15. Cytoskeleton and Cell Movement
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Hi in this video we're gonna be talking about micro tubules. So micro tubules are side of skeleton elements which we've already talked about but essentially their function is they act as cellular tracks. So kind of like train tracks um that move vesicles and organelles throughout the cell. So micro two wheels, they are made up of two billion, which is the small sub unit that makes them up. And so each two billion is actually a dime er composed of an alpha subunit and a beta subunit. And the positions of the sub unit are very, very carefully controlled because each position provides the two billion with a polarity. So the plus end is where the beta part is and the minus in is where the alpha part is. And so this is important because that means that one side is going to have a plus in and one side is gonna have a minus in. So that means that each side does different things and has different abilities. So one of those abilities is that usually the plus end is where growth occurs for the most part, doesn't have to but very fast growth happens at the plus in and so um I'm not, not only our micro tubules made up of just tumbling like long strings of tooling. Instead they have micro tubules associated proteins or maps um that bind them and stabilize them against this assembly. We and they don't need to, this is simple. So here's an actor to bill. You can see that there's a dime er here you have your alpha and red and your beta and yellow and these are very orderly positioned all the way down the micro tubules sub unit. And that gives them polarity where you have your beta subunit which is going to be the plus in and your alpha side which is going to be what's called the minus end. Now Tulin is um attached to G. T. P. So this is we keep seeing this in cell bio. It's a big regulator. And so this G. T. P. Or this binding GTP affects migratory, Real preliminary ization and in case you don't remember what polarization is, it just means growth. So I'm going to walk you through the steps of microbial growth. So the first thing is that proto filaments which are going to be the small aggregates of turbulence of these early lake filaments of two billion form at my real organizing centers. So this is where micro tubules are nuclear hated or undergo nuclear station. Which if you remember, has absolutely nothing to do with the nucleus, but instead has to do with the small aggregation of turbulence sub units which allow for the microbial to grow. Nuclear nation is like the first step in microbial growth. It happens at microbial organizing centers in the south, Then from here two million Plymouth Ization occurs, which is just growth. And um these two million diners can actually be added to either side. They can be added to the plus in or they can be added to the minus in. But you're like wait, you just told me they add faster at the plus in which is true and I'm gonna tell you why. So the reason that they do this has to do with why it's how it's bound to GTP. So because two billion diners are bound to GTP, they can also hydrolyzed by G. D. P. So the G. D. P. Which is called the T. Form, which because it's T. Or the G. D. P. Which you may see is the D. Form depends on kind of how fast the two billion diners at each end. So so if um so two million diners have the ability to hide relies GTP to GDP and they do it quickly upon addition. But if they're hydrolyzed slowly, so it means that they when they bind to the microbial, the growing microbial, they're like wait just a little not long but just a little bit there just slow about it. About hydra lies ng from GTP to GDP, then what happens is the next timer is added before hide relation or hydrolyzed, This can occur. And so when this happens, you actu get the stacks two million die MERS that are all have GTP and haven't yet gotten to the process of hydra lies ng. So when this happens it forms a GTP cap. And so you have multiple two million G. T. P. S. That are bound to each other on this growing microbial end and they're not immediately hydrolyzed. So they're going to be hydrolyzed but not yet because they're slow about it. Alternatively if it's hydrolyzed quickly then G. D. P. Is just gonna be super hydrolyzed so fast that before the next dime er can add it's actually going to just be hydrolyzed. Now the growth can happen on either end but the growth is much easier at this end because it has the GDP cap and none of these are hydrolyzed yet. And it's just the two billion binds this side much faster. Much easier. So this is going to be the quickest way which if you remember what I said above is also going to be the plus end. Whereas here it's going to be the minus and usually and if there's a bunch of G. D. P on the end, the two million is like well I don't have any energy to stay together so it's actually gonna start destabilizing it and breaking apart. So what this looks like is this image here. So you have your two billion sub units. You have some that are bound to GTP and others that are bound to GDP. Now these form together to form these small proto filaments but eventually they form micro tubules and in the process of polymers they grow. So you can see that there's this GTP cap with all these subunits that have GTP bound to them and you can see that in blue. So the T. Billing wants to add to this in which is usually the plus in. And so it's problem arising, it's growing at this end. These subunits are adding whereas this end it's hydrolyzed so quickly when the subunits are added. So it is then you have a bunch of GDP bound to Berlin and subunits don't want to buy into that. They want to buy into the GTP cap. And so when nothing is being added to it as none of this energy coming in, it's gonna de polarize. The sub units are gonna start breaking off. And this whole thing is gonna be like unraveling kind of a zipper um from the from the minus end. But it's being created at the plus end. So that's how to will employ more ization. And deep polarization works. Now there's two terms that are very commonly used to describe this and they sound so similar. It's kind of hard to separate them. So hopefully I'll be able to explain this. So the first is dynamic instability. So this is when a microbial end. So one of the ends either the plus or minus, which is between liberation and depolymerization. Now tread milling is different because this is when subunits are recruited to the plus end and shed or gotten rid of at the minus end. So this deals with one end and this deals with what's happening at both ends. So for dynamic instability, this is what you see. So you have GTP and GDP, we're focusing specifically on one end. So when there's a bunch of GTP here it's going to be growing. And then what's called a catastrophe, which I think is a little dramatic, but it's just called a catastrophe. When all these things get hydrolyzed really quickly, then it starts destabilizing and shrinking. But then occasionally it can be rescued by adding a G. T. P. Onto it and if it's rescued then it's going to start growing again. And so this is dynamic instability, what's happening at one end. Whereas tread milling is what I've showed you previously, which is what's going on at both ends. So hopefully that was clear. So with that let's now move on.
Microtubules and Cell Division
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Okay, so now we're going to talk about micro tubules and cell division. This is going to be really short because we're going to go over a lot of this when we actually get to cell division. But I just want to introduce it here. So the first thing I want to introduce is the central zone and these are part of these are kind of locations in the cell organelles in the cell that actually are responsible for organizing microbial arrays during cell division. And these are rays are really important because micro tubules are what move everything around. They move replicated organelles, they move replicated D. N. A. And they make sure that everything is sorted into cells when they divide. And that's super important because you don't want one cell with like 75% of the D. A. And one with only 25%. And you don't want one with all the micro tubules, our all the mitochondria and one of none. I think it's got to be equally distributed. And so central zones and micro tubules do that. So central zones contain central pairs which I know you've seen in your bio one on one class. And central pairs are super important because they act as the nuclear station site for microbial growth. So remember Nuclear Nation has nothing to do with the nucleus. Instead it has to do with where these side of skeleton elements form. So Central's have Nuclear Nation sites for micro tubules growth during cell division. And so these two billion diners come into the central's and they're added with their minus in towards the Central's and plus in out towards the side of plaza where they grow and attached to D. N. A. Or or organelles or whatever they're gonna attach, shoot during cell division. So this is That's what they look like. They have this very unique structure with this triplet. So you see the 123123 which is arranged around. So they have these triplet. Now there's a pair. So you can see there's one here and two here. Um and this is where nuclear station of micro tubules occurs during cell division. So like I said, super short but also super important. So now let's move on.
Under which condition is a GTP cap formed during microtubule formation?
If the microtubule end is hydrolyzed slowly
If the microtubule end is hydrolyzed quickly
Treadmilling is when a microtubule end switches from polarization to depolarization.
A single tubulin subunit is composed of which of the following components?
An alpha tubulin
A beta tubulin
A dimer of alpha and beta tubulin
A tetramer of alpha and beta tubulin