So this module deals with the cytoskeleton and what
you are now looking at is a cell that has been stained
with Coomassie Blue. It's a fibroblast from skin.
Coomassie Blue is a general stain for proteins. You may
have already been familiar with electrophoretic gels of
proteins that have been stained with Coomassie Blue.
Well, here the whole cell can pick up Coomasie Blue
wherever there is protein, and in this case what you're
looking at is a meshwork of insoluble cytoskeletal rods
and filaments that are picking up the stain.
And what you see is a network of fibers, giving you
the impression that the cell is not just a bag of liquid,
but is actually a meshwork...
(think of them as intracellular bones) - that's why we
call it a cytoskeleton.
The cytoskeleton gives cells shape and also allows cells
themselves to move, or components within cells
(organelles, vesicles and the like) to move as well.
and we'll see specific examples of all of this shortly.
Let's look at the structure, polarity and the assembly of
cytoskeletal elements. Here we have mictrotubules.
Microtubules are built from tubulin monomers and we
see here that one of the monomers, the beta tubulin monomer
actually binds GTP (that's the red circle., the red ball if you will
So a heterodimer forms when an alpha and a
GTP-bound beta tubulin monomer come together, and
and microtubules are formed by the aggregation of
these heterodimers containing GTP, at least to start with.
Microtubules have polarity, a plus end and a minus end,
The plus end is the end at which the heterodimers
come together to make a microtubule. And the minus
end is where the microtubule might come apart, where
the heterodimers come off the microtubule. You'll
notice that there is a difference in color between the
heterodimers that are adding to the microtubule and
those that are coming off. That's because that soon
after you establish several rows of these heterodimers
in a microtubule, the GTP is hydrolyzed. And so in fact
most of the length of the microtubule consists of
alpha GDP/beta tubulin heterodimers.
I don't have very many of them showing here, but by far
the longest component of a microtubule is bound with
GDP, not GTP.
Here is an electron micrograph of a microtubule.
And when you take the measurements, the diameter of
this cylinder (this tubule) is 25 nm, making it the
largest of the 3 principal cytoskeletal components.
Now we're going to be talking about stable and unstable
microtubules and we need to just say here if
microtubules in a structure are stable, they grow
to a certain length and then they stay at that length and
their minus end is basically inactive. There's no
disassembly going on.  But we'll also see examples of
dynamic microtubules which first perhaps form by
growing faster than they come apart, but then later
the process reverses itself and they come apart faster
than they form. That's an example of a dynamic
microtubule, and we're going to see that soon.
Microfilaments are composed of actin monomers
called G-actin, meaning globular actin (remember that
polypeptides can be globular or fibrous). Well, G-actin is
globular actin. These are the monomers.They can be
radioactively labeled for an experiment of the sort that
I'll show you in a bit. And you can show that there's an
assembly end. They add at one end and come off at the
other end much like microtubules. So what is an actin
filament. It's called F-actin.
It is actually two intertwined polymers of G-actin. They
are 7nm in diameter. Here's an electron micrograph
(a very nice one) showing this intertwined pair
of F-actin polymers in a kind of a helical configuration.
And the diameter of that pair of polymers is 7 nm, and
that's the narrowest or smallest diameter structure in the cytoskeleton.
I won't have time to explain more about the pulse-chase
experiment with microfilaments, but I will do so for
microtubules.  Finally the third major component of the
cytoskeleton are intermediate filaments and they're
simply formed from monomers that are pretty well
extended. They are not globular. They have mostly
secondary structure, these polypeptides. And they
come together first to form dimers, and the dimers
come together to form tetramers, and then the
tetramers assemble into somewhat larger structures of
of about 10 nm in diameter, making them intermediate
between 25 and 7. And these are very strong rope-like
bundles that provide a lot of strength to cells and we've
already seen some of that and we'll see it a bit more.
Here is an electron micrograph of intermediate
filaments, or bundles of intermediate filaments at
10 nm in diameter. We can localize these 3 major kinds
of filaments, and you will see that they're localized
differently in cells.  So let's look at microtubules.
In all four of these cells, if you look closely you will see
that the microtubules (almost not visible, but the yellow structures)
seem to emanate from something around the nucleus
(or at the nucleus) and that is very small, but it's the
pair of centrioles that characterize animal cells
And, the microtubules - you may remember centrioles
are made up of microtubules - so that they are the focal
points for assembling microtubules in animal cells.
And so you see that in two out of the three cases, very
clearly, the microtubules are radiating out from the centriole.
They do so in the neuroin as well, but it's a special case
where microtubules also lie parallel to the long axis of
axons in neurons. In pigment cells and in neurons, you
have a very dramatic example of the role of microtubules
in moving vesicles around. And you'll see in a little bit
that motor proteins use microtubules as tracks and they
essentially 'walk' along these tracks and they can carry
different vesicles. In the case of a pigment cell in the
skin of a chamelion, when the chamelion darkens
because it's against a dark background, pigments that
are otherwise concentrated in the middle of the cell
move out and spread and darken the cell. And that's
how a chamelion can change color. It responds neurally
and the pigment cells then move their vesicles
containing melanin and other pigments outward in this
fashion so that the pigments radiate throughout the cell
and darken the cell. In the case of the neuron, the
vesicles that are moving from the cell body at the upper
left down to the nerve ending at the lower right are
of course the vesicles containing neurotransmitters.
Neurotransmitters are then synthesized in the cell body,
and conveyed along microtubule tracks by motor
proteins to the nerve ending where they're going to
sit around and wait for a nerve impulse to cause the
vesicles to fuse with the nerve ending and release their
neurotransmitter to affect eitehr another nerve or
a muscle cell.
Here is a fluorescent micrograph in which fluorescent
antibodies to microtubules are added to a cell
(and actually a living cell) where it actually picks up the
location of the microtubules. The location in the cartoon
is in fact taken from images like this.  This is a partial
picture so it's not really showing that the highest
concentration of microtubules ,or fluorescence in this case,
is at a single point around the nucleus which is down
at the lower right of the picture. And that single point is
actually the centriole.
Here are cells showing where the microfilaments are.
Here's our epitelial cell for example. In the columnar cell,
The microfilaments are organized roughly around the
cell in what's called the cell cortex, which is the
cytoplasm immediately below the cell membrane. If this
is a cell lining your small intestine, then the structure
you're looking at at the top of the cell of course are
microvilli. And the actin filaments not only are in the
cortex of the cell as a whole, but also penetrate the
microvilli.
Here's our neuron again. The shape of the cell,
especially the structure of the axon, is in part due to the
alignment of actin filaments also along the long axis.
But this doesn't seem to be involved too much in
mobility of vesicles - that's a function of the microtubules.
To the right of the neuron are an attached and an
unattached fibroblast. Now microfilaments have a very
different organization depending on whether the cell is
attached to a surface (called a substratum) or whether
the cell is suspended in medium. An unattached
fibroblast takes on an oval or spherical shape and like
the columnar cell, the actin is organized largely in the
cortex just under the cell membrane to form a cortical
ring of actin filaments or microfilaments which allow
the cell to have this sort of rounded shape. When a cell
like this attaches to a surface however, one of the first
things it's going to start to do is to flatten out and then
start to move. That will require a re-organization of
the microfilaments, and that's what you see looking so
different in the attached fibroblast.
In the picture, by the way, of the microtubules,
I neglected to point out that whether a fibroblast for
example is attached or unattached, the distribution of
microtubules is going to be pretty much just the same.
So it's the microfilaments that re-arrange and allow
cells to change shape readily when they are attached
versus when they are unattached.  And here is a
fluorescence micrograph using fluorescent antibodies
against actin and localizing then, actin bundles in cells.
Look at the attached fibroblast in the cartoon and look
at the fluorescence micrograph and you get a sense of
these criss-crossing fibers that look like they are
penetrating the processes that this cell is extending.
Those are the pointy parts, right, the pointy parts, the
processes being extended by the cell. You see that in
the picture as well as in the cartoon.
Finally, intermediate filaments...
We've already seen that intermediate filaments are often
associated with cell junctions, making the junctions very
tight and firm. This would be for example either a spot
desmosome or a belt desmosome that we described in
another module. And so intermediate filaments have a
function in strengthening cell attachments. Intermediate
filaments are found throughout a neuron, but also along
the long axis of the axon, conferring stability to this
long, extended shape of the axon. And here we have a
fibroblast which is basically a mesh-work
of intermediate filaments that surround the nucleus,
and then penetrate all over the cell in all directions.
And that's indeed what you see if you use fluorescent
antibodies to intermediate filaments to localize them,
to localize intermediate filaments in a cell.
Let's take a look at a single microtubule.  You'll see that
there are 2 different colors here because the cartoon of
of a microtubule on the left is blue - it comes from a
different textbook. I wanted to show you how
microtubules assemble. The alpha-beta heterodimers
will add to each other to form a flat sheet of
microtubules, called a protofilament. When it gets long
enough and wide enough, that sheet will curl. And that's
what you're seeing here looking at the blue microtubule
image. And eventually a seam will form and you will get
an actual tubule, and that's now shown on the right.
And the alpha-beta heterodimers that aggregate to form
the protofilament, continue to add to a fully formed
microtubule, and they add of course to the 'plus end' to
grow the microtubule. If you look in cross-section at a
microtubule (and that's shown in the cartoon as item B),
but also shown as an electron micrograph as item D,
you count 13 tubulin subunits in that cross-section.
And indeed, all microtubules in eukaryotes are a ring of
13 tubulin subunits. And again, you see this longitudinal
section that's also going to be 25 nm in diameter.
So here we see the alpha-beta heterodimers continuing
to add even after the microtubule has fully formed by
forming a seam in a curling protofilament. Even after
that, the microtubule can continue to grow longer by
the addition of more alpha-beta subunits.
Remember that these alpha-beta subunits are adding
as alpha-GTP-beta subunits. And you'll see in a moment
in another image, that this growth by addition to the
plus end eventually results in GTP hydrolysis.
So let's take a look. Here we have an experiment
in which the alpha-beta tubulins were made radioactive.
That's the black and white balls, to just simply indicate
alpha-GTP-bound beta tubulin heterodimers. You see in
this partially formed microtubule the alpha-GTP-beta
tubulin heterodimers that are already there, and
therefore not radioactive, and behind them you see the
alpha-GDP subumit bound to beta to make the
heterodimer, that has been part of the microtubule for a
period of time. So the experiment then is to add
radioactive alpha-beta heterodimers bound to GTP
(that should be a black ball, not a red ball in that sentence up there)
and to add them to isolated microtubules for a very, very
short time. After that short pulse of labeling, allowing
the microtubules to grow a little bit using some of the
radioactive heterodimers, the sample is centrifuged.
The microtubules are brought down to the bottom of
the tube and the supernatant is thrown away (meaning
you're throwing away any remaining radioactive
alpha-beta heterodimers. And to the sedimented
microtubules in the pellet, you add fresh solution
containing non-radioactive alpha-beta heterodimers.
And during the time that the non-radioactive
heterodimers are present (that's called the 'chase'),
the microtubule will continue to grow. But now...
when they grow during the chase period, they are NOT
adding radioactive heterodimers; they are adding
non-radioactive heterodimers.  If you sample
some of the microtubules at different times of the
chase, you can make autoradiographs and you would
be able to see a set of autoradiographs like those
represented here. Right after the pulse, you might see
microtubules with radioactivity, that is dark silver grains
on the autoradiograph, at one end of the microtubule.
And most of the microtubules will be labeled at an end.
We'll talk about what that means in a second.
If you take some of the microtubules out a little later
during the chase period, and that would be the second
autoradiograph from the top, you would find that the
silver grains are now no longer at the very end of the
microtubule, but are somewhere within the microtubule.
If you wait a little longer, you will find that most of the
microtubules that you can sample by autoradiography
like this might show radioactivity somewhere near the
center of the microtubule. And if you wait still longer
during the chase, you will find that the radioactivity is
now once again near an end of the microtubule. If you
think about for a little while  (remember that what
you're looking at is many autoradiographs of
microtubules at each of the different times during the
chase. So what this is interpreted to mean is shown in
the cartoon. The alpha-beta heterodimers are going to
to be adding to one end and coming off the other.
So let's see what happens. We have the assembly end
shown here (the plus end). We have the radioactive
heterodimers (with the black balls) now adding, and if
follow them, we interpret the autoradiographs with this
cartoon. If you wait long enough after the chase, you'll
find that the moicrotubules are no longer radioactive
at all, and they don;'t become radioactive again
because what's coming off at the minus end (at the
disassembly end) might be radioactive, but they are
GDP-bound, and GDP-bound heterodimers cannot
participate in adding to the plus end.
Microtubule-based motility is based on molecular
motors (that I mentioned earlier) that walk along
microtubules and in some cases is predicated on
the very dynamic nature of microtubules. In fact their
instability. And were going to look at examples.
Let's talk first about motor proteins. Motor proteins are
gonna require free energy, so they're all in fact ATP-
fueled. This is an example of a protein that has been
studied. It has multiple domains and it's made up of
several polypeptides. This is dynein. And we'll see that
dynein functions for example in cilia and flagella
of eukaryotic cells, but it also functions as a molecular
motor, carrying vesicles in a neuron.
And we'll see that in more detail in just a bit.
So I said that molecular motors typically move vesicles.
Here we have the interior of a cell that has been dually
immunostained. That is, there are two fluorescent
antibody preparations: one against lysosomal proteins,
and another prep that's a different color fluorescence
against microtubules. And what you should see here
is... the green is a fluorescent antibody against lysosomal proteins...
so wherever you see green, that's one or more lysosmes
and you can see that wherever you see green, they are
associated with (attached to) the red stuff, which is the microtubules.
You do not see lysosomes sitting out in the middle of
the black spaces because they are not really free in the
cytoplasm of the cell. They are attached to microtubules.
As a part of the cytoskeleton, microtubules (and you'll
also see this is true of actin and intermediate filaments)
not only give a cell shape, but they function as a kind of
scaffold on which various cellular structures and
organelles are hung. Dynein is one of these motor proteins;
kinesin is another. And we know that one of the key
differences between dynein and kinesin are that they
carry vesicles in opposite directions.  This is useful
by the way, in a nerve cell because if you want to carry
neurotransmitters from the cell body to the nerve
terminus, use kinesin because it's going to carry
vesicles to the plus end of these microtubules at the
nerve ending. And if you have to carry empty vesicles
back in order to pick up more neurotransmitter, then
the motor protein that is used is dynein.
There are other proteins that are associated with these
complexes as well. But the main take-home message
here is that dynein carries vessels back to the cell body
where they can be recycled or refilled with a neurotransmitter, and
kinesin carries the neurotransmitter-filled vesicle in the
other direction to the axon terminus, or nerve terminus.
Now what's shown here for a nerve cell is also true of
other cells. In the case for example of a pigment cell,
the pigments will be carried by dynein in one direction,
and kinesin in the other, depending on whether the cell
has to darken or lighten. Dynein is also attached
to microtubules in cilia and flagella and also in spindle
fibers of mitosing cells, where they allow one
microtubule (or one microtubule complex) to walk along
(if you will, slide along) the other. So this now is a
cross-section of a cilium or a flagellum. It can be either
one, because in fact in eukaryotes, cilia and flagella show
the same cross-sectional structure of microtubules.
Here is our cross-section of a single microtubule
(remember, with its 13 tubulin monomers as an inset)
showing that in the cross-section of a cilium or
flagellum, there are many microtubules, one of which is
just highlighted here.
Now, taking a cartoon from your textbook, (which is
essentially a cartoon of what we just saw in the high
resolution electron microscope).. and this structure has
many parts.  Let's go back one. So I want to show you
that there are doublet microtubules, and if you
count them there are 9 of them in the circumference of
this cilium or flagellum. There are two single
microtubules in the middle. And then there's all this
fuzzy stuff. These fuzzy things have different structures,
and I can illustrate them better looking at the cartoon.
So here are all the parts that we would recognize.
The doublet microtubules are connected to one another
by these blue structures called nexin. The doublets also
have motor proteins, dynein... in fact 2 dynein
molecules on each of the doublets. We talk about an
outer and an inner dynein arm. These are the motor
proteins that can extend from one doublet to the next
allowing one doublet to move along the other.
And again we're going to see that in more detail in the
context of an experiment that led us to a sliding
microtubule model for the bending of a cilium or a
flagellum. So it's the dynein arms (outer and inner) that
are gonna allow cilia or flagella to bend. There are other
structures here that you can see on your own.
When you look at the electron micrograph, it looks like
each doublet is at the end of a radial spoke that is
that is projecting from the center of the cilium or
the flagellum. Surrounding the central pair of
microtubules, or single microtubules, is a gray
substance which is sometimes called the central, or
inner sheath. And this entire structure as you can see, is
also surrounded by the plasma membrane. So a cilium
or a flagellum is actually an extension of the cell,
complete with a membrane surrounding this
complex-looking structure, which by the way is called
"9-plus 2" (9+2), meaning 9 double microtubules plus
2 single ones, the 9+2 microtubule array characteristic
of eukaryotic cilia and flagella.
Well, you can dissect a cilium or a flagellum and analyze
the structures inside. So here we have a sperm, with
various parts shown. But we're gonna concentrate on
the flagellum, which you can actually 'pop' off the sperm
using a high-speed blender. Now what you have is a
membrane-bound 9+2 array of microtubules, and this
isolated flagellum will actually beat just like a real
flagellum, if you add ATP.
So it is a kind of model for the intact sperm in terms of
the movement of the sperm tail, right?
Now, if you treat with some detergents that will disrupt
the phospholipids of the membrane, you can actually
strip the membrane off.  If you want to then centrifuge,
you can collect the structures you have left behind,
the axoneme, which is going to move to the pellet, and
then you can throw away the supernatant which has all
the phospholipids and other membrane components.
And then you can re-suspend this structure called the
axoneme, and look at it in the electron microscope.
The axoneme is the inner microtubular component of
a flagellum or a cilium.
Now if you add ATP, it is hydrolyzed and the free energy
does in fact enable this axoneme to beat. Now without
the membrane, that beat is a little jerky and not quite as
smooth as it would be in the flagellum itself, but it
definitely is whipping around.
Now, you can use different detergents and different
chemical treatments to break the axoneme apart or to
remove components of the axoneme. And you can do
this...  right....you will actually separate the microtubules
in the axoneme into single and double microtubules,
or doublet microtubules. So here we have axoneme and
an appropriate detergent and we've broken it apart.
And what do you see here? You see the single
microtubule that would have been derived from the
middle component of an axoneme, and you see
the doublets. And illustrated on these doublets are
the dynein arms as well. If you dialyze this preparation
to remove the detergents and chemicals which had
disrupted the microtubule structure of the axoneme,
the microtubules will re-aggregate, not into an axoneme,
but into a sheet of doublets, shown here.
The single microtubules don't have the capacity to
associate with one another because they have nothing
that allows them to bind. On the other hand the doublets
with their dynein arms (which account for sliding of one
doublet against another) would have to bind the
doublets to one another, at least at some point. So you
can explain the production of this sheet by removing
the detergent. Now if you add ATP to this stuff, they
come apart again. Let's take a look at how they
come apart. We know that they come apart because
when you add ATP, the ATP will be hydrolyzed and the
microtubule doublets will actually slide past
one another, like this. And I'll show you that again...
They slide past one another and they come apart.
How do we know this!? If we just added ATP we would
watch it get hydrolyzed and a couple of seconds later
we'd look in the electron microscope and we would see
all these individual doublets, and we would say "Oh! we
added ATP; the microtubule sheet comes apart. But
it comes apart by sliding. We know that- there are
actually electron microscope pictures which I wasn't
able to find in time for this presentation. But I'm
showing you a cartoon that is very much what the
electron micrograph would show. Add ATP and if you
stop the action almost as soon as you add the ATP,
you can actually see microtubules that have walked
along one another but have not yet dissociated.
And so you can imagine that if you allowed that to
continue, the microtubules that are walking along
one another would reach the end of the microtubule that
they are walking on, and would fall off.
And so you would get this tube full of dissociated
individual doublets.  But you can actually see in the
electron microscope a partially disassembled, or dissociated, sheet of
microtubules that you formed in the way I described,
if you stop the action almost immediately after you add
the ATP. This is what led to the 'sliding microtubule'
mechanism to explain how microtubules enable cilia
or flagella to move. So here we have just a pair of
doublets with the dynein arms linking them. And if you
just imagine one of the doublets moving along the other
(movement is restricted e.g., by linking proteins),
if the microtubule on the right is extended upward,
the effect will be to bend the entire structure.
And now you just have to imagine that in the 9+2 array
of microtubules in cilia and flagella, that the beat of a
cilium or of a flagellum is going to be based on
alternately walking the microtubule on one side of
the 9+2, or walking of microtubules on the other side.
Now, the question at the bottom is something for you
to think about.
We use detergents at various steps. In one case, we
use detergents to strip the cell membrane off of the
flagellum or the cilium and reveal the axoneme.
So that axoneme is a flagellum without a membrane.
We then use chemical treatments to cause the axoneme
microtubules to come apart So the real question is
we have introduced some new components here... what
do you think some of these detergents are doing to the
axoneme.  And you can think about that.
You may remember that E. coli bacteria (and many
other bacteria) also have flagella, but these flagella
are not at all related, even evolutionarily,  to those of
eukaryotes. The flagellum of, say E. coli, is a single kind
of protein called flagellin. It's not an array of them.
It's just a bundle of these these proteins attached to
the surface of the cell, in fact to a little motor. And the
And motor is powered by a protein pump.
So here we have a proton pump in the membrane.
We have a concentration gradient of protons that can
be relieved. And as the protons at high concentration
outside the cell are allowed back in, the motor is fueled
and the flagellin structure twirls around back and forth.
So it doesn't actually have a beat. It's more like a
propeller turning around, rather than something that
can bend and unbend.
OK... I said that microtubules can be stable or unstable.
The axoneme of a cilium or flagellum is a
stable structure.
What that means is that during the formation of a
cilium or flagellum, the alpha-beta heterodimers build a
microtubule by adding first to a protofiament, and then
to a microtubule. And they grow until they are the right
length for a flagellum or they'ere the right length for
a cilium. And then they stop.
At that point for as long as the cell is active and healthy,
these 9+2arrays for example, are stable. They don't
assemble and disassemble. And we've just seen that
sliding if these stable microtubules accounts for
bending of a cilium or a flagellum in a eukaryotic cell.
There are other kinds of motility, for example the
movement of chromosomes during mitosis, that
involve a dynamic microtubule, that in fact involve
assembly and disassembly sites. Let's take a look at
the dynamic mitotic spindle. You should recall that the
spindle apparatus is actually made up largely of
microtubules.
If you look, in this cartoon, you will see that there are
microtubules that extend from the centrioles of this
animal cell towards the metaphase plate - you may
remember that's the place around the equator of a cell
where the chromosomes line up just before the
duplicated chromosomes are pulled apart, and the blue
and black and pink structures are these paired
chromatids, which are duplicates of chromosomes
that are formed. And we're going to pull those
chromatids apart to oppposite poles of the cell where
we will now call them chromosomes.
So there are two kinds of microtubules in this structure.
There are microtubules that are attached to the
centromeres of the chromosomes. These are
terminologies that you should look up again if you don't.
remember. They are attached to the centromeres via
proteins that assemble at the centromeres, called the
kinetochore. Again we'll look at that in detail later.
But I'm bringing those terms up to you now so you can
look them up in the textboook if you need to.
The kinetochore is where the microtubules attach to
the centromeres of chromosomes. There are also
microtubules that don't attach to any chromosomes or
chromosomal material, but instead attach to each other.
So we refer to the polar microtubules as those which
are extending from the poles of the cell towards
each other and do not contact chromosomes,
but rather, contact each other. They have dynein arms,
or dynein motors. Those motors will function to push
microtubules apart, which will cause the poles of the
cell to separate, in effect causing what is otherwise a
round cell to become stretched and become ovoid and
then eventually, to separate entirely. The kinetochore
microtubules are those which are actually attached to a
kinetochore at the centromere of chromosomes.
The plus ends of microtubules in a mitotic spindle are
at the kinetochore. That means that's the site of spindle
microtubule assembly, because that's where the
alph-beta tubulin heterodimers are going to add.
So that means the opposite end, at the centriole...
(it's counterintuitive that the opposite end at the centriole)... is the site of disassembly.
And in fact, the disassembly of the microtubules at the
minus end (at the centriolar end) pulls
the chromosomes, or chromatids, apart, and then
eventually pulls the chromosomes to opposite ends of
the cell. Now we know that there is force being
generated on the chromatids, which is going first of all
to separate the chromatids and eventually to pull the
chromatids to opposite sides of the cell. This was a very
dramatic and and very clever experiment. We have a
device, the micro-laser beam, that is capable of being
aimed at a single mitotic spindle fiber, which is really a
bundle of microtubules in this fashion... let's go back
again. We aim it. And what we're going to do is sever
the bundle. We're going to break the microtubule bundle.
One more time, and watch what happens - it happens very fast.
After the is severed you can no longer exert force
equally on both chromatids. So instead, the entire
chromosome with its two chromatids are drawn to the
left side of this dividing cell.
One more time... I want to show you that as that's
happening, the alph-beta heterodimers
(wrong color here! - they're going to be the GDP-bound ones)
are actually coming apart. So the take-home message is
that microtubule disassembly at the minus end is what
actually powers the movement of the chromosomes
from the metaphase plate during anaphase (and
eventually telophase) to that left and then right pole
of the cell.
So dynein motors separate the polar microtubules
at the same time as microtubule disassembly occurs
at the minus end. Let's talk briefly about
two drugs (there are several drugs that can disrupt
microtubule function). Colchicine is one.
Adding colchicine to cells will cause microtubules to
depolymerize. In other words, the microtubules will
in effect dissolve, will disappear.
Taxol, which is isolated from the bark of a particular
tree (and now can be synthesized) blocks de-polymerization.
It blocks the disassembly of microtubules.
So ask yourself what would happen to dividing cells that
have reached metaphase if they were then exposed
either to colchicine or to taxol.
And that brings us to the end of this module.
We focused on microtubules here. Other modules will
focus on microfilaments. Intermediate filaments have
been dealt with in the context of cell-cell junctions
and communication.
