Microtubules

- Let's talk about microtubules in more detail. So, first we'll discuss the structure. So, microtubules are made up of two proteins. The first is called alphatubulin, and the second similar protein is called betatubulin, and the alphatubulin and betatubulin will join together to form a dimer. A dimer's simply when you have two molecules that are similar or identical, and you put them together. And then these dimers are gonna form long chains or polymers, and these polymers will be put together into a sheet, and then that sheet is gonna be rolled up to form a tube. And here we have a microtubule, and if you recall, the diameter of a microtubule is approximately 25 nanometers. And at one end of the microtubule, it's going to be anchored to this place called the microtubule organizing center, or in short, we could call it the MTOC, and at the other end, it's actually really intersting, dimers can be added very, very quickly to this end of the microtubule, making it longer, or dimers could be taken off that end of the microtubule, making it shorter. So, it can become longer and shorter very, very quickly. So, microtubules are dynamic. They change, and they can change very quickly, and you'll see as we go along, it's important for microtubules to be able to become longer or shorter in order for them to fulfill their functions. Let's go back to the microtubule organizing centers. So there are actually different types of microtubule organizing centers, and we're gonna talk about two. The first is the centrosome, and the second is called a basal body, and centrosomes and basal bodies are pretty similar in structure, but the microtubules attached to them carry out different functions. Let's talk about the centrosome first. The centrosome is an organelle that's found near the nucleus of a cell. It's made up of a lot of these different types of proteins, but we're gonna focus mainly on two rods that are found in the centrosome. These are the two blue rods that I'm shading in, and each one of these rods is called a centriole, and if we took a closer look at the structure of the centrioles, they would look something like this. They're made up of these triplets of microtubules. So each one of these triplets are three microtubules that are attached to each other, and there are nine of these triplets that make up one centriole. So I'm just gonna circle them. Eight and nine, so that means it takes 27 microtubules to make one centriole, and what purpose do these centrioles serve? Well, when a cell is replicating, or undergoing mitosis, these centrioles are going to duplicate, and a pair of centrioles will land up on either side of the cell, and we're gonna fast forward to the metaphase part of mitosis right now. So here we are, in the metaphase part of mitosis, and we're looking all what's called the mitotic spindle. The blue lines that kind of look, maybe like I don't know the web of a spider. So those are all microtubule fibers, and they're holding on to the chromosomes in a very specific way. So, let's go through this mitotic spindle step by step. So we have in the center of the cell, the chromosomes. Then at the center of the chromosomes in that magenta, that's the centromere, and then outside of the centromere in light blue, that is the kinetichore. The kinetichore is a protein on the chromosome that's gonna serve as an anchoring site for the fibers. So, coming out of the kinetichore, those blue little fibers, those are the kinetichore fibers, and then the kinetichore fibers turn into the microtubules. So, let's pick one right over here. So these are the microtubules, and if I wanted to be more specific I'd say that these are the interpolar microtubules. So I'm just going to point out a couple more to make it clear. This would be an interpolar microtubule, this would be an interpolar microtubule, this would be not an interpolar microtubule. We'll see in a minute what that is. But anyway, they're called interpolar microtubules because they are between the two poles of the cell, this being one pole, and this being another pole, and you can see, the interpolar microtubules are attached to our centrioles. I'm just gonna highlight them to make it more clear. Here's one pair of centrioles, and here's another pair of centrioles. So the centrioles are an anchoring site for the interpolar microtubules. Let's just go through a couple of other structures. So, these microtubules that are kind of coming out of the centrioles, those are called astral microtubules, and they're called astral microtubules because this part, the centrioles plus those fibers that are kinda coming out of it. So, each one of these is called an aster, and it's called an aster because it forms a shape that looks something like a star, and the word aster means star, and you can see some of the interpolar microtubules are actually attached to the astral microtubule. That's a pretty complex network going on. But, what's the point of this entire mitotic spindle? Well, if you recall, I mentioned before that microtubules can shorten or become longer very, very quickly. So what's going to happen during the next phase of mitosis, during anaphase, is the microtubules are going to become shorter and pull the chromosomes apart so that one half of all the chromosomes ends up on one side of the cell, and the other half of all the chromosomes ends up on the other side of the cell. So it helps to separate the chromosomes, and then eventually this cell is gonna be split down the middle, and two different cells are going to be formed. So the purpose of the centrioles is they serve as an anchoring site for the microtubules that are attached to the chromosomes, and then the microtubules will become shorter pulling the chromosomes apart having half of them in one half of the cell, the other half in the other half of the cell. So, let's just recap. We mentioned that there are two different types of microtubule organizing centers. We said the first was the centrosome. So I'm just gonna point it out in the diagram. The centrosome would this area, and recall the centrosome is composed of the centrioles plus other proteins, and then we said the other type of microtubule organizing center is called a basal body. So, we just discussed the centrosome, now we're gonna move on to basal bodies. So, basal bodies are the microtubule organizing center in cells that either have cilia, in singular that would be a cilium, or flagella, and in singular, that would be a flagellum. So cilia are these hair-like projections that come out of a cell. For example, in cells in the respiratory tract have cilia, and they beat in an upward direction, and they help push mucus up our respiratory tract and that's why you'll sometimes cough up mucus, and a flagellum is a tail-like projection that comes out of a cell, and it helps the cell move, and in fact, the only human cell that has a flagellum is the sperm cell. So, sperm cells move with the help of their flagella. And, I'd like you to keep in mind when we talk about flagella that we're speaking about flagella that's found in eukaryotic cells because the flagella that's found in prokaryotic cells has a different structure than what we're describing. So just keep that in mind. So here we have a cell. We have a nucleus in the center, and right over here in blue is the microtubule organizing center, or the basal body. Remember it has pretty much the same structure as a centriole, and anchored to the basal body is a flagellum or a cilia. So, flagellum and cilia have pretty much the same structure. The only difference is that cilia tend to be shorter, and flagella are longer. So, we're gonna pretend like this is a flagellum, but if it were a cilia, it's the same structure anyway. So, if we were to cut the flagellum, or cilia for that matter, just like that, and look at a cross section of it, it would look something like this. So, it's made up of microtubules that are in a very specific arrangement, and we call this the "9 + 2" arrangement. Let's see why it's called this way. So it's made up of these pairs of microtubules and they're actually nine pairs. So, one, two, three, four, five, six, seven, eight, nine pairs, and in the center, there is one lone pair, and that's what that two refers to. So, that's why this arrangement is called the "9 + 2" arrangement, and between the pairs of microtubules we have this protein called nexin. It helps to keep the microtubules in their place, and then coming out of the microtubules we have this protein called dynein, and dynein is a protein that breaks down ATP and uses that energy to help the microtubules move past each other, and that's actually what drives the movement of the flagella and cilia. So, that takes care of basal bodies, and now let's talk about one more function of microtubules. Microtubules play a really important role in the internal transport in neurons. So, here we have a nerve cell. Let's just label the various parts. We have the dendrites here. We have the soma, or the cell body. We have the nucleus. We have the axon, and then we have the synaptic terminal. And most of these substances in the cell are made in the soma, and these substances have to get to the axon or to the synaptic terminal. And how does that happen? Well, with the help of the microtubules. So the microtubules form this network that runs from the soma, all the way down to the synaptic terminal, and they kind of act like a railroad track, and different substances are moved along this railroad track of microtubules with the help of two proteins, kinesin and dynein. So, kinesin and dynein help to shuttle different things from the soma down to the axon or the synaptic terminal. So, what are some things that can be shuttled down this microtubule railroad? So, synaptic vesicles, which actually contain neurotransmitters. Different proteins that the cell needs. Different lipids that might be necessary, and even organelles, such as mitochondria. And, kinesin and dynein are able to transport substances in this direction from the soma to the synaptic terminal, but also in the other direction, going from the synaptic terminal to the soma, and this process is called axonal transport, or another way to say that is axoplasmic transport. That means transporting substances down this microtubule railroad track. So, microtubules play a very important role in the transport in nerve cells. In fact, they even help to transport nerve signals because synaptic vesicles, which contain neurotransmitters are shuttled down microtubule railroad tracks all the way to the synaptic terminal where the neurotransmitters that they contain are released into the synapse.