Hello everyone in this lesson. We are going to be talking about the cell cycle and the mechanisms that are utilized to control the processes in the cell cycle. Okay, so the cell cycle is going to be these different major stages that the cell goes through when it is deciding and preparing for cellular division. And of course cellular decision needs to be controlled. Right because what is uncontrollable cellular division, it's going to be cancer. So obviously we don't want that. So our cells are going to have many regulators and many control mechanisms to ensure that the division of cells doesn't get out of control. So cell cycle controls rely on a core group of switches. You can think of them as switches but they're gonna be proteins that turn things on and off like a switch does. And they're going to turn certain proteins on certain proteins off. And this is going to determine the steps in the cell cycle. So the most important determinant and control of the cell cycle that you are going to need to know are going to be the cyclones and these are going to be these little proteins that turn protein penises on and off and they are going to vary in their concentration but they're not going to vary in their activity. So they're going to vary in concentration in the cell but they're not really going to vary in activity. So I'm going to tell you why this is important in just a second. So there are many different types of cyclones. I'm going to teach you about four types of cyclones and the different types of cyclones are going to become incredibly prominent and then they're going to die down in different stages of the cell cycle and this is going to control those stages of the cell cycle. So that's why I said they vary in concentration but the job of each cyclone is dependent on the type of cycling that it is and it doesn't change. So what does the Cyclone do? A Cyclone is this little protein that it's going to affect? The cycling dependent penises or C. D. K. S. Cycling dependent penises do exactly what their name says. Remember, penises, phosphor relate or D. Phosphor, a late proteins and this is going to turn proteins on or turn proteins off and these kindnesses are switched on or off by cycling. That's why they are cycling dependent keenness. A cyclone will bind to the C. D. K. And it will allow the C. D. K. To activate itself and begin doing its job. So the cell cycle controls the protein kindness is based on the types of cyclones that are activating that cyclone dependent penis. So you may be wondering what that would look like, what that would look like would be something like this. We have our C. D. K. So C. D. K. And then we're going to have the little Cyclone Cyclone and the Cyclone dependent kindness is going to turn on. So this is cycling and it is turned on and now this is activated and this means that the cycling dependent penis can then phosphor elit or d phosphor relate certain proteins which will have different jobs in the process of the cell cycle. So the important thing to know about the C. D. K. S is they do not vary in concentration, so do not vary in concentration. They are always in the cell and their levels within the cell remain the same. But the cycling concentrations do not, the cycling concentrations constantly change, but the C. D. K s are always there, they're always waiting to be activated. So what's that going to look like that's going to look like something like this. So you can see that these are different cyclones, I'm going to go out of the picture. So you can see all of this gray. These are four different cyclones and we have a B. D and E. Cycling. And you can see that during the different phases G one S. G two mitosis of the cell cycle, These are different phases, you can see that these cyclones greatly vary in their concentration. Now, if I were to show you the concentration of cd case, it would be something like this. This would be C D C D K concentrations and the C D. K concentrations don't change, they're always there in the same amount, but the cycling concentrations do change and it depends on which stage in the cell cycle the cell is in. So that is some basis on the C. D. Case and the cyclones. So now let's scroll down and let's look at those different stages of the cell cycle. Let's talk about something interesting and very important for those stages of the cell cycle. We are going to talk about checkpoints, checkpoints are stages within the cell cycle that the C D. K. S actually do their job. So there are many different checkpoints, there are four here, but I'm going to go over five. I didn't list the m checkpoint because it's pretty self explanatory. I tell you guys about it in just a second. So there's the G one check point, the start checkpoint, the S checkpoint and the G two checkpoint. So remember in G one, which we can see down here in G one. Let me show you this cell cycle diagram. So this is G one right here, G one phase and then here is the G one checkpoint. Remember in G one phase, the cell is deciding whether it wants to divide or not and the way it determines this is it looks at its cell size, its nutrients level and its growth factors. And if it has any D. N. A damage, if it sees that any of these characteristics are not good in for cellular division, it will not go into cellular division. So that's what the G one checkpoint is looking for, its checking all of these things, it's activating cd Ks to check all of these different characteristics and once it has passed through the checkpoint and seeing that everything is fine then it will proceed on to S phase. So S phase is going to be here in green and the S checked is right at the end of S phase. Now, once the cell has decided it wants to go and divide itself it can't go back, there's no going back at this point, it's all or nothing. Once it enters S phase it's done, it's going to start dividing. So s phase, remember that this is DNA replication phase or synthesis replicate or synthesis of D. N. A. Phase. And basically you're replicating the D. N. A. Inside of the cells. So there's enough D. N. A. In there for the two new daughter cells. And the S checkpoint is going to ensure that the DNA replication went through properly and nothing really detrimental happened and that replication has ended. That's the S checkpoint. Once the S checkpoint has been completed then the cell will move on to the G two phase which is right here and we have our G two checkpoint. Now, G two is going to be whenever the cell is getting ready for the physical division process of mitosis. So this is going to ensure that the D N. A. Has completely been replicated. Any D N. A damage that was created during S phase days is going to be fixed. Any mutations are going to be fixed and it's going to repair anything that needs to be repaired before the cell enters into mitosis. So then we have M. Phase which is relatively short. Oh you can't really see the M. Whenever I use that color. So this is in phase. I'll write it in here. M phase in phase is mitosis. And we're going to have our spindle checkpoint or R. M. Checkpoint. Basically the spindle checkpoint is between meta phase and anna phase. And it's just ensuring that the chromosomes are correctly attached to the spindles before they are separated and pulled to the poles of the cell. So the spindle checkpoint doesn't have as much to do with C. D. Ks and cyclones as the other checkpoints. So that's why we didn't really listed above. But it is an important checkpoint to know. But this is gonna be the overview of the cell cycle and this is going to be the cycle that a cell will take whenever it wants to divide into two daughter cells. So if we scroll up all of these checkpoints I just talked about. So we have the G. One checkpoint which pauses to ensure that the D. N. A. Is repaired and ready to go before S. Phase. And it makes sure that the cell actually wants to divide and has all the nutrients that it needs. Then we have the starting point at the end of G. One. Remember I talked about this. If the cell decides to go into S. Phase it's going to continue on through mitosis, there is no stopping after this point. Then we have the S checkpoints and this is a pause in the S. Cycle to ensure that the DNA replication is happening correctly and that it is coming to completion and then G two checkpoint is a pause right before mitosis just to make sure that everything is ready to go before the self physically divides itself during the process of mitosis. Now there are are going to be cycling proteins that are associated with these transitions and stages in the cell cycle. And I listed those for you here. We have the G one S. Cyclones, we have the G one Cyclones and the cyclones and the M. Cyclones and they're going to be very important for these different stages. So whenever we have the G one cyclones these are going to control the G. One and S. Cyclones and they're really just these cyclones are very important for ensuring the G one phase goes correctly and actually ensures that all of the phases of the cell cycle actually proceed correctly and I'll show you an image of that in just a second and then we are going to have the G one one, S. Cyclones and these are going to activate Cd Ks in late G one and this is going to trigger the beginning of the S phase. This is going to trigger that transition into cellular division. This is the starting point where we can't turn back and these G one S. Cyclones are going to bind to cd case and trigger that start point. Then we're going to have the S. Cyclones which are going to activate seed pKS after the start to stimulate the replication of DNA. So these S. Cyclones are going to bind to the C. D. K. S. And then the C. D. Ks are then going to phosphor relate and turn on all of the proteins that are needed for S. Phase or the replication of DNA. And then we're going to have our M. Cyclones and these are going to trigger mitosis. So these M. Cyclones are going to bind to the C. D. K. S. And this is going to to activate all of the proteins that are needed for mitosis. So see here that the type of cyclone that is utilized is going to activate the C. D. K. Which will then activate proteins associated with that. Cyclone M. Cyclones, activate M proteins or mitosis proteins. S. Cyclones activate proteins for cellular DNA replication. G. The one Cyclones are going to control that transition from G. One into S. And then G. One S. Cyclones are going to do the same thing and they're going to activate proteins that are needed to begin the process of cellular division. I hope that makes sense. So we have these different cyclones, what is their concentration going to look like? Well remember this picture we saw up here. These are actually referring to these different cyclones. So the G one Cyclones are actually here in red G one. Cyclones are kind of unique because they're pretty much needed for the whole process in the different stages of the cell cycle. But they're incredibly an integrally important for the G. One phase. But they're unique because they're needed for all of the phases. And then you can see in green these are going to be the G one S. Cyclones which are needed for that transition from G. One into S. Phase. And then you're going to have the S. Cyclones here in blue which are needed for the replication of D. N. A. And the repair of G. N. A. D. N. A. That is happening in G. Two phase before mitosis. So that's why there's high concentrations of S. Cyclones in S. Phase and G. Two phase because the DNA is being replicated in S. Phase and it's being repaired in G. Two phase. Now, the final set of cyclones here in orange are going to be the M. Cyclones and they are going to come to a peak in concentration right at the start of mitosis. So all of those mitosis proteins have been phosphor elated and turned on and that whole process can be again. So this is how the cyclones that are different and unique to each stage are going to change and concentration throughout the cell cycle. Now I know that I went over a lot in this video. I hope it was helpful. Just remember that cyclones are utilized for the control of the cell cycle. There are different types of cyclones that control the different stages of the cell cycle, and cyclone activate C. D. K. S. And the type of cycling determines which proteins will be activated by the C. D. K proteins. And this is going to ensure that the cell cycle goes correctly, that none of the phases are messed up and that the process of cellular division is controlled and doesn't get out of hand. Okay, everyone, let's go on to our next topic.
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Okay, so now we're going to talk about regulation but specifically regulation of the cycle independent penises themselves. So this is kind of a double level of regulation, Right? So we talked about, we're talking about cell division mitosis and how the cD case regulate that. So how do we regulate the cD case? So there are three main ways and the first is a little weird, but also probably the most important. And so this is um through the C D. K inhibitor phosphate. Now this might sound a little weird, right, because when we think of adding phosphates, the proteins, what do we think of? We think of activating them, kindnesses add phosphates they activate. But this is a special type of phosphate and it acts as an inhibitor. So instead of activating it does the exact opposite. So instead that phosphate has to be removed so it has to be the phosphor related in order to become active. So that means that C. D. K. S actually need to be foss for elated at one site and de fosse for elated at another site in order to become active. So the protein responsible for de phosphor awaiting or removing these phosphates is called C. D C. 25 that removes the inhibitory phosphate. So this is a little different than anything else we've mentioned in this course or really probably anything you've seen before but it's a really important mechanism of C. D. K. Regulation. Now the second one is kind of, it just I feel like makes sense. So there are C. D. K inhibitors. So these are proteins they bind A C. D. K. S usually when they're in complex with Cyclones and they blocked their function. So this acts like every other inhibitor we've talked about right, it just it's an inhibitor it binds to it, it blocks its function. But it is the second way of C. D. K regulation. And then finally the third way is going to be through cycling levels. So if cycling levels are high then you're gonna have like it's going to be active cd case. But if they're low cycling levels are low then you're gonna have inactive C. D. K. S. And this is because Cyclones activate C. D. K. S. So if you have high cycling levels you're gonna have high activity. But if you have low cycling levels you're gonna have low activity. So then the question is, well how do you regulate cycling levels? Well, typically they just get degraded. So there's this complex called the ana phase promoting complex and this actually degrades M. And S. Cyclones by labeling them with ubiquity in and targeting them for destruction. And this is this is really really, really important complex because it happens at the end. So remember this, we're in an a phase here, it's degrading these m. It's degrading these s. Cyclones that have stimulated the cell cycle but are no longer needed. Um So then slowing down the cell cycle and getting ready to stop it essentially. So here is that unique mechanism that I've talked about, you've probably never seen before. So here we have a C. D K C D. K. One and it has an activating phosphate, but it also has this inhibiting phosphate. So if we were just to take this complex, what's in my arrows, Not anything to do with this one down here, just what's in my bracket? Is this C. D. K. One in my bracket. Is this active or inactive? Right, so this is an inactive Complex because it has the inhibiting phosphate. Now, once the protein, remember what protein this is. c. d. c. 25. Once it comes in and removes this phosphate that leads to activation. So as soon as this phosphate is gone, it's no longer inhibiting. And so you get activation of C. D. K. One and this happens for a lot of different cD case. So those are the three ways that C. D. K. S. R. Regulated so that then they can go on and regulate this area cell cycle. So now let's turn the page
Often, Cdks have to be dephosphorylated to become activated.
Which of the following cell cycle checkpoints occurs immediately before the start of S phase?
Which of the following proteins is responsible for removing the inhibitory phosphate on Cdks?