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.