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
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in the comments below.