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Video transcript

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 alpha tubulin and the second similar protein is called beta tubulin and the alpha tubulin and beta tubulin will join together to form a dimer a dimer simply when you have two molecules that are similar or identical and you put them together and then these dimers are going to form long chains or polymers and these polymers will be put together into a sheet and then that sheet is going to 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 MTO C and at the other end it's actually really interesting dimers can be added very very quickly to this end of the microtubule making it longer or diamas 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 going to talk about two the first is the centrosome and the second is called a basal body in 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 cells made up of a lot of these different types of proteins but we're going to 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 it 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 I'm just going to circle them eight and nine so that means it takes 27 microtubules to make one central and what purpose do these centrioles serve well when a cell is replicating or undergoing mitosis the centrioles are going to duplicate and appear centrioles will land up on either side of the cell and we're going to fast forward to the metaphase part of mitosis right now so here we are in the metaphase part of mitosis and we're looking at 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 onto 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 kinetochore the kinetochore is a protein on the chromosome that's going to serve as an anchoring site for the fibers so coming out of the kinetochore those blue little fibers those are the kinetochore fibers and then the kinetochore 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 inter polar microtubules so I'm just going to point out a couple more to make it clear this would be an interpolar microtubules this would be an interpolar microtubule this would not be an interpolar microtubule we'll see them in 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 going to highlight that 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 kind of coming out of it so each one of these is called an Astor and it's called an Astor because the forms a shape that looks something like a star and the word Astor means star and you can see some of the interpolar microtubules are actually attached to the astral microtubules it'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 it'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 core Soames and then eventually the cell is going to 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 on 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 going to point it out in the diagram the center someone to be this area and recall the centrosome is composed of these 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 going to move on to basal bodies so basal bodies are the microtubule organizing Center in cells that have either cilia and Cingular that would be a cilium or flagella and in singular that would be a flagellum so cilia are these here like projections that come out of cell for example in cells in the respiratory tract have cilia on they beat in an upward direction and they helped push mucus up our respiratory tract and that's why you'll sometimes a cough up mucus and flip a flagellum is a tail like projection that comes out of the 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 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 with 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 central 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 going to pretend like this is a flagellum but if there were 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 piers of microtubules and they're actually nine piers one two three four five six seven eight nine piers and in the centre there is one lone pair and that's what that two refers to so that's why this arrangement is called the nine plus two arrangement and between the pairs of microtubules we have this protein called Nexen it helps to keep the microtubules in their place and then coming out of the microtubules we have this protein called dining and dining is a protein that breaks down ETP 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 a 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 of 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 see that is axial plasmic 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