Microtubules | Cells | MCAT | Khan Academy
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Microtubules | Cells | MCAT | Khan Academy

– 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.

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75 thoughts on “Microtubules | Cells | MCAT | Khan Academy

  1. How did the inner pull microtubual become shorter when they pulled the chromosome apart? Did they take off the middle part or the end parts?

  2. How do u spell dionene correct way and what would happen of some consumes more then that is required by the body and where is it devised from plants or animal what is the structure of it compound element ?
    What would happen if tubular over grow do that me brain mass grows

  3. Very well presented. It is very helpful when so many details are presented in a way that is easy to retain. Thank you for a great video lesson.

  4. Correct me if I'm wrong, but aren't interpolar microtubules the ones that do not attache to kinetochores, but go from one MTOC directly to the other?

  5. URGENCY QUESTION: in the fungi, how chromosoms move in their nucleuses?
    does the strips of duke transformed from cytoplasm to nucleus?

  6. How do most cells transport vesicles? Just in the cytosol, no railroads? Also what holds the organelles of a cell (nucleus ER, etc) in place? Is that microtubules?

  7. There is a mistake in your video. Interpolar microtubules are antiparallel microtubules that originate from opposite poles, and overlap at the midzone of mitotic spindles. They don't attach to kinetochores. The ones you showed are kinetochore microtubules. Would be good to make it clear that there 3 types of microtubules taking part during mitosis and they all have different functions.

  8. Reminded me of school again. Lots of labelling of parts less actual explaining of how stuff actually works. Are those tubules in the neuron as she calls them "tracks" transporting electrical signals? Or something else.

  9. Both the mitotic spindle and the interphase cytoskeleton are formed from rapidly tuning-over microtubule populations with half lives of less than a few minutes, which grow from and shrink towards the microtubule organizing centres.

  10. Microtubule is a much more complex molecule. The building block of this protein is a dimer called tubulin, which is composed of two sub units: α-tubulin and β-tubulin. α-tubulin and β- tubulin form a filamentous chain called “protofilament”. Microtubules are built by arranging 13 such protofilaments around an empty core. This gives rise to a tube-like construction (hence the name microtubule), which is stiffer, longer and wider than actin. Microtubules have a distinct organizing site called the “centrosome”. Microtubule polymerization begins at this organelle. The end where faster polymerization occurs is called the plus terminus. The end where slower polymerization takes place is called the minus end. Microtubules grow from the centrosome towards the membrane, by anchoring their minus end to the organelle. Once microtubules reach the membrane they detach from the centrosome and create a highly dynamic network. The formation of this network is assisted by a group of proteins with microtubule binding domains called Microtubule Associated Proteins (MAP).

    -Creative BioMart

  11. does this come up on the grade 12 course or biology 30? im in process of needed to redo mine dont wanna waste my time learning it, if i dont need to

  12. 9:19 I think there is something incorrect, in the 9+2 arrangement of the basal body the central microtubules are NOT paired as the other ones (1 complete and 1 incomplete microtubule, who attach each other). Indeed, the central "pair" is more like TWO SEPERATE and both complete microtubules, just connected by a bridge and not direcly attaching, as your drawing suggests. Pls correct me if I'm wrong.

  13. Proof for Hollow gram theory. You can actually try this yourself. Find flash light with strobe light. Close your eyes. The electron waves will strike micro tubular structures in the eye. Your consciousness will collapsing the light wave. You will see the information With your on eyes. Try it! Best wishes Graphite God

  14. Tnx for the teaching but signals did not go back they go forward from d dendrite to the axon terminal and then d axon terminal takes received signal to another neuron and so on continuously,,,,tnx again ma'am

  15. "Soul/Consciousness/Fluctlight/Quantum Fluctuation of Photon" runs inside these Microtubules ? Just like binary states inside transistors !

  16. There is a mistake in your video. The 'interpolar' microtubules in the video are actually kinetochore microtubules. The interpolar microtubules extend from opposite poles and meet each other somewhere around the center where they overlap and associate with motor proteins.

  17. Let me clear,, During synthesis phase(a phase of interphases) do the whole centrosome become double or only centriole become double?

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