Mitosis: Splitting Up is Complicated – Crash Course Biology #12


Hey, check this out. Cool, huh? I bet you wish you could do this
have a clone clean up around the apartment for you go to class,
maybe take your Mom to dinner on her birthday? Well, you can’t do that.
And actually there are some really good reasons why you can’t do that. We’re going to talk about
those in the next episode. But, do you know what
CAN clone themselves? Your cells. Like, almost every single
one of them. And in fact they’re doing it right now! For any creature bigger than a
single-celled organism, all of life stems from cells’
ability to reproduce themselves, because that’s what allows organisms to develop, grow, heal and keep from
dying, for as long as possible. This particular kind of cell
division is called mitosis, and it’s responsible for a whole
lot of your body’s key functions. If you get a cut, your body needs
to make new cells. Mitosis. Have too much to drink
and damage your liver? You gotta replace those
cells. Mitosis. Tumor growing in your spine?
Unfortunately, again mitosis. While you go from a seven pound baby
to a seventy pound child it’s not your cells that are
increasing in mass; you’re just getting more of them. Over and over and over again. That’s mitosis. This process is so central to your
life that it will take place in your body, over your lifetime,
about 10 quadrillion times. That’s 10 thousand billion times! Like all split ups, it’s not easy.
It’s going to maybe be a little bit messy,
there’s a lot of drama, and it can take a surprisingly
long amount of time. But trust me, after we’re done
with it we’ll all be better off. So you are made of trillions
of cells just like giraffes and redwood trees. And remember that inside each cell
there’s a nucleus that stores your DNA, which contains
all of the instructions on how to build you. That DNA is organized
into chromosomes and as we’ve mentioned before, in your body cells, or somatic
cells, you have 46 chromosomes grouped into 23 pairs, one in each pair is from your mom,
and the other one’s from your dad. Cells with all 46 chromosomes
are called diploid cells, because they have 2 sets each.
And that’s what we’re focusing on today. You also have haploid cells that
have half as many chromosomes (23). And those are your sex cells.
They’re produced in an equally fantastic process called meiosis,
which we’ll be talking about in the next episode. But for now, the main thing to
remember about mitosis is that it allows one cell with
46 chromosomes to split into two cells that are
genetically identical, each with 46 chromosomes.
All in order to keep the party of life going. Now, the nucleus in your cell
controls everything that goes on in the cell. It has all of the instructions
necessary for making the cell survive so you don’t
need to duplicate the whole cell. All you need to do is duplicate
the DNA, get it wrapped up, and then if you have two separate
pockets of DNA, that’s all you need to have two new cells. Mitosis takes place in a series
of discrete stages called prophase, metaphase,
anaphase, and telophase. And you can just say that
over and over again, and let it sink into your head. And part of what’s really amazing
about this whole process is that, while we know what these stages are,
we don’t always know the underlying mechanisms that make
all of them happen. And this is part of science.
Science isn’t all the stuff we know, it’s how we’re trying to
figure all this stuff out. Consider it job security
if you want to be a biologist; there is a lot of stuff that future
biologists have to still figure out, and this is one of them. Alright, let’s get our clone on. So, most of their lives,
cells hang out in this limbo period called interphase,
which means they’re in between episodes of mitosis, mostly growing
and working and doing all the stuff that makes them useful to us.
During interphase, the long strings of DNA are loosely coiled and messy,
like that dust bunny of dog fur and laundry lint under your bed. That mess of DNA
is called chromatin. But as the mitosis process
begins to gear up, lots of things start
happening in the cell to get ready for the big division.
One of the more important things that happens is that this little set of protein cylinders next to the nucleus,
called the centrosome, duplicates itself. duplicates itself. We’re going to have to move a lot
of stuff around in the nucleus and that’s going to be regulated
by these centrosomes. The other thing that happens
is all of the DNA begins to replicate itself too, giving the cell two copies
of every strand of DNA. To brush up on how DNA
replicates itself like this, check out this episode
and then come on back. Now the cell enters the first phase,
or the prophase, when that mess of chromatin condenses and
coils up on itself to produce thick strands of DNA wrapped around proteins – those
my friends, are your chromosomes. Instead of dust bunnies,
the DNA is starting to look a little bit more like dreadlocks. And the duplicates that have been
made don’t just float around freely; they stay attached to the original,
and together they look like little X’s; these are called the chromatids
and one copy is the left leg and arm of the X, and the other
copy is the right leg and arm. Where they meet in the
middle is the centromere. Just so you know, these X’s are
also called chromosomes sometimes double chromosomes,
or double-stranded chromosomes. And when the chromatids separate,
they’re considered individual chromosomes too. Now, while the chromosomes are
forming, the nuclear envelope gets out of the way by
completely disintegrating. And the centrosomes then peel away
from the nucleus, and start heading to opposite ends of the cell.
As they go, they leave behind a wide trail of protein ropes
called microtubules running from one centrosome to the other. You might recall from our
anatomy of the animal cell that microtubules help provide
a kind of structure to the cell; and this is exactly what
they’re doing here. Now we reach the metaphase,
which literally means “after phase” and it’s the longest
phase of mitosis.; It can take up to 20 minutes. During the metaphase,
the chromosomes attach to those ropey microtubules
right in the middle, at their centromeres. The chromosomes then begin to be
moved around, and this seems to be being done by molecules
called motor proteins. And while we don’t know too much
about how these motors work, we do know, for instance,
that there are two of them on each side of the centromere. These are called
Centromere-associated protein E. So, these motors proteins attach
to the microtubule ropes and basically serve to spool up the
tubules’ slack. At the same time, another protein, dynein, is pulling
up the slack from the other ends of the ropes near the cell membranes.
After being pulled in this direction and that, the
chromosomes line up, right down the middle of the cell. And that brings us to the latest
installment of Bio-lography. So how do those chromosomes
line up like that? We know that there are motor proteins involved
but like, how? What are they doing?
Well, remember when I said earlier that there are a lot of things
that we don’t totally understand about mitosis? It’s sort of weird
that we don’t, because we can literally watch mitosis happening
under microscopes, but chromosome alignment is a good example of a
small detail that has only very recently been figured out,
and it was a revelation about 130 years in the making. Mitosis was first observed by a
German biologist by the name of Walther Flemming, who in 1878
was studying the tissue of salamander gills and fins when
he saw cells’ nuclei split in two and migrate away from each other
to form two new cells. He called this process mitosis,
after the Greek word for thread, because of the messy jumble of
chromatin, a term he also coined, that he saw in the nuclei. But Flemming didn’t pick up on
the implications of this discovery for genetics, which was still a
young discipline. And over the next century, generations of
scientists started piecing together the mitosis puzzle,
by determining the role of microtubules, say, or identifying
motor proteins. Now, the most recent contribution
to this research was made by a postdoctoral student named
Tomomi Kiyomitsu at MIT. He watched the same process that
Flemming watched, and figured out how at least one of the motor
proteins helps snap the chromosomes into line. He was studying a motor protein
called dynein, which sits on the inside of the membrane. Think of the microtubles as
tug-of-war ropes, with the chromosomes as the flag
in the middle. What Kiyomitsu discovered was that
dynein plays tug of war with itself. Dynein grabs onto one end of the
microtubules and pulls the tubules and chromosomes toward one
end of the cell. When the ends of microtubules
come too close to the cell membrane, they release
a chemical signal that punts the dynein to the other side
of the cell. There, it grabs onto the other end of the
microtubles and starts pulling, until SMACK it gets
punted back again. All of this ensures that the
chromosomes will line up exactly in the middle,
so that they will be split evenly. That discovery was published in
February 2012, a couple of weeks before I sat in this chair, and
134 years after mitosis was first observed. If you want to join the ranks
of scientists who are answering the many questions left
about mitosis, and lots of other things about our lives
maybe someday I’ll do a Bio-lography about you. Now so far we’ve gone through
the interphase, when the centrosomes and DNA replicate
themselves and get ready for the split; the prophase, when the chromosomes
form and the centrosomes start to spread apart; and metaphase, where the
chromosomes align in the middle of the cell. And now it’s time to separate
the chromosomes from their copies. This time, motor proteins start
pulling so hard on the ropes that the X-shaped chromosomes
split back into their individual, single chromosomes. Once they’re
detached from each other, they’re dragged toward either
end of the cell. The prefix ‘ana’ means ‘back’
that may help you remember the name of this phase,
called anaphase. After this, it’s just a matter
of using all of that genetic material to rebuild, so that the
copied genetic material has all the accouterments of home. In the last phase, telophase,
each of the new cell’s structures are reconstructed. First, the nuclear membrane re-forms,
and nucleoli form within them. And the chromosomes relax
back into chromatin. Then a little crease forms
between the two new cells, which marks the beginning of
the final split. That division between the two new cells
is called cleavage. All that’s left is to
make a clean break. This is done by cytokinesis
literally “cell movement” by which the two new nuclei move
apart from each other, and the cells separate. We now have two new cells, each
with the full set of 46 chromosomes. These clones are called the
daughter cells of the original cell,
and like identical twins they are genetic copies of each other
and also of their parent. But, that’s obviously not
the case for you. Even if you are an identical twin. Shout-out to identical twins! See me in the comments. while you kind of are a clone
of your sibling you are not a clone of your parents. Instead, half of your DNA in each
of your cells is from your mom, and half is from your dad. To understand why that is,
we have to understand how eggs and sperm are formed. And that
is meiosis, and that’s what we’re going to be talking about
next week on Crash Course. Until then, you can just watch
this video over and over again or you can just watch the bits
that you want to re-watch using our table of contents,
which is also available in the description for people
who are using iPhones and can’t click annotations If you have any questions,
you can reach us on Facebook or on Twiiter
or of course, in the comments below.

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