Hello everyone. In this lesson, we are going to be talking about how a cell enters into the process of mitosis and how the chromosomes are going to be condensed and ready for the process of separation. Okay. So we know that mitosis is cellular division and we know that cellular division requires the chromosomes to be separated from one another and then divided into 2 daughter cells. But remember that chromosomes have to be really condensed to do this process. But first, let's talk about how a cell actually gets into this phase. Because we've talked about the cell cycle. Right? And the cell cycle is going to be the stages and steps that a cell takes before it goes into cellular division. So the cell has to go through G1, S, and G2 phase before entering into M phase or mitosis. Remember, G1 phase is basically the first growth phase where it's getting ready and preparing its size and its nutrients and its growth hormones. And then S phase is going to be the synthesis of DNA phase, or DNA replication phase, where the cell is making enough DNA to turn into 2 cells. And then G2 phase is basically just the preparatory phase for M phase which then follows. So how does a cell actually go into this process into the M phase of mitosis? Well, it's going to utilize those M Cyclins and CDKs that we talked about in earlier lessons. Remember M Cyclins or mitosis cyclins are going to be proteins that are going to increase in concentration before and during mitosis. And these cyclins are gonna bind to and activate cyclin-dependent kinases or CDKs. And M cyclins are going to tell the CDKs to activate proteins that are needed for mitosis. So they are going to be responsible for the cell entering into mitosis. Now, these MCDKs or the cyclin-dependent kinases that will bind to the M Cyclins are gonna be activated by the CDC25. CDC25 is going to be a protein, specifically it's going to be a phosphatase protein that removes inhibitory phosphate groups from the CDK's active site so that the CDK can actively bind to the cyclin. Now, if you're wondering what CDC actually stands for, CDC is going to stand for Cell Division Cycle protein. So it is the cell division cycle protein number 25, and it is going to be a phosphatase, which are proteins that remove phosphate groups. Kinases add phosphate groups. Phosphatases remove phosphate groups, and it is going to remove a phosphate group from the active site of the CDK allowing the CDK to bind to the cyclin. Now, MCDKs instigate the process of chromosomal condensation. Chromosomal condensation is incredibly needed and important for mitosis and meiosis because these chromosomes need to be separated and they need to be moved around the cell. So they have to be compact and packaged nicely so it's not just a big mess of DNA that's being dragged around the cell. So MCDKs are going to begin the process of condensation by activating proteins that are needed for this process. So condensation is required for mitosis or those chromosomes just won't be untangled, and they won't be able to move. Now, the different proteins that are needed for this condensation process that are going to be activated are going to be the condensins, their name is pretty self-explanatory, and the cohesins. So the condensins are going to be protein complexes that assist in chromosomal condensation and segregation. Basically think of condensins as condensing the chromosome. They're gonna actually take that chromatin and pack it really tightly together. And cohesins are gonna be protein complexes that hold 2 sister chromatids together and regulate their separation during anaphase. Remember sister chromatids are going to be identical copies of the replicated chromosome. Whenever the cell goes through S phase, the DNA is replicated. It is copied, and there are going to be 2 copies of each chromosome, and those are going to be called sister chromatids. They are going to be attached to one another via a specialized amount of DNA called the centromere. And these sister chromatids are going to be also held together by these protein complexes called cohesins. Now if you think about this, what does the word cohesion mean? It means to stick together, right? Cohesins are playing off that word. So their job is to make the sister chromatids cohesive. So now, let's go down and let's actually look at what this looks like in an image. Okay. So, let's start with this one right here. So this big guy right here is going to be a chromosome, but this thing is gigantic. Right? It's huge and it's gonna have to shrink down if we want that thing to be able to be a condensed chromosome that is easily movable. So, look at this. This is a chromosome and I want you to realize that this is a sister chromatid here in black, and this is a sister chromatid here in black. And, they are going to be attached via the centromere, which I will highlight in blue. Now, the centromere is going to attach them together but so is the cohesin. And the cohesin is actually going to be these little green dots that you can see. So these green dots here are going to be the cohesin protein complexes that actively keep the sister chromatids together. This is very very important, and they will stay here until anaphase happens and those sister chromatids are pulled apart. And cohesin is really important for maintaining this structure, and it's really important because if the sister chromatids aren't held together really tightly in a really nice and regulated way, those sister chromatids won't be separated correctly, and they may not go into the correct daughter cells which can create a big problem, and then the genetics of the daughter cells will be all messed up. So Cohesin is very very important. Excuse me. Okay. So now, let's go on to the next image right here. What happened? This is the same chromosome but it's shrinking. It's getting shorter and shorter and shorter and that is because of the condensin, and the condensin are going to be these red complexes here. And they are going to be these big protein complexes that basically grab onto the DNA and start twisting and turning it and making it really compact, and then you can see the next step is even shorter. And you can see that this is a really fully condensed chromosome. It's really small. It's a lot more movable than the one over there, right? It's really easy to grab and move and transition into the daughter cells. This is going to be called a metaphase chromosome because during metaphase the chromosomes are extremely compacted, and they are ready for movement in the next stage called the anaphase. So the condensins sorry the condensins will stay on these chromosomes until the sister chromatids have been separated and then transported into the daughter cells, and the daughter cells nuclear envelope has reformed. Then the condensins will leave the chromosomes and allow them to begin to uncondense or get much looser inside of their new nuclei. But the condensin and cohesin are very very important for the compaction and the connection of the sister chromatids so that they are ready for transport during the process of mitosis and meiosis. Okay, everyone. Let's go on to our next topic.
Mitosis - Online Tutor, Practice Problems & Exam Prep
Created using AIMitosis is a critical process of cellular division involving several stages: prophase, prometaphase, metaphase, anaphase, and telophase. During prophase, the mitotic spindle forms, and chromosomes condense, becoming visible. In prometaphase, the nuclear envelope disassembles, allowing microtubules to attach to kinetochores on sister chromatids. Metaphase aligns chromosomes at the spindle equator, ensuring proper separation. Anaphase sees sister chromatids pulled apart by the spindle, while telophase reforms the nuclear envelope around each set of chromosomes. Key proteins like cohesin and condensin play vital roles in chromosomal organization and separation throughout this process.
Mitosis Entry
Video transcript
Mitosis Steps
Video transcript
Okay. So now, let's go over the individual steps of mitosis. Now, I know you went over this in your intro bio classes, but hopefully, I'll be giving a little bit extra information, and maybe even an extra step for some of you. So the first step is prophase. And the important thing to know about prophase is this is when the mitotic spindle forms. So you may say, okay, well what's the mitotic spindle? I don't remember. Well, the mitotic spindle is a network of microtubules, you can call these aster microtubules sometimes, and centrosomes that control mitosis. And so, the mitotic spindle has two spindle poles where microtubules are attached to the centrosomes. Now, if you remember what centrosomes are, they're those like they're always drawn like this. Essentially, but they're made up of microtubules as well. And those microtubules come out of the centrosomes, and there's two of them at each end of the cell, and they those microtubules stick out. And so, the mitotic spindle actually forms because there are centrosome duplication occurring during S phase when the DNA is duplicated. And so each one of these centrosomes then moves to opposite poles of the cell. And so this is what happens during prophase, where the mitotic spindle is forming and moving to the other side of the cell. Then you have a potentially new step for some of you. Some of you may have gone over this, but it's prometaphase. And, this is the step where the nuclear envelope is disassembled. So, remember the nuclear envelope surrounds the nucleus and it breaks down during prometaphase. So when it breaks down, that exposes the chromosomes. Right? So those microtubules that are coming from the mitotic spindle begin to attach to the chromosomes at a specific location called the kinetochore. And so, because there are two poles and each of those microtubules is coming in at different ways, each one of the poles attaches to each one of the sister chromatids. So we say that as like being by, having a bi-orientation, so each sister chromatid is attached to the opposite poles. So here we have an example of what this looks like. We have prophase. You can see, an interphase just here. You have, chromatin and chromosomes, but you can't really see the chromosomes. Right? They're just kind of it just looks like spaghetti almost because it's not condensed yet. So in prophase, you start seeing these actual chromosomes, and this is because they're condensing, and that makes them visible. You also see the mitotic spindle, so here's one and here's one with those, microtubules that kind of look like spider legs extending out from them, so that also happens during prophase. Then you have prometaphase, and this is where the nuclear envelope, which is keeping the keeping the microtubules from the mitotic spindle and the chromosomes separate breaks down. So now, the microtubules begin to interact with the chromosomes and each sister chromatid of a chromosome. So each one of these travels is attached to an opposite spindle. That's what that looks like. So one is going to be attracted over here, and the other one's going to be attracted over here. So then we get to metaphase. So metaphase is when the duplicated chromosomes align at the spindle equator. So this forms a metaphase plate, which is just a line of chromosomes straight along the in equator. So it forms this line, essentially, in the middle of the cell. And so there's a really important checkpoint here called the spindle assembly checkpoint, and this checkpoint makes stops the cell cycle and makes sure that the chromosomes are aligned properly, so they're straight in the middle. And if they're not, then it delays entry into anaphase because they aren't aligned properly. There must be a space there. So after that, they're now aligned in the middle, so now we can move on to anaphase. And this is where the cohesin, which if you remember what cohesin is, right, it's that protein holding the sister chromatids together. Well, another protein called separase comes in, breaks that, so the sister chromatids can now separate and they do because they pull pull towards each spindle pole. So, the first thing that happens is that the sister chromatids begin moving towards the poles, right. Then the second thing that happens is the spindle poles actually move further apart. And so this not only are the sister chromatids just moving with the microtubules, but the spindles themselves are moving further, so they're moving really far apart. And so, this is also where the anaphase-promoting complex works, right, it's an anaphase. Do you remember we talked about this in the cell cycle control, that it works here to prevent to degrade some of these other proteins to prevent the repeat of mitosis because it's coming to the end, so it wants to degrade those things that are promoting it. So, that's what happens in anaphase. And then, in telophase, what you get is the nuclear envelope. Now, it's starting to reform, the mitotic spindle is starting to disassemble, and then things will eventually break into two cells. So here we have metaphase. You see they're aligned in the middle. I like to think m middle m metaphase. Then, during anaphase, each one of the sister chromatids is being pulled to the spindles, as the spindles move forward too, so this separates the sister chromatids. Then in telophase, what you get is they keep moving and the nuclear envelope is going to start reforming here. The spindles are going to start breaking down, and eventually, you're going to get two separate cells. So, on the next page, I'm not going to go through this just because it's exactly what I just went over, but there is a full diagram of the entire overview of mitosis with all these images together starting here at anaphase and going through prophase, prometaphase, metaphase, anaphase, and telophase before breaking into two different cells here. So if you want to review the overview of mitosis, make sure to come to this page and see if you can figure out what's going on without the notes. So now, let's move on.
Which of the following is the correct order of mitosis?
In which of the following steps do the sister chromatids separate?
In which of the following steps does the cell cross the spindle assembly checkpoint?
The nuclear envelop begins to reform in which of the following steps?
Here’s what students ask on this topic:
What are the stages of mitosis and their key events?
Mitosis consists of five stages: prophase, prometaphase, metaphase, anaphase, and telophase. In prophase, the mitotic spindle forms, and chromosomes condense, becoming visible. During prometaphase, the nuclear envelope disassembles, allowing microtubules to attach to kinetochores on sister chromatids. Metaphase aligns chromosomes at the spindle equator, ensuring proper separation. Anaphase sees sister chromatids pulled apart by the spindle, while telophase reforms the nuclear envelope around each set of chromosomes. Key proteins like cohesin and condensin play vital roles in chromosomal organization and separation throughout this process.
Created using AIWhat role do cohesin and condensin play in mitosis?
Cohesin and condensin are crucial proteins in mitosis. Cohesin holds sister chromatids together, ensuring they are properly aligned and separated during anaphase. It maintains the structural integrity of chromatids until separase cleaves it, allowing chromatids to move to opposite poles. Condensin, on the other hand, is responsible for chromosomal condensation. It compacts chromatin into dense, movable structures, facilitating their segregation during cell division. Both proteins ensure accurate chromosome segregation, preventing genetic abnormalities in daughter cells.
Created using AIHow does the mitotic spindle function during mitosis?
The mitotic spindle is a structure composed of microtubules and centrosomes that orchestrates chromosome movement during mitosis. It forms during prophase and consists of two spindle poles. In prometaphase, microtubules attach to kinetochores on sister chromatids. During metaphase, chromosomes align at the spindle equator. In anaphase, the spindle pulls sister chromatids apart to opposite poles. Finally, in telophase, the spindle disassembles as the nuclear envelope reforms around separated chromosomes. The spindle ensures accurate chromosome segregation, crucial for producing genetically identical daughter cells.
Created using AIWhat is the significance of the spindle assembly checkpoint in mitosis?
The spindle assembly checkpoint (SAC) is a critical regulatory mechanism in mitosis, occurring during metaphase. It ensures that all chromosomes are properly aligned at the spindle equator before anaphase begins. The SAC prevents the cell from progressing to anaphase until each chromosome is correctly attached to spindle microtubules from opposite poles. This checkpoint is vital for preventing chromosome missegregation, which can lead to aneuploidy and genetic disorders. By ensuring accurate chromosome alignment and attachment, the SAC maintains genomic stability in daughter cells.
Created using AIWhat happens during the prometaphase stage of mitosis?
During prometaphase, the nuclear envelope disassembles, allowing spindle microtubules to interact with chromosomes. This stage follows prophase and precedes metaphase. The breakdown of the nuclear envelope exposes chromosomes, enabling microtubules to attach to kinetochores on sister chromatids. Each chromatid is connected to opposite spindle poles, establishing a bi-orientation. This attachment is crucial for the subsequent alignment of chromosomes at the spindle equator during metaphase, ensuring accurate segregation during anaphase.
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