Hey, check this out! 

[Hank splits into two Hanks]

Ah! 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 out 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 and grow and keep 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? 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. 

(1:07) 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 gonna 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. 

[Intro music]

(1:40) 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 two sets each. And that's what we're focusing on today.

(2:11) 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. 

(2:35) 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. 

(3:02) 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, like, all the stuff we know, it's how we're trying to figure all this stuff out. Consider it job security, if you ever 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 live 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. 

(3:52) 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. 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 [www.youtube.com/watch?v=8kK2zwjRV0M] 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. 

(4:58) Now, while the chromosomes are forming, the nuclear envelope gets out of the way by, like, 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; 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 ropy 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.

(5:52) So these motor proteins attach to the microtubule ropes and basically serve to spool up the tubules' slack. At the same time, another protein, called 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 instalment of Bio-lography. 

[Biolo-graphy music]

So how do 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 were 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 like 130 years in the making. 

(6:43) 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 jungle 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. 

(7:22) Now the most recent contribution to this research was made by a post-doctoral 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 microtubules 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 the 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 microtubules 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. 

(8:12) That discovery was published in February of 2012, a couple of weeks before I sat down on 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 Biolo-graphy about you. 

(8:34) Now so far we've gone through the interphase, where the centrosomes and DNA replicate themselves and get ready for the split; the prophase, where the chromosomes form and the centrosomes start to spread apart; and the 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.
 

Since 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 of the accoutrements 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. 

And 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. 

(9:45) 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, of course, 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 you mom, and half is from your dad. 

(10:12) 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 gonna 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 Twitter, or, of course, in the comments below.