what's called those mitotic chromosomes. And these Xs are the sister chromatid pairs that we had discussed before. These sister chromatid pairs, which are held together at the centromere by cohesin. proteins. Remember that? So what you should know is that during prophase, sister chromatid pairs form. Okay, they show up as these little X's. And the nucleus is still present. Here are your two centrosomes. Okay, the centrosomes are kind of moving towards opposite ends of the cell. All right, but what I really want you to know is that the nucleus is still present and sister chromatid pairs form as these little X structures. All right now what happens during pro-metaphase? Look at this. Do you still see a nucleus or has the nucleus broken down? The nucleus has broken down. I don't see a nucleus. In fact the sister chromatid pairs are floating around in the cytoplasm. And I want you to take a close look here. Not only are the sister chromatids floating around, but it looks like microtubules spanning from the centrosome here. The microtubules have attached to the sister chromatid pairs. One microtubule attaches to this sister. Another microtubule from the adjacent or from the opposite centrosome is attached to this. sister here. So take a look. I just want to show you what's going on. Do you remember when I said that there are these proteins on the outside of the centromeres as well? Not the cohesin proteins that hold the sisters together, but these proteins on the outside of the sister chromatid pairs. These are called kinetochore. And kinetochore, you can think of them as docking sites for microtubules. There are microtubules growing from the centrosome at one end of the new cell, and there are microtubules growing from the other end of the cell, from the other centrosome at the other end of the cell. So what happens during prometaphase is that there's this docking that happens. Microtubules attach to kinetochore on the sister chromatid pairs. And look at this. One sister is connected. to one centrosome via microtubule and the other sister is connected to the other centrosome via microtubule. So each sister is connected to the opposite end of the cell, right? Each sister is connected to a microtubule from the opposite ends of the cell. And those sisters begin to make their way to the center of the cell. Are the sisters at the center of the cell yet? That's right, Wicket. Not yet, but they try to make their way to the center of the cell. So remember, what do I need you to know about prometaphase? The nuclear envelope breaks down and the sister chromatids start moving towards the center of the cell. Next, we have metaphase. During metaphase, those sisters have made their way to the center of the cell. In fact, they've lined up down this imaginary plate called the metaphase plate. It's an imaginary plate down the center of the cell. And those sister chromatids have lined up down that metaphase plate. And there you go. And remember, each of these Xs is two identical sisters. One sister is connected to this centrosome with a microtubule. The other sister is always connected to the opposite centrosome with a microtubule. And that's what's happened during metaphase. So what do you need to know about metaphase? You should know that sister chromatid pairs line up down the metaphase plate, down the center of the cell. You know what happens next during anaphase? Do you remember those...... Those cohesin proteins that are holding these sisters together at their narrow waist, the centromere, those cohesin proteins separate, they let go, and the sisters move in opposite directions, right? So each sister moves to an opposite end of the cell. Again, what do I need you to know about anaphase? During anaphase, sister chromatids separate from one another and head to opposite poles of the cell. And then lastly telophase. Those sisters are now, they've reached the opposite ends of the cell and daughter nuclei start to form. That means a nucleus starts to form around these chromosomes. A nucleus starts to form around these chromosomes and the DNA will start to spaghettify. The DNA will start to decondense at this point. So that leads to the end of mitosis. Mitosis results in two genetically identical cells. And that makes sense if you think about it. Remember, in humans, we had 46 chromosomes, none of which are identical. Those chromosomes were copied into 46 identical chromosomes, right? So we copied our chromosomes and we now had 92 chromosomes. Those sisters, those copies lined up down the center of the cell, and then those copies separated. So this cell got a copy, the other cell got a copy. For example, this cell got a mom's chromosome 1, the other cell got an exact copy of mom's chromosome 1. This cell got a dad's chromosome 1, the other cell got an exact copy of dad's chromosome 1. So every one of these chromosomes was copied. and then separated into the new daughter cells. This results in two genetically identical cells. So each daughter cell will have 46 chromosomes, and those chromosomes are identical to each other as well as to the original parent. All right, before we move on from mitosis, I just want to share a little more terminology with you that we didn't delve too much into, but we should at least address it. Remember the centrioles? The centrioles are bundles of microtubules at the core of the centrosome. At this end of the centrosome, you see these short microtubules that almost look like a little starburst right here? Those short microtubules radiating out from the centrosome is called the aster. In fact, aster means star. Then there are microtubules that grow from the centrosome. This is known as the mitotic spindle. Here's more terminology for you. Remember during anaphase, I said that the cohesin proteins let go and the sister chromatids move to opposite ends of the cell? Well, this is thanks, during anaphase, this is thanks to an enzyme called separase. Separase cleaves the cohesin proteins. releasing the sisters from one another. Those sisters then separate and move to opposite ends of the cell. Now you might be wondering, how do the sisters separate and move to opposite poles of the cell? Well, you're in for a treat because they've actually determined this, and it's really interesting how it works. At the core of the centrosome, you see here the kinetochore, at the core of the kinetochore, no pun intended, but if this is a sister chromatid in the kinetochore, which is connected to the microtubules that come from the centrosome, those, those kinetochore contain, look at this, motor proteins. These motor proteins, hi, wicked, wicked's here pawing at the screen. Hi, hi, buddy. Hi, buddy. Hi. Hey, buddy. Hi. Hi, buddy. Okay. Um, I'll get. right with you, Wicket. Now, these microtubules, what happens is these motor proteins, which are found inside of the kinetochore, they actually walk along the microtubule. And that's how the sisters move. So if you've ever wondered how the sisters move to opposite ends of the cell, It's actually because of motor proteins inside of the kinetochore. Isn't that neat? So the sisters move themselves to opposite ends of the cell. And look what I have here highlighted at the top. Sister chromatids move to opposite poles of the cell by motor proteins found inside a kinetochore. They make their own way towards the poles of the cell. They're not being pulled towards the poles of the cell. They make their own way via motors in the kinetochore. So now that we understand how mitosis works, remember mitosis is the separation of the chromosomes resulting in two genetically identical cells. Now it's time for the cell itself to separate. This is known as cytokinesis. So let's discuss how cytokinesis works in animal cells versus plant cells. Alright, so take a look here at cytokinesis in the animal versus the plant cell. On the left, we have an animal cell, and you can see here cytokinesis in action. What happens is there's a constriction of the cell membrane where there's like this pinching that occurs, and that's called a cleavage furrow. A cleavage furrow is forming. And remember when we learned about microfilaments? Microfilaments are actually responsible for producing the cleavage furrow. Isn't that interesting? Microfilaments form what's known as a contractile ring, and that contractile ring is responsible for forming the cleavage furrow. And there's like this constriction that occurs until you have two daughter cells that are separated from one another. And again, this is occurring in animal cells. Now in plant cells, what you have are little vesicles full of cell wall material. Remember cell wall material is cellulose? Well, little vesicles full of cellulose line up down the center of the cell. And you can kind of see that in this cartoon here, little vesicles lining up. And these vesicles are actually from the Golgi. They originated from the Golgi and they line up down the center of the cell. And then do you remember when vesicles meet? Remember it's like two soap bubbles that meet, they pop, they kind of fuse to one another. And so these vesicles will fuse to one another, forming what's known as an early cell plate, which is like a new nascent cell wall in the center of the cell, which will then mature into a mature cell wall, a new cell wall. So again, cytokinesis occurs via cleavage furrow formation, thanks to microfilaments in animal cells. And cytokinesis occurs via vesicles, vesicles that form a cell plate, a cell plate in plant cells. And here you can see the complete M phase or mitotic phase in a plant cell. Remember mitotic phase consists of cytokinesis as well as mitosis. Now what you should realize is mitosis and cytokinesis are not the same thing. However, together they make up M-phase. Here you can see a plant cell. The plant cell is rectangular, right? You've got the rectangular cell wall. The plant cell is undergoing prophase, where the sister chromatid pairs form. Then we have prometaphase, where the nuclear envelope breaks down and the sister chromatids begin to migrate towards the metaphase plate. And then you have metaphase, where the sister chromatid pairs have lined up down the metaphase plate at the center of the cell, then anaphase, where the separase protein or enzyme has broken apart the cohesin, and the cohesin lets go of the sisters. The sisters migrate to opposite ends of the cell. Thanks to those motor proteins in the kinetochore, those sister chromatids start moving to opposite ends of the cell. And then remember, During telophase, your daughter nuclei form and the DNA can begin to spaghettify, to unravel, to relax. And usually telophase is somewhat concurrent. That means occurs at roughly the same time as cytokinesis. So you can see a cell plate also forms. So again... Cell plate formation or cytokinesis can begin before telophase is finished, but that does not mean that it is part of telophase. Okay, just keep that in mind. Awesome. Well, let's take a quick break time with Gizmo and Wicket, see what these two little guys are up to. And when we come back, I'm going to go through mitosis one more time with you at the board, and then we're going to get into some other topics as well. All right, welcome back from break time with Gizmo and Wicket. Let's go to the board one more time where I just want to give you another comprehensive look at mitosis and cytokinesis and interphase and the cell cycle in general, just so you have an idea of everything that's going on one more time. Alright, so the cell cycle begins with a process known as interphase. Interphase takes up about 90% of the cell's cycle time. This is essentially where the cell is growing, the cell is functioning, and the cell is preparing to divide. It starts with G1. Alright, G1 is a sub-phase of interphase. During G1, the cell grows. The cell is just growing, it's producing proteins it needs, organelles it needs. Remember animal cells have one pair of centrioles at this time and if you look inside of the nucleus there are homologous chromosomes. Dad and mom chromosome one, dad and mom chromosome two. 2 for example, there would be 46 chromosomes in a normal cell at this point. Now usually a cell, if the cell wants to reproduce or divide, the cell will proceed to S phase, But if the cell does not get that clearance to go into S phase, the cell will exit the cell cycle and actually enter what's known as G0. Just be aware of that. The cell can exit the cell cycle and go from G1 to G0. But when the cell does get the go-ahead to go forth into S phase, it'll copy each and every chromosome. Notice that you started with 46 chromosomes total in a human cell. Well, you're going to copy each and every chromosome. So you're going to have two copies of chromosome 1 from the dad, two copies of chromosome 1 from the mom, two copies of chromosome 1 from, sorry, chromosome 2 from the dad, two copies of chromosome 2 from the mom. You're going to go from 46 chromosomes to 92. You're making a copy of each and every chromosome. And just as a quick review, just remember that homologous chromosomes are not identical, but they share the same chromosome number. So if this was chromosome 7 from the dad, that would be the paternal chromosome 7. this would be the homolog which would be the maternal chromosome 7. Paternal is inherited from the dad. Maternal is inherited from the mom. And a normal cell does has homologous chromosomes inside. Okay a normal cell. Now only after S phase where the DNA has been replicated do you have what is known as sister chromatids. This would be two identical copies of the same exact chromosome. So for example that would be chromosome 7 from the mom, that would be chromosome 7 from the mom. They're both exactly identical in every way and the sequence is identical. So at the end of S phase you have sister chromatids formed, identical copies. Next we enter G2, growth phase 2. At this point look the centrioles where there was one bundle of centrioles they replicate. Now you have two bundles of centrioles and they start to drift apart. And that's about it. The DNA starts to condense into those mitotic chromosomes. Remember, DNA is loose and then DNA starts to condense in preparation for mitosis. Okay, this would be the end of interphase. We are now moving over to mitosis, which starts with a process known as prophase. During prophase, notice that those centrioles are now drifting further and further apart. apart towards opposite poles of the cell. And look at these little green lines I drew. These little green lines represent the mitotic spindle. That's microtubules that are actually being synthesized or produced from the centrioles. Okay, at this point, you wouldn't call these centrioles anymore. It's more accurate to call them centrosomes because centrosome means centrioles plus microtubules. Okay, and if you look inside of the nucleus, what do you have? You have sister chromatid. pairs form. Sister chromatid pairs form and condense into those mitotic chromosomes. Mitotic chromosomes are when the chromosomes have condensed into their most tight form and they look like those X structures. Why do they look like X's? Because one sister is now connected to another sister at the narrow waist. At the narrow waist called the centromere. And the reason the two sisters are connected to one another at the centromere is because cohesin proteins are holding them together as a pair. Okay, so you have, for example, both of dad's chromosome 1, both of mom's chromosome 1 as a pair, both of dad's chromosome 2, and both of mom's chromosome 2. Now you're ready for prometaphase. And look, the nuclear envelope, the nucleus has broken down. The sister Chromatids are essentially floating around the cytoplasm. at this time. The nuclear envelope breaks down and microtubules attach to kinetochore. So what does that mean? Notice that the centrosome here and the centrosome here, those microtubules have really grown. And when the microtubules microtubule encounters the sister chromatids at the centromere, it can actually connect to the sister chromatid. Microtubules connect to the sister chromatids at the centromere, at the narrow waist. And the The way they connect is that there are these proteins there called kinetochore, or kinetochore microtubules attached to kinetochore. Kinetochore are proteins at the centromere that are docking sites for the microtubules. Microtubules attach to them. Okay. So again, what's important about prometaphase, nucleus breaks down, microtubules attach to kinetochore on the sister chromatid pairs. And notice that the sister chromatid pairs. chromatid pairs start making their way to the center of the cell. By metaphase, they have made their way to the center of the cell. It's called this imaginary metaphase plate, right? Sister chromatids line up at center of cell, okay? Sister chromatids line up at the center of the cell. And all of the sister chromatids are attached at their kinetochores to microtubules from opposite poles of the cell, opposite centrosomes. Now, do you remember the cohesin proteins holding the sisters together? The cohesin proteins holding the sisters together? Well, those break apart. Those let go. And look at this. this sister chromatid separates from that sister chromatid so anaphase sister chromatids separate from one another when those cohesin proteins that held the sisters together release one sister chromatid heads to the left and one sister chromatid heads to the right so why is that important remember these sisters are exact same information right so what that means is the same information is going to this cell as is going to this cell. If this is dad's chromosome one going to this cell, this is another exact copy of dad's chromosome one going to that cell. And there's mom's chromosome one heading to this cell, mom's chromosome one heading to that cell. So whatever information is heading to the left, that same exact information is heading to the right because its sister chromatid is heading to the right and that is an exact copy. And just to be clear, the reason these are traveling apart, the reason they are moving is not because the centrosomus is pulling the the sisters the sisters themselves make their way to the opposite poles of the cell you know why do you remember those little kinetochore proteins attached to the centromere those little kinetochore proteins have little motor molecules inside that walk so these they're they're walking okay there's kinetochore those kinetochore proteins have little motors inside that walk so the sister chromatids are actually walking themselves to the opposite poles of the cell. It's a pretty interesting fact. So what happens in the next step, telophase, this is the final phase, subphase of mitosis. Now you have two cells with 46 chromosomes each and whatever DNA is in this new nucleus is found in this new nucleus. Okay, notice that the new nuclear envelope starts to form so a daughter nucleus forms here and a daughter nucleus forms here. daughter nuclei form. And usually cytokinesis can start at telophase, but what you should realize is cytokinesis is not part of telophase. Cytokinesis is not part of mitosis, but it is concurrent with mitosis. So it can happen at roughly the same time. There's some overlap there, but mitosis plus cytokinesis is actually called M phase or mitotic phase. Just be clear on that. All right. So what is the product of mitosis? Mitosis results in two genetically identical cells. All right. Mitosis results in two genetically identical cells. And those are somatic cells, two genetically identical somatic cells. So this cell and this cell are identical to one another. And they are identical to the original cell that made them way back. Now, did you know that prokaryotes do not undergo mitosis when they divide? Instead, prokaryotes, including the bacteria and the archaea, they undergo a process known as binary fission. It's an asexual type of reproduction. In binary fission, the chromosome replicates. Remember, usually prokaryotes only have a single circularized chromosome. And that single circularized chromosome begins replicating at a spot called the origin of replication. And then two daughter chromosomes actively move apart. So let me show you how this process works. Again, here you have a bacterial cell or a prokaryotic cell of some sort. Remember that its DNA only consists of one chromosome, and it's a circularized chromosome. So if I were to untangle this chromosome, there would be no beginning and no end. It's a closed circle of DNA. Now, this DNA needs to be copied, right? Because the cell is going to be dividing soon. So at this specific point called the origin of replication. That's where the DNA replication occurs. Essentially, the DNA is unwound and DNA is copied in both directions. See, there's one direction where you're copying the DNA going this way, and then you're also copying the DNA going the other way. And then at some point, you have copied the DNA and you reach the termination site. Those two copies of the chromosome act as a link between the DNA and the chromosome. move to opposite ends of the cell. And then there's this kind of this inward growth that's happening. It's analogous to cytokinesis, and it's called septum formation. So you should know that cytokinesis occurs via septum formation. This is known as the septum. It's an inward growth of the bacterial cell wall. And that results in two cells, two daughter cells. Now let's turn our attention back to the eukaryotic cell. Now you and I are what's known as multicellular organisms. We are made up of billions and billions of different types of cells. And did you know that some of the cells in your body divide all the time, while other cells in your body divide much more rarely? And yet other cells don't divide at all, right? So for example, the cells of your spinal cord and your heart, they do not divide, if ever. Whereas the cells in your hair follicles and the cells of your bone marrow that give rise to the blood, those cells are dividing all of the time. So what is the difference between, you know, oops. the cells that are growing all the time versus the cells that are never growing, these cells are being controlled. Does that make sense? There is something known as the cell cycle control system, and this cell cycle control system directs a cell on how frequently to divide, how frequently to undergo the cell cycle. And what happens is if there's a problem with this control, This can lead to cancer cells. Cancer cells manage to escape those controls on the cell cycle. Essentially, that's what cancer is. Cancer involves cells that have escaped the cell cycle control. So instead of dividing every other day, for instance, or only dividing when there's cell damage, those cells divide and divide and divide. So let's talk a little bit about the cell cycle control system. And then we can discuss how it can go awry to result in cancer. So again, inside of each and every one of our cells is what's known as the cell cycle control system, which is made up of various checkpoints. Checkpoints where the cell cycle stops until a go-ahead signal is received. So let me show you kind of a drawing that will help. to make this more clear in your mind. Imagine if this is the cell cycle. It kind of looks like a roulette table, you know, but and this this this this rod is going around the table. Well, eventually this rod runs into a flap, right? One of these plastic flaps. These are checkpoints. These represent checkpoints. And the whole table represents the cell cycle. Remember the cell cycle, which is made up of interface G1. Interphase S phase, interphase G2, followed by M phase, M phase, which includes mitosis and cytokinesis, right? So this table, this represents the cell cycle. And notice how this wheel will spin as long as there's no flap in its way. But when it runs into a flap, that's known as a checkpoint, and that's where the cell cycle will arrest. until a go-ahead signal is given. So a go-ahead signal would be like if this flap were to plop down, right? Then the rod could continue its journey. Now, there are three main checkpoints during the cell cycle. That's why you see three flaps here to represent the three main checkpoints of the cell cycle. The most important checkpoint is this one right here. This is known as the G1 checkpoint, sometimes called G1. the G1 slash S checkpoint because it occurs between G1 and S. This is where the cell decides whether to divide or not, right? So if a cell decides to divide, this flap will go down and the cell will copy its chromosomes and proceed into mitosis. If the cell does not decide to divide, this flap will... remain engaged and something interesting happens. Let me show you what happens if the G1 checkpoint is not cleared. All right, look at this. If that G1 checkpoint is not cleared without a go-ahead signal, the cell exits the cell cycle and enters what's known as G0. I haven't mentioned G0, but G0 is when the cell actually exits the cell cycle. The cell has it. At this point, the cell has no business or no interest in dividing anytime soon. And in fact, those cells of your spinal cord and those cells of your heart, they are stuck in G0 because no go-ahead signal is given, right? So when a go-ahead signal is given, that's when a cell can come back from G0 and re-enter the cell cycle. Isn't that neat? Alright, so at this point if the G1 checkpoint, which is the main checkpoint, were to clear, the cell would proceed into S phase, at which point it would copy all of its chromosomes. So it would copy the 46 chromosomes into 92, and then it would enter G2 phase where, you know, the centrioles would be copied, then the DNA would start to condense, and then it would smash into this flap right here, so it would arrest here. This is known as the G2 checkpoint, and during the G2 checkpoint, the cell makes sure that the DNA was copied correctly during S phase, and that there's no real problems with the DNA. And if that's true, this flap will, you know, disengage, and the wheel will continue until it bumps into this checkpoint called the M checkpoint, also known as the metaphase checkpoint, the metaphase, late metaphase checkpoint. Remember what happened during metaphase? All of the sister chromatids are now connected to microtubules from opposite ends of the cell. Well, at this point, if all of those little sisters are not connected to microtubules, then there won't be this flap will not disengage. This flap ensures that every sister chromatid pair is connected. to the proper number of microtubules, you know, one microtubule per sister chromatid. And so everyone's going to separate correctly. Otherwise, if you don't have this quality control, then the cells might divide, even if one of the sister chromatids is not connected properly to a microtubule. And then that would result in problems where you have weird numbers of chromosomes in the cells. Isn't that neat? So again, The G1 checkpoint, this is the main checkpoint where the cell decides to divide or not. At this point, the cell will, you know, if that clears, the cell will enter, you know, S phase. If it does not clear, the cell will exit into G0. The next checkpoint checks to make sure that the DNA was copied correctly and free of mutations. And then the next checkpoint makes sure that in late metaphase, everything is ready to separate. All the chromosomes are ready to separate because all the sister chromatids have a microtubule attached at the kinetochore. All right, and just to wrap up this chapter, I want to ask you a little bit about cancer, to think about how cancer works, because it has everything to do with the cell cycle control system. These flaps, these checkpoints are responsible for regulating how often cells divide in your body. But do you think that inside of your cells, you have... actual plastic flaps that prevent the cell cycle from proceeding? No, that's right, wicked. That would be hilarious though. Yeah, you don't have actual plastic flaps inside of your cells that stop the cell cycle from proceeding. So you have to ask yourself, in your cells, what are these plastic flaps made of? What are these checkpoints made of? Because something must be serving as checkpoints inside of your cells, okay? And I'll let you think about it for a second, but it's actually different proteins. Remember I said in a previous chapter that proteins are so important in your cells? Different proteins have all different kinds of jobs, like hemoglobin protein, its job is to carry oxygen in your red blood cells. Insulin proteins job is to help regulate your blood sugar. Well, you have a host of proteins and these are known as, you know, collectively you could call them checkpoint proteins. These proteins play a role as checkpoints. They play a role in maintaining the cell cycle control system. And when these checkpoint proteins are not functioning correctly, that's when the flap can fail. That's when the cell does not realize that it should stop until a go-ahead signal is given, and the cell will continue and continue and continue around the cell cycle. And by the way, remember what I said earlier, that proteins are coded for by your genes. So the genes that code for the checkpoint proteins are called tumor suppressor genes, proto-oncogenes. These are important genes in your cells that code for very important checkpoint proteins, cell cycle proteins, proteins that when, you know, they become misregulated or mutated, you know, they can cause problems in these proteins. So what you need to know is that the reason cancer occurs is because of a disruption in the checkpoint proteins. And that means that you have a mutation in these genes, either tumor suppressor genes or proto-oncogenes. Mutations in these types of genes result in cancer. Okay, so it's not just any gene mutations that result in cancer. It's gene mutation that affects these very important... checkpoint genes and these proto-oncogenes and tumor suppressor genes because those are the genes that code for the checkpoint proteins. Well I hope this was informative, a very interesting chapter on cell cycle, mitosis, binary fission, cancer. So I hope you learned a lot and took away a good deal from this chapter. If you have any questions please leave them in the comment box below and I'll catch you guys next time. Dr. D, Dr. D, Dr. D, Dr. D, Dr. A Dr. D, Dr. A Dr.

So what is the cell cycle? The cell cycle is based on the reproduction of cells or cell division. This is when one cell divides into two. And there are numerous reasons for cell division.

These include asexual reproduction, which we will talk about later in this chapter, growth, development, tissue renewal, and healing. All of these are requiring cell division. Now, before we delve too deep into cell division, we need to understand how our DNA works.

What is our genome? What are chromosomes? And how does our DNA look?

The reason we need to understand this is because it's not until we totally understand what our DNA consists of that we can totally understand how cell replication works. So first of all, all of the DNA in a cell constitutes the cell's genome. Okay, so your genome consists of all of your DNA and each of your nucleated cells in your body, you know how you have billions upon billions of cells in your body, each of those cells with a nucleus contains your entire genome, all of your different genes. A genome can consist of a single DNA molecule or chromosome in prokaryotic cells or a number of DNA molecules or chromosomes in eukaryotic cells.

DNA molecules in a cell are packaged into chromosomes. So do you remember when we talked about chromosomes, when we discussed cell structure? A chromosome is a continuous piece of double-stranded DNA in the genome and Remember chromosomes?

The DNA is wrapped around those histone proteins. Remember the histone core that's made up of eight histone proteins? And together, the DNA plus the histone proteins were called chromatin. Do you recall that from the chapter on cells? So you can think of each continuous piece of double-stranded DNA plus histone protein is called a chromosome.

And Some organisms only have one chromosome. Remember we discussed this before, how prokaryotes, you know, bacteria, have a single circularized chromosome with no beginning and no end? Kind of like a rubber band, right? However, eukaryotes typically have numerous chromosomes.

So, for example, how many chromosomes does a human have? Can you beat Wicket? Right, as always, Wicket.

46 chromosomes. A typical human has 46 chromosomes. And so we have 46 pieces of DNA inside of our nucleus, right?

And so a DNA molecule of a chromosome carries several hundred to a few thousand genes. So a gene, do you recall what we said a gene was? A gene is just a segment of a chromosome that codes usually for a protein.

Now there are between, you know, depending on the textbook you read, between 23,000 and 25,000 different genes. Well, if there are only 46 chromosomes, then that means that each chromosome contains hundreds or thousands of genes. Think about it.

The human genome has between 23,000 and 25,000 genes. and only 46 chromosomes, this means that each chromosome houses hundreds or thousands of genes. Okay, and again, humans have 46 chromosomes, whereas prokaryotes like bacteria and archaea, they typically only have one chromosome.

Here you can see a eukaryotic cell that where the DNA is visible. And you can see the chromosomes. You can see those chromosomes inside of the nucleus. This would be the nucleus and inside of the chromosomes.

All right, so let me share with you in this image what's known as the lab procedure karyotyping. Here a scientist would lay out every chromosome side by side. These are the 46 chromosomes of a human being.

I want you to notice something. I want you to notice that although there are 46 chromosomes, two of your chromosomes are called chromosome 1, two of your chromosomes are called chromosome 2, two of your chromosomes are called chromosome 3, and so on and so forth. There are only 23 different chromosomes, and you have two of each type.

So here's some terminology I'd like you to know. A chromosome 1 through 23 is called a set of chromosomes. So if chromosome 1 through 23 is called a set, then how many sets of chromosomes do humans possess typically?

That's right, Wicket, two sets. Two sets of 23. Now let me ask you this, why do you think you have two chromosome 1s, two chromosome 2s? two chromosome threes.

In a typical cell, why would you have two of each chromosome? Well, if you guessed that you got, that you inherited one set from mom, your mother, and one set from dad, your father, and that's why you have two chromosome ones, well then you'd be right. One set of chromosomes, one through 23, was present in the sperm And one set of 23, a chromosome 1 through 23, was present in the egg. So when the sperm fertilized the egg, that's how you inherited your two sets of chromosomes. You essentially inherited one set of 23, you know, a number 1 through 23 from your father.

and one set of 23, a number 1 through 23 from your mother. So this is the reason why you have two sets of chromosomes. Now let me ask you this, why do you think you have two chromosome 1s, two chromosome 2s? Do you think these two chromosome 1s are identical? That's right, Wicket, these are not identical chromosomes.

You inherited one chromosome 1 from your mom and one chromosome 1 from your dad. You inherited one chromosome 2 from your mom and one chromosome 2 from your dad. So again, let me ask you, do you think these are identical? Do you think these two chromosome 1s are identical?

No, they are not identical. Okay, they are not identical. So let me ask you this. Why do you think they're both called chromosome 1? Why do you think they are both called chromosome 1?

Well, it's because of the types of genes present on chromosome 1. So, for example, I'm just making things up right now, but just to make a point. For example, if eye color gene is found on my chromosome 1, you know, it's on everyone's chromosome 1. So eye color gene, if it's on chromosome 1, in my genes, in my chromosomes, then it's on everyone's chromosome 1. So eye color gene might be right here on the chromosome 1 from your dad, and it's right here on the chromosome 1 from your mom. But does that mean that both of those eye color genes would say the same information, would dictate the same eye color? No, you might inherit blue eye color gene from your mom, and you might inherit... brown eye color gene from your dad.

These are known as alleles or versions of a gene, okay? So just because it's the same types of genes that appear on the chromosomes doesn't mean it's the exact same information that appears on the chromosomes. So you can think of chromosome one, chromosome one, these two chromosomes would have the same types of genes, hair color, eye color. But that doesn't mean it's going to be the exact same information.

It's not going to be the exact same hair color because you inherited one of those genes from your mom and one of those genes from your dad. And by the way, these have a name. When you have these two chromosome ones that are not identical, okay, they are not identical. One was inherited from your mom and one was inherited from your dad.

What they do. do share in common are the types of genes, but not the exact same genes, not the exact same information, right? These are known as homologous chromosomes.

Does that make sense? Homologous chromosomes. So what do I need you to know about homologous chromosomes?

One homologous chromosome is inherited from your mom. The other homologous chromosome is inherited from your dad. One was from the sperm. One was from the egg.

They have the same chromosome number because they share the same types of genes, but they are not identical because they don't, you know, one's from your mother and one's from your father. So even though they both may share, I don't know, hair color gene, one might say brown hair color while the other says blonde hair color, if that makes any sense. So are homologous chromosomes identical?

No, that's right, Wicket. They are not because one's inherited from your mom and one's inherited from your dad. So now let me ask you this. If 46 chromosomes, which you see right here before your eyes, if 46 chromosomes are present in each and every one of your cells, you know, that has a nucleus, then what do you think needs to happen to these chromosomes? Before the cell can divide, what do you think we need to do with these chromosomes before cell division?

And remember, the purpose of cell division, like let's say one of your skin cells divides to become two skin cells, you know, the purpose of that is to make two genetically identical cells, right? So let's say the cell just wants to copy itself. You want to make two copies of the cell. That's that.

typical cell has 46 chromosomes, what do you think we need to do first? Can you beat Wicket? That's right, Wicket. We need to copy the chromosomes.

We need to make a carbon copy of every chromosome. Does that make sense? Because think about it. If one of my cells wants to divide to form two cells, well, I want to make sure that those two cells also have the 46 chromosomes of my genome, right? If I don't copy all my DNA and the cell just divides, well then I'm going to dilute my chromosomes.

I'm not going to have two genetically identical cells, am I? I'm going to have two cells with only 23 chromosomes, which doesn't make any sense. So before a cell can divide, right, for cell division, we need to go through and look at this. We need to make a copy of this chromosome right here.

And we need to make a copy. of this chromosome. So we need to make a copy of mom's chromosome 1, and we need to make a copy of dad's chromosome 1, and we need to make a copy of mom's chromosome 2, and we need to make a copy of dad's chromosome 2. We need to go through and copy each and every chromosome, make a carbon copy.

And do you know what that's called? Those are called sister chromatids. Here we go, sister chromatids. So again, for example, let me show you something.

Let's just take this example right here. See this example right here? This is the homologous chromosome pair, chromosome one.

One's from mom, one's from dad. They're both called chromosome one because they share the same types of genes, but they're not identical because obviously you inherited different information from mom than dad. So again, let's just look at chromosome one.

Now remember chromosome one. Here's chromosome one. Okay, let's pretend this is chromosome one. This one's dad's chromosome one, right?

This one's mom's chromosome one. They're both chromosome one, but you inherit one from the paternal lineage, you know, your dad. And one is maternal from mom. These are known as what?

With respect to each other, what are these? That's right, they are homologous chromosomes. And again, what did I say we need to do with these chromosomes before the cell can divide? That's right, Wicket. We need to make a copy.

So see this dad's chromosome one? I am going to make an exact copy of this dad's chromosome one. I'm going to make a copy of it and watch that.

So now I have two copies of dad's chromosome one right here. These are identical now. In every way, they are identical copies of each other.

And I'm going to do the same thing with mom's chromosome one here. Look. I'm going to take mom's chromosome 1 and I'm going to make an identical carbon copy of it as well.

Here we go. On the right, you can see I've made two copies, two identical copies of mom's chromosome 1. By the way, what are these two copies called? The copies are called sister chromatids. Sister chromatids.

So I need you to really, really understand this. I need you to understand this. I want you to understand the difference between homologous chromosomes and sister chromatids.

So I just want to take you to the board to really drive this home and then we'll be right back here to talk a little bit more about important features of this process. What are homologous chromosomes? As you know, humans have 46 total chromosomes, right?

Humans have 46 chromosomes total. But you inherited 23 of those from your dad and you inherited 23 from your mom. All right, I'm going to use blue for dad, red for mom.

Okay, the egg from your mom's egg, the egg housed 23 chromosomes. One chromosome one. one chromosome two, one chromosome three, one chromosome four, you get the idea. 23 different chromosomes. The sperm from the dad housed 23 chromosomes as well.

This included a chromosome one, a chromosome two, a chromosome three. So every person then has 46 total chromosomes but they actually have two sets of chromosomes. So what's a set?

A set means 1 through 23. Having chromosomes 1, chromosome 2, chromosome 3, chromosome 4, a set of chromosomes 1 through 23, that's a set. So you inherited a set from your dad and you inherited a set from your mom and so you have two sets of chromosomes. You know what that means?

That means, let me draw chromosome 1 for example. Let's say this is chromosome 1. from your dad. You also have a chromosome 1 from your mom.

Now both of these are called chromosome 1. Okay both are chromosome 1 but are they identical? If you were to sequence the DNA would it have the exact same sequence? No. That one's from your dad and that one's from your mom. Your mom and dad are not related.

So they're not going to have a similar DNA. It's not going to be the exact same. So these are not the exact same chromosome.

They don't have the exact same sequence. But they're both called chromosome 1. Why are they called chromosome 1? They're both chromosome 1. and you have two chromosome twos and you have two chromosome threes and you have two chromosome fours so what constitutes the chromosome number well here's the thing these two do have something in common but it's not the exact same sequence what they have in common is that the same types of genes that are on this chromosome are found on that chromosome does that make sense You remember what chromosomes are?

Chromosomes are one continuous piece of chromatin or DNA. One continuous piece. And remember the chromosomes house the genes, right?

There are genes along the chromosomes. Each chromosome contains thousands or at least hundreds, I should say, hundreds of genes on each chromosome. All right, so whatever genes appear on this chromosome one, also appear on that chromosome 1. For example, if eye color, the gene for eye color was on this chromosome, it would also be on the other chromosome 1. was on this chromosome 1 from mom, it would also be on that chromosome 1 from your dad.

So what we could say is the same types of genes appear on these chromosomes, but not the exact same sequence. So again, you have two chromosome 1s, two chromosome 2s, two chromosome 3s, but they're not identical. One's from your mom, one's from your dad. But what they do share in common is types of information, the same types of genes. Okay.

And, and there's a term for this. You see how there's the paternally inherited from your dad and the maternally inherited chromosome one, they are called homologous chromosomes. Okay, homologous chromosomes. Homologous chromosomes are not identical.

Homologous chromosomes share the same chromosome number, but remember one was inherited from your dad, one from your mom, so they have the same types of genes on there, the same types of information, but they're not identical. Okay, now again you've inherited 23 chromosomes from your dad. 23 chromosomes from your mom. You have two chromosome 1s, two chromosome 2s, two chromosome 3s. Okay, now what happens if the cell wanted to divide, right?

A normal cell has 46 chromosomes, right? Well, what if that cell wants to divide into two cells? Well, it can't just go ahead and divide because if a cell went ahead and just went ahead and divided, Each new daughter cell would have 23 chromosomes. Does that make sense? That's not good.

You don't want your daughter cells to have 23 chromosomes each if one cell divides into two. You want it to divide so that it retains its 46 chromosomes. So in that case, you know what you need to do?

Before the cell divides, Before the cell divides you need to copy all the DNA don't you need to copy all the chromosomes So for example, if this is dad's chromosome 1 you need to make a copy of it You need to have two dad's chromosome ones right see you make a copy of dad's chromosome one So these are now identical copies of dad's chromosome one and then you also need to copy mom's chromosome one You see that you need to make a copy of that as well. So this is an exact carbon copy of mom's chromosome 1. Now you have 2 of mom's chromosome 1, 2 of dad's chromosome 1, and you're going to have a copy of all your 46 different chromosomes. So how many total chromosomes do you have at this point? You have 92 total chromosomes, right? 92. That way, that way, if you were to divide at this point, you would have 46, 46. You'd be back where you started, which is what you want.

Okay, there's a term for this. You need to understand this. You see how Now you have a copy of every chromosome.

Those copies have a very specific name. Those copies are called sister chromatids. Okay?

Sister chromatids are identical in every way. Sister chromatids are identical in every way. Sister chromatids. And here's the thing, a normal skin cell doesn't have sister chromatids, right?

It just has 46 chromosomes, one set from mom, one set from dad, okay? The only time a cell has sister chromatids is when the cell is about to divide, the cell has considered it is about time to divide the cell into two. At that point it makes a comp- of all the 46 chromosomes and only at that point do you have sister chromatids. Okay so hopefully this was helpful.

You should definitely, definitely understand the difference between homologous chromosomes which are not identical. One was inherited from mom, one was inherited from dad. They share the same chromosome number because they have the same types of genes and then the difference between that and sister chromatids which only exist after the DNA has been replicated by the way foreshadowing this is going to happen during S phase of interphase okay only at that point do you have a sister chromatids All right, welcome back from the board. I just wanted to give you a little more terminology here when it comes to these sister chromatid pairs.

This is a pair of sister chromatids in blue. Next to it is a pair of sister chromatids in red. Now, it is proper to say this pair of sister chromatids on the right, this pair of sister chromatids is homologous to this other pair of sister chromatids. You see, so that is proper terminology.

The two blue ones are still homologous to the two red ones, as long as they all have the same shared chromosome number. So for example, if all four of these blue and red chromosomes are chromosome one, if all four are chromosome one, then yes, these two sister chromatids are homologous to these two sister chromatids, if that makes sense. Also, you see how chromosomes have this narrow waist?

Every chromosome has a narrow waist, and that narrow waist is called the centromere, centromere. And the centromeres are where the two sister chromatids join together. See, so this sister chromatid and this sister chromatid, they're connected at the the hip, you could say, at the centromere.

And the proteins that hold the two sisters together are called cohesin proteins. So you see cohesin proteins at the centromere hold sister chromatids together at the centromere. Okay, and there are other proteins on the outside of the centromere called kinetochore, and I'll explain what these do later on.

So these X structures are probably what pops into your mind when you think of chromosomes. However, I want to tell you something really important. These chromosomes, they only look like this during cell division, when the cell is actively dividing. In fact, that's called mitosis, right?

Mitosis is what we're going to be going over in just a few slides here. And mitosis is where the chromosomes segregate to result in two genetically identical cells. But what I want you to understand is this, the chromosomes only look like this during mitosis when the cell is actively dividing.

Okay, the rest of the time, the DNA looks nothing like this. The DNA is very, very, very condensed here in this mitotic chromosome. But normally, when the cell is not actively dividing, when the cell is not undergoing mitosis, cell division, the DNA is very loose.

It's nowhere near this tightly packed up. The DNA unravels and becomes loose. And when it does, I call it a bowl of spaghetti, right?

The DNA just unravels into a bowl of spaghetti. And at that point, you can't see these X structures. You can't see the individual chromosomes. When a cell is not in cell division, when the cell is not undergoing mitosis, the chromosomes look nothing like this.

In fact, you can't see the chromosomes at all because they're just kind of like a diffuse blob of DNA. You can't see how many chromosomes there are. You can't see individual chromosomes.

It's just a bowl of spaghetti, right? It's just a bowl of chromatin. You can think of it that way, okay?

So just realize that. And for that reason, these chromosomes here and these chromosomes here and these chromosomes here, you see how they all look like little, you know, individual Xs or individual structures. These are known as mitotic. chromosomes, okay? Mitotic chromosomes, because they only appear like this during mitosis.

When the cell is not actively in mitosis, you do not see these structures because the DNA is a bowl of spaghetti. All right, so what happens during the cell cycle? Well, you need to know that there are two major phases to the cell cycle.

There is what's known as the mitotic phase or M phase where the cell is actively dividing. And then there's interphase where the cell is not actively dividing. In fact, that's when the cell grows, the cell copies its chromosomes, the cell prepares for cell division, right? All of that occurs during interphase. And by the way, interphase takes up most of the cell cycle timeline.

On average, interphase takes up 90% of the timeline of the cell cycle, whereas the mitotic phase, M-phase, where the cell is actually dividing, this takes, you know, around 10% of the cell cycle timeline. M-phase includes what's known as mitosis. Mitosis is where the chromosomes separate or sister chromatids separate into new daughter cells. And then cytokinesis, meaning where the cell itself separates, the cytoplasm separates to form two distinct cells. Again, mitosis is the separation of the chromosomes to result in two genetically identical cells, and cytokinesis is the separation of the cells themselves, right?

And this is together they are known as M-phase. So let me tell you more about what goes on during interphase. During interphase, 90% of the cell cycle, there are three sub-phases. G1, called first gap, S-phase, or synthesis phase, and G2-phase, second gap.

These are three sub-phases of interphase. So let me tell you a little more about what's happening during these sub-phases. Look, if this is the cell cycle, then the big, the big teal arrow is interphase. And remember that takes up about 90% of the cell cycle. At first, you have G1 phase or the first gap.

During G1 phase, you need to know that the cell is growing. There's metabolic activity. The cell is growing.

Then the cell will enter what's known as S phase or synthesis phase. This cell continues to grow, but what's most important is that DNA is synthesized, or the DNA is copied. Remember when I said the 46 chromosomes in human, they need to be copied to form sister chromatids? This is occurring during S phase, DNA synthesis.

So here is when the sister chromatids form, because we copy all 46 chromosomes. In fact, at this point, the cell has 92 total chromosomes. And next you have G2 phase, or the second gap phase. There's more metabolic activity and growth, and there's preparation for cell division.

Specifically, I want you to know that not only is there preparation for cell division, the centriole pairs, or the centrosomes they're called, are replicated during G2 phase. Alright, so these are the subphases of interphase. Remember, interphase is made up of subphases G1, S, and G2.

During G1, the cell undergoes growth. There's growth. During S phase, the DNA is replicated.

And at G2, this is where the chromosomes start. to coil more tightly. Remember what I was saying is that the DNA is usually loose, but then the DNA can coil up before mitosis, right during mitosis. And the centrioles are also replicated.

So turn your attention to this image on the right here. In the nucleus, you don't see individual chromosomes, do you? Remember, this is the bowl of spaghetti.

Right here you have a bowl of spaghetti. The DNA is all loose and strewn about. This chromatin is very loose.

And so because the DNA is not compact, the DNA is really indistinguishable. The individual chromosomes, I should say, are indistinguishable. And look at this. Remember these centriole bundles where animal cell usually has one centriole bundle and this forms the centrosome.

Well, at this point during G2, the centrioles replicated. So you have two centrosomes. Okay.

Again, that's 90% of the cell cycle called interphase. And what's left? That's right.

Mitotic phase. This is where the cell actually divides. During mitosis, the chromosomes segregate. During cytokinesis, the actual cell separates into two.

So let's go into that in more detail. Speaking of mitotic phase or M phase, let's go ahead and start with mitosis and see what are the steps that make up mitosis. Mitosis is conventionally broken down into the following five stages. Prophase.

Prometaphase, metaphase, anaphase, and telophase. So let's start with prophase. Remember, the cell has just completed interphase with G1, S, and G2, and the DNA is now fully condensed. So let's talk about what's happening during prophase, which is the first subphase of mitosis.

Look inside of the chromosomes. Do you see individual chromosomes now? Yes, you do, right?

You see those. X structures. Why do you see the X structures and not just a bowl of spaghetti? Well, because at this point, the DNA has condensed into what's called those mitotic chromosomes. And these Xs are the sister chromatid pairs that we had discussed before.

These sister chromatid pairs, which are held together at the centromere by cohesin. proteins. Remember that?

So what you should know is that during prophase, sister chromatid pairs form. Okay, they show up as these little X's. And the nucleus is still present. Here are your two centrosomes.

Okay, the centrosomes are kind of moving towards opposite ends of the cell. All right, but what I really want you to know is that the nucleus is still present and sister chromatid pairs form as these little X structures. All right now what happens during pro-metaphase? Look at this.

Do you still see a nucleus or has the nucleus broken down? The nucleus has broken down. I don't see a nucleus.

In fact the sister chromatid pairs are floating around in the cytoplasm. And I want you to take a close look here. Not only are the sister chromatids floating around, but it looks like microtubules spanning from the centrosome here.

The microtubules have attached to the sister chromatid pairs. One microtubule attaches to this sister. Another microtubule from the adjacent or from the opposite centrosome is attached to this.

sister here. So take a look. I just want to show you what's going on.

Do you remember when I said that there are these proteins on the outside of the centromeres as well? Not the cohesin proteins that hold the sisters together, but these proteins on the outside of the sister chromatid pairs. These are called kinetochore.

And kinetochore, you can think of them as docking sites for microtubules. There are microtubules growing from the centrosome at one end of the new cell, and there are microtubules growing from the other end of the cell, from the other centrosome at the other end of the cell. So what happens during prometaphase is that there's this docking that happens.

Microtubules attach to kinetochore on the sister chromatid pairs. And look at this. One sister is connected.

to one centrosome via microtubule and the other sister is connected to the other centrosome via microtubule. So each sister is connected to the opposite end of the cell, right? Each sister is connected to a microtubule from the opposite ends of the cell.

And those sisters begin to make their way to the center of the cell. Are the sisters at the center of the cell yet? That's right, Wicket.

Not yet, but they try to make their way to the center of the cell. So remember, what do I need you to know about prometaphase? The nuclear envelope breaks down and the sister chromatids start moving towards the center of the cell.

Next, we have metaphase. During metaphase, those sisters have made their way to the center of the cell. In fact, they've lined up down this imaginary plate called the metaphase plate. It's an imaginary plate down the center of the cell.

And those sister chromatids have lined up down that metaphase plate. And there you go. And remember, each of these Xs is two identical sisters.

One sister is connected to this centrosome with a microtubule. The other sister is always connected to the opposite centrosome with a microtubule. And that's what's happened during metaphase. So what do you need to know about metaphase?

You should know that sister chromatid pairs line up down the metaphase plate, down the center of the cell. You know what happens next during anaphase? Do you remember those......

Those cohesin proteins that are holding these sisters together at their narrow waist, the centromere, those cohesin proteins separate, they let go, and the sisters move in opposite directions, right? So each sister moves to an opposite end of the cell. Again, what do I need you to know about anaphase? During anaphase, sister chromatids separate from one another and head to opposite poles of the cell.

And then lastly telophase. Those sisters are now, they've reached the opposite ends of the cell and daughter nuclei start to form. That means a nucleus starts to form around these chromosomes. A nucleus starts to form around these chromosomes and the DNA will start to spaghettify.

The DNA will start to decondense at this point. So that leads to the end of mitosis. Mitosis results in two genetically identical cells. And that makes sense if you think about it. Remember, in humans, we had 46 chromosomes, none of which are identical.

Those chromosomes were copied into 46 identical chromosomes, right? So we copied our chromosomes and we now had 92 chromosomes. Those sisters, those copies lined up down the center of the cell, and then those copies separated.

So this cell got a copy, the other cell got a copy. For example, this cell got a mom's chromosome 1, the other cell got an exact copy of mom's chromosome 1. This cell got a dad's chromosome 1, the other cell got an exact copy of dad's chromosome 1. So every one of these chromosomes was copied. and then separated into the new daughter cells.

This results in two genetically identical cells. So each daughter cell will have 46 chromosomes, and those chromosomes are identical to each other as well as to the original parent. All right, before we move on from mitosis, I just want to share a little more terminology with you that we didn't delve too much into, but we should at least address it. Remember the centrioles?

The centrioles are bundles of microtubules at the core of the centrosome. At this end of the centrosome, you see these short microtubules that almost look like a little starburst right here? Those short microtubules radiating out from the centrosome is called the aster.

In fact, aster means star. Then there are microtubules that grow from the centrosome. This is known as the mitotic spindle. Here's more terminology for you.

Remember during anaphase, I said that the cohesin proteins let go and the sister chromatids move to opposite ends of the cell? Well, this is thanks, during anaphase, this is thanks to an enzyme called separase. Separase cleaves the cohesin proteins. releasing the sisters from one another.

Those sisters then separate and move to opposite ends of the cell. Now you might be wondering, how do the sisters separate and move to opposite poles of the cell? Well, you're in for a treat because they've actually determined this, and it's really interesting how it works.

At the core of the centrosome, you see here the kinetochore, at the core of the kinetochore, no pun intended, but if this is a sister chromatid in the kinetochore, which is connected to the microtubules that come from the centrosome, those, those kinetochore contain, look at this, motor proteins. These motor proteins, hi, wicked, wicked's here pawing at the screen. Hi, hi, buddy.

Hi, buddy. Hi. Hey, buddy. Hi.

Hi, buddy. Okay. Um, I'll get. right with you, Wicket.

Now, these microtubules, what happens is these motor proteins, which are found inside of the kinetochore, they actually walk along the microtubule. And that's how the sisters move. So if you've ever wondered how the sisters move to opposite ends of the cell, It's actually because of motor proteins inside of the kinetochore.

Isn't that neat? So the sisters move themselves to opposite ends of the cell. And look what I have here highlighted at the top.

Sister chromatids move to opposite poles of the cell by motor proteins found inside a kinetochore. They make their own way towards the poles of the cell. They're not being pulled towards the poles of the cell.

They make their own way via motors in the kinetochore. So now that we understand how mitosis works, remember mitosis is the separation of the chromosomes resulting in two genetically identical cells. Now it's time for the cell itself to separate.

This is known as cytokinesis. So let's discuss how cytokinesis works in animal cells versus plant cells. Alright, so take a look here at cytokinesis in the animal versus the plant cell. On the left, we have an animal cell, and you can see here cytokinesis in action. What happens is there's a constriction of the cell membrane where there's like this pinching that occurs, and that's called a cleavage furrow.

A cleavage furrow is forming. And remember when we learned about microfilaments? Microfilaments are actually responsible for producing the cleavage furrow.

Isn't that interesting? Microfilaments form what's known as a contractile ring, and that contractile ring is responsible for forming the cleavage furrow. And there's like this constriction that occurs until you have two daughter cells that are separated from one another.

And again, this is occurring in animal cells. Now in plant cells, what you have are little vesicles full of cell wall material. Remember cell wall material is cellulose?

Well, little vesicles full of cellulose line up down the center of the cell. And you can kind of see that in this cartoon here, little vesicles lining up. And these vesicles are actually from the Golgi. They originated from the Golgi and they line up down the center of the cell. And then do you remember when vesicles meet?

Remember it's like two soap bubbles that meet, they pop, they kind of fuse to one another. And so these vesicles will fuse to one another, forming what's known as an early cell plate, which is like a new nascent cell wall in the center of the cell, which will then mature into a mature cell wall, a new cell wall. So again, cytokinesis occurs via cleavage furrow formation, thanks to microfilaments in animal cells. And cytokinesis occurs via vesicles, vesicles that form a cell plate, a cell plate in plant cells. And here you can see the complete M phase or mitotic phase in a plant cell.

Remember mitotic phase consists of cytokinesis as well as mitosis. Now what you should realize is mitosis and cytokinesis are not the same thing. However, together they make up M-phase.

Here you can see a plant cell. The plant cell is rectangular, right? You've got the rectangular cell wall. The plant cell is undergoing prophase, where the sister chromatid pairs form.

Then we have prometaphase, where the nuclear envelope breaks down and the sister chromatids begin to migrate towards the metaphase plate. And then you have metaphase, where the sister chromatid pairs have lined up down the metaphase plate at the center of the cell, then anaphase, where the separase protein or enzyme has broken apart the cohesin, and the cohesin lets go of the sisters. The sisters migrate to opposite ends of the cell.

Thanks to those motor proteins in the kinetochore, those sister chromatids start moving to opposite ends of the cell. And then remember, During telophase, your daughter nuclei form and the DNA can begin to spaghettify, to unravel, to relax. And usually telophase is somewhat concurrent.

That means occurs at roughly the same time as cytokinesis. So you can see a cell plate also forms. So again...

Cell plate formation or cytokinesis can begin before telophase is finished, but that does not mean that it is part of telophase. Okay, just keep that in mind. Awesome. Well, let's take a quick break time with Gizmo and Wicket, see what these two little guys are up to.

And when we come back, I'm going to go through mitosis one more time with you at the board, and then we're going to get into some other topics as well. All right, welcome back from break time with Gizmo and Wicket. Let's go to the board one more time where I just want to give you another comprehensive look at mitosis and cytokinesis and interphase and the cell cycle in general, just so you have an idea of everything that's going on one more time.

Alright, so the cell cycle begins with a process known as interphase. Interphase takes up about 90% of the cell's cycle time. This is essentially where the cell is growing, the cell is functioning, and the cell is preparing to divide.

It starts with G1. Alright, G1 is a sub-phase of interphase. During G1, the cell grows. The cell is just growing, it's producing proteins it needs, organelles it needs.

Remember animal cells have one pair of centrioles at this time and if you look inside of the nucleus there are homologous chromosomes. Dad and mom chromosome one, dad and mom chromosome two. 2 for example, there would be 46 chromosomes in a normal cell at this point.

Now usually a cell, if the cell wants to reproduce or divide, the cell will proceed to S phase, But if the cell does not get that clearance to go into S phase, the cell will exit the cell cycle and actually enter what's known as G0. Just be aware of that. The cell can exit the cell cycle and go from G1 to G0. But when the cell does get the go-ahead to go forth into S phase, it'll copy each and every chromosome. Notice that you started with 46 chromosomes total in a human cell.

Well, you're going to copy each and every chromosome. So you're going to have two copies of chromosome 1 from the dad, two copies of chromosome 1 from the mom, two copies of chromosome 1 from, sorry, chromosome 2 from the dad, two copies of chromosome 2 from the mom. You're going to go from 46 chromosomes to 92. You're making a copy of each and every chromosome. And just as a quick review, just remember that homologous chromosomes are not identical, but they share the same chromosome number.

So if this was chromosome 7 from the dad, that would be the paternal chromosome 7. this would be the homolog which would be the maternal chromosome 7. Paternal is inherited from the dad. Maternal is inherited from the mom. And a normal cell does has homologous chromosomes inside.

Okay a normal cell. Now only after S phase where the DNA has been replicated do you have what is known as sister chromatids. This would be two identical copies of the same exact chromosome. So for example that would be chromosome 7 from the mom, that would be chromosome 7 from the mom.

They're both exactly identical in every way and the sequence is identical. So at the end of S phase you have sister chromatids formed, identical copies. Next we enter G2, growth phase 2. At this point look the centrioles where there was one bundle of centrioles they replicate.

Now you have two bundles of centrioles and they start to drift apart. And that's about it. The DNA starts to condense into those mitotic chromosomes.

Remember, DNA is loose and then DNA starts to condense in preparation for mitosis. Okay, this would be the end of interphase. We are now moving over to mitosis, which starts with a process known as prophase. During prophase, notice that those centrioles are now drifting further and further apart.

apart towards opposite poles of the cell. And look at these little green lines I drew. These little green lines represent the mitotic spindle. That's microtubules that are actually being synthesized or produced from the centrioles. Okay, at this point, you wouldn't call these centrioles anymore.

It's more accurate to call them centrosomes because centrosome means centrioles plus microtubules. Okay, and if you look inside of the nucleus, what do you have? You have sister chromatid. pairs form.

Sister chromatid pairs form and condense into those mitotic chromosomes. Mitotic chromosomes are when the chromosomes have condensed into their most tight form and they look like those X structures. Why do they look like X's?

Because one sister is now connected to another sister at the narrow waist. At the narrow waist called the centromere. And the reason the two sisters are connected to one another at the centromere is because cohesin proteins are holding them together as a pair.

Okay, so you have, for example, both of dad's chromosome 1, both of mom's chromosome 1 as a pair, both of dad's chromosome 2, and both of mom's chromosome 2. Now you're ready for prometaphase. And look, the nuclear envelope, the nucleus has broken down. The sister Chromatids are essentially floating around the cytoplasm. at this time.

The nuclear envelope breaks down and microtubules attach to kinetochore. So what does that mean? Notice that the centrosome here and the centrosome here, those microtubules have really grown. And when the microtubules microtubule encounters the sister chromatids at the centromere, it can actually connect to the sister chromatid.

Microtubules connect to the sister chromatids at the centromere, at the narrow waist. And the The way they connect is that there are these proteins there called kinetochore, or kinetochore microtubules attached to kinetochore. Kinetochore are proteins at the centromere that are docking sites for the microtubules. Microtubules attach to them.

Okay. So again, what's important about prometaphase, nucleus breaks down, microtubules attach to kinetochore on the sister chromatid pairs. And notice that the sister chromatid pairs. chromatid pairs start making their way to the center of the cell. By metaphase, they have made their way to the center of the cell.

It's called this imaginary metaphase plate, right? Sister chromatids line up at center of cell, okay? Sister chromatids line up at the center of the cell. And all of the sister chromatids are attached at their kinetochores to microtubules from opposite poles of the cell, opposite centrosomes. Now, do you remember the cohesin proteins holding the sisters together?

The cohesin proteins holding the sisters together? Well, those break apart. Those let go.

And look at this. this sister chromatid separates from that sister chromatid so anaphase sister chromatids separate from one another when those cohesin proteins that held the sisters together release one sister chromatid heads to the left and one sister chromatid heads to the right so why is that important remember these sisters are exact same information right so what that means is the same information is going to this cell as is going to this cell. If this is dad's chromosome one going to this cell, this is another exact copy of dad's chromosome one going to that cell.

And there's mom's chromosome one heading to this cell, mom's chromosome one heading to that cell. So whatever information is heading to the left, that same exact information is heading to the right because its sister chromatid is heading to the right and that is an exact copy. And just to be clear, the reason these are traveling apart, the reason they are moving is not because the centrosomus is pulling the the sisters the sisters themselves make their way to the opposite poles of the cell you know why do you remember those little kinetochore proteins attached to the centromere those little kinetochore proteins have little motor molecules inside that walk so these they're they're walking okay there's kinetochore those kinetochore proteins have little motors inside that walk so the sister chromatids are actually walking themselves to the opposite poles of the cell. It's a pretty interesting fact.

So what happens in the next step, telophase, this is the final phase, subphase of mitosis. Now you have two cells with 46 chromosomes each and whatever DNA is in this new nucleus is found in this new nucleus. Okay, notice that the new nuclear envelope starts to form so a daughter nucleus forms here and a daughter nucleus forms here.

daughter nuclei form. And usually cytokinesis can start at telophase, but what you should realize is cytokinesis is not part of telophase. Cytokinesis is not part of mitosis, but it is concurrent with mitosis.

So it can happen at roughly the same time. There's some overlap there, but mitosis plus cytokinesis is actually called M phase or mitotic phase. Just be clear on that.

All right. So what is the product of mitosis? Mitosis results in two genetically identical cells. All right. Mitosis results in two genetically identical cells.

And those are somatic cells, two genetically identical somatic cells. So this cell and this cell are identical to one another. And they are identical to the original cell that made them way back. Now, did you know that prokaryotes do not undergo mitosis when they divide? Instead, prokaryotes, including the bacteria and the archaea, they undergo a process known as binary fission.

It's an asexual type of reproduction. In binary fission, the chromosome replicates. Remember, usually prokaryotes only have a single circularized chromosome. And that single circularized chromosome begins replicating at a spot called the origin of replication.

And then two daughter chromosomes actively move apart. So let me show you how this process works. Again, here you have a bacterial cell or a prokaryotic cell of some sort.

Remember that its DNA only consists of one chromosome, and it's a circularized chromosome. So if I were to untangle this chromosome, there would be no beginning and no end. It's a closed circle of DNA.

Now, this DNA needs to be copied, right? Because the cell is going to be dividing soon. So at this specific point called the origin of replication.

That's where the DNA replication occurs. Essentially, the DNA is unwound and DNA is copied in both directions. See, there's one direction where you're copying the DNA going this way, and then you're also copying the DNA going the other way. And then at some point, you have copied the DNA and you reach the termination site.

Those two copies of the chromosome act as a link between the DNA and the chromosome. move to opposite ends of the cell. And then there's this kind of this inward growth that's happening.

It's analogous to cytokinesis, and it's called septum formation. So you should know that cytokinesis occurs via septum formation. This is known as the septum.

It's an inward growth of the bacterial cell wall. And that results in two cells, two daughter cells. Now let's turn our attention back to the eukaryotic cell. Now you and I are what's known as multicellular organisms. We are made up of billions and billions of different types of cells.

And did you know that some of the cells in your body divide all the time, while other cells in your body divide much more rarely? And yet other cells don't divide at all, right? So for example, the cells of your spinal cord and your heart, they do not divide, if ever.

Whereas the cells in your hair follicles and the cells of your bone marrow that give rise to the blood, those cells are dividing all of the time. So what is the difference between, you know, oops. the cells that are growing all the time versus the cells that are never growing, these cells are being controlled. Does that make sense? There is something known as the cell cycle control system, and this cell cycle control system directs a cell on how frequently to divide, how frequently to undergo the cell cycle.

And what happens is if there's a problem with this control, This can lead to cancer cells. Cancer cells manage to escape those controls on the cell cycle. Essentially, that's what cancer is. Cancer involves cells that have escaped the cell cycle control.

So instead of dividing every other day, for instance, or only dividing when there's cell damage, those cells divide and divide and divide. So let's talk a little bit about the cell cycle control system. And then we can discuss how it can go awry to result in cancer. So again, inside of each and every one of our cells is what's known as the cell cycle control system, which is made up of various checkpoints. Checkpoints where the cell cycle stops until a go-ahead signal is received.

So let me show you kind of a drawing that will help. to make this more clear in your mind. Imagine if this is the cell cycle.

It kind of looks like a roulette table, you know, but and this this this this rod is going around the table. Well, eventually this rod runs into a flap, right? One of these plastic flaps.

These are checkpoints. These represent checkpoints. And the whole table represents the cell cycle.

Remember the cell cycle, which is made up of interface G1. Interphase S phase, interphase G2, followed by M phase, M phase, which includes mitosis and cytokinesis, right? So this table, this represents the cell cycle.

And notice how this wheel will spin as long as there's no flap in its way. But when it runs into a flap, that's known as a checkpoint, and that's where the cell cycle will arrest. until a go-ahead signal is given.

So a go-ahead signal would be like if this flap were to plop down, right? Then the rod could continue its journey. Now, there are three main checkpoints during the cell cycle. That's why you see three flaps here to represent the three main checkpoints of the cell cycle. The most important checkpoint is this one right here.

This is known as the G1 checkpoint, sometimes called G1. the G1 slash S checkpoint because it occurs between G1 and S. This is where the cell decides whether to divide or not, right?

So if a cell decides to divide, this flap will go down and the cell will copy its chromosomes and proceed into mitosis. If the cell does not decide to divide, this flap will... remain engaged and something interesting happens.

Let me show you what happens if the G1 checkpoint is not cleared. All right, look at this. If that G1 checkpoint is not cleared without a go-ahead signal, the cell exits the cell cycle and enters what's known as G0.

I haven't mentioned G0, but G0 is when the cell actually exits the cell cycle. The cell has it. At this point, the cell has no business or no interest in dividing anytime soon.

And in fact, those cells of your spinal cord and those cells of your heart, they are stuck in G0 because no go-ahead signal is given, right? So when a go-ahead signal is given, that's when a cell can come back from G0 and re-enter the cell cycle. Isn't that neat? Alright, so at this point if the G1 checkpoint, which is the main checkpoint, were to clear, the cell would proceed into S phase, at which point it would copy all of its chromosomes.

So it would copy the 46 chromosomes into 92, and then it would enter G2 phase where, you know, the centrioles would be copied, then the DNA would start to condense, and then it would smash into this flap right here, so it would arrest here. This is known as the G2 checkpoint, and during the G2 checkpoint, the cell makes sure that the DNA was copied correctly during S phase, and that there's no real problems with the DNA. And if that's true, this flap will, you know, disengage, and the wheel will continue until it bumps into this checkpoint called the M checkpoint, also known as the metaphase checkpoint, the metaphase, late metaphase checkpoint. Remember what happened during metaphase? All of the sister chromatids are now connected to microtubules from opposite ends of the cell.

Well, at this point, if all of those little sisters are not connected to microtubules, then there won't be this flap will not disengage. This flap ensures that every sister chromatid pair is connected. to the proper number of microtubules, you know, one microtubule per sister chromatid.

And so everyone's going to separate correctly. Otherwise, if you don't have this quality control, then the cells might divide, even if one of the sister chromatids is not connected properly to a microtubule. And then that would result in problems where you have weird numbers of chromosomes in the cells.

Isn't that neat? So again, The G1 checkpoint, this is the main checkpoint where the cell decides to divide or not. At this point, the cell will, you know, if that clears, the cell will enter, you know, S phase.

If it does not clear, the cell will exit into G0. The next checkpoint checks to make sure that the DNA was copied correctly and free of mutations. And then the next checkpoint makes sure that in late metaphase, everything is ready to separate.

All the chromosomes are ready to separate because all the sister chromatids have a microtubule attached at the kinetochore. All right, and just to wrap up this chapter, I want to ask you a little bit about cancer, to think about how cancer works, because it has everything to do with the cell cycle control system. These flaps, these checkpoints are responsible for regulating how often cells divide in your body.

But do you think that inside of your cells, you have... actual plastic flaps that prevent the cell cycle from proceeding? No, that's right, wicked.

That would be hilarious though. Yeah, you don't have actual plastic flaps inside of your cells that stop the cell cycle from proceeding. So you have to ask yourself, in your cells, what are these plastic flaps made of? What are these checkpoints made of?

Because something must be serving as checkpoints inside of your cells, okay? And I'll let you think about it for a second, but it's actually different proteins. Remember I said in a previous chapter that proteins are so important in your cells?

Different proteins have all different kinds of jobs, like hemoglobin protein, its job is to carry oxygen in your red blood cells. Insulin proteins job is to help regulate your blood sugar. Well, you have a host of proteins and these are known as, you know, collectively you could call them checkpoint proteins. These proteins play a role as checkpoints.

They play a role in maintaining the cell cycle control system. And when these checkpoint proteins are not functioning correctly, that's when the flap can fail. That's when the cell does not realize that it should stop until a go-ahead signal is given, and the cell will continue and continue and continue around the cell cycle.

And by the way, remember what I said earlier, that proteins are coded for by your genes. So the genes that code for the checkpoint proteins are called tumor suppressor genes, proto-oncogenes. These are important genes in your cells that code for very important checkpoint proteins, cell cycle proteins, proteins that when, you know, they become misregulated or mutated, you know, they can cause problems in these proteins.

So what you need to know is that the reason cancer occurs is because of a disruption in the checkpoint proteins. And that means that you have a mutation in these genes, either tumor suppressor genes or proto-oncogenes. Mutations in these types of genes result in cancer.

Okay, so it's not just any gene mutations that result in cancer. It's gene mutation that affects these very important... checkpoint genes and these proto-oncogenes and tumor suppressor genes because those are the genes that code for the checkpoint proteins. Well I hope this was informative, a very interesting chapter on cell cycle, mitosis, binary fission, cancer.

So I hope you learned a lot and took away a good deal from this chapter. If you have any questions please leave them in the comment box below and I'll catch you guys next time. Dr. D, Dr. D, Dr. D, Dr. D, Dr. A Dr. D, Dr. A Dr.

Transcript for:Understanding the Cell Cycle and Its Phases

Transcript for:
Understanding the Cell Cycle and Its Phases