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Studying for a test? Prepare with these 2 lessons on Crash Course: Biology and Ecology.
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Blonde Man: Now, check this out. 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 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 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, damage your liver, you got to replace those cells, mitosis. Tumor growing in your spine? Unfortunately, again, mitosis. Why you go from a 7 pound baby to a 70 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 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. (music) 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 are somatic cells. You have 46 chromosomes grouped into 23 pairs. One in each pair is from your mom and the other ones are 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. 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 that 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 discreet stages called prophase, metaphase, anaphase and telophase. And you could 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 like all the stuff we know. It's how we're trying to figure all this stuff out. Consider 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. All right, 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. 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 that 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 dread locks 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 called 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 like, completely disintegrating and the centrosomes then peel away from the nucleus, start heading to the 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 the 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 could 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 motor proteins attached to the microtubule ropes basically serve to spool up the tubule slack. Now at the same time, another protein called dynein is pulling up the slack from 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 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 is only very recently been figured out. And it was a revelation like 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 cell's 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 on 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. And 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 the 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 the dynein plays tug-of-war with itself. Dynein grabs on to one end of the microtubules and pulls the tubules and chromosomes towards 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 on to 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 in the middle, so that they will be split evenly. That discovery was published in February 2012, a couple of weeks before I sat down 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 biolo-graphy about you. Now so far, we've gone through the interphase where the centrasomes 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 that genetic material to rebuild, so that the copied genetic material has all of the accoutrements of home. And the last phase, telophase, each of the new cell structures are reconstructed first. The nuclear membrane reforms and the nucleoli form within them and the chromosomes relax back into chromatin and 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. We now have two new cells, each with a 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 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 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.