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Zygote differentiating into somatic and germ cells

Learn how a zygote, the single cell produced by fertilization, divides by mitosis to produce all the tissues of the human body (including germ cells, which can undergo meiosis to make sperm and eggs). Created by Sal Khan.

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Video transcript

Voiceover: In the last video, we saw an egg from our mothers fuse with a sperm from our fathers to form a zygote that would keep replicating and turn into us, if this... Let's say this was the sperm from my father that fuses with the egg from my mother, and then this zygote will keep replicating and eventually turn into Sal, and make a video about fertilization. So how does that actually happen? So now that this zygote has the diploid number of chromosomes, and once again they're referring to that as 2N, where N would be the haploid number, 2N would be the diploid number, and in the case of human beings, N is 23, so in the case of human beings, N is 23 and two times N of course would be 46 chromosomes. I have my full contingency of chromosomes here. I got a Y sex chromosome from my father, X of course from my mother, so I'm going to be a male. And so what then happens? Well now, through mitosis, this zygote is going to keep replicating. So it will, you know, after one, so after one, and we're going to go into the details of the mechanics of mitosis, but after one round of mitosis, it is now two cells. It is now two cells. And I'm going to draw it, once again, I'm not going to draw it at scale. It's now two cells. I want to make sure I have enough space on my little chalkboard here. It has two cells. Instead of drawing all of the chromosomes, let me just say that each of these, in my nucleus, I still have 2N. I have the diploid number. So each of these two cells that it has differentiated to still have the full contingency. That's what mitosis does. It essentially replicates the entire cells. You have the same number of chromosomes. And then this process is just going to keep happening. These two characters are going to replicate, are going to replicate, and so then you're going to have, through mitosis, and now you're going... So this is another round of mitosis right over here. Mitsosis. So they just keep duplicating themselves. And each of these cells have the full contingency. 2N, the diploid number of chromosomes for, well, in this case it's going to be 46 for a human being. And then this process is just going to keep happening. So this process is going to keep happening. I'll do dot dot dot to show that, you know, a lot of this has been going on. So mitosis is just going to keep happening. And so eventually you're going to have thousands of these cells, and eventually as we'll see, you're going to have millions and ten millions of them. So let me draw them really really really small. There's a bunch of them there. And each of them, each of them are going to have the diploid number of chromosomes. They're going to have 46 chromosomes. 23 pair of homologous chromosomes. So we now have a big ball of these here. And these cells, some of them are going to, they're going to differentiate into me. They're going to differentiate into the different parts of my body. So for example, these cells right over here might eventually, they'll keep replicating, but then it's them and their offspring might eventually differentiate into my brain cells. These cells here will keep replicating, and them and their offspring, I guess you could say, or the things that they replicate into, might differentiate into my heart. These right over here might differentiate into my lungs, and of course all of these eventually will differentiate into all the different, and they and their offspring will differentiate into all of the things that make me me. And so you have a lot more of this mitosis. You're eventually going to have a human being. So let me just say this is more mitosis going on. Mitosis. And now let me make an attempt to draw a human being. That doesn't really look like me, but... Well, I have a lot of hair, so that's my... I have big eyebrows as well. Still doesn't look like me, but anyway. You get the point. And so, and I'll try to draw fairly quickly. But this is obviously not my best rendering of a human being. But you get the general idea. Nice broad shoulders, that's nice. All right, so let me, let me... I'm focusing too much on drawing this human being. Anyway, you get the general idea. My stomach isn't quite that flat, but anyway, you get the general idea. So this, it'll eventually differentiate into a human being. These cells here in pink eventually differentiate into the cells in the brain. These cells here eventually differentiate into the cells into the lungs, and obviously at this scale, the cells are way too small to even see. These cells differentiate into the cells of the heart. Now, I want to draw an important distinction here. Because most of the cells that I've just depicted here that are just a product of mitosis, these are your, I guess you could say these are your body cells, or these are your somatic cells. So these, all of these cells that I'm pointing out in your heart, your lung, your brain, these are somatic cells, or body cells. Somatic cells. And so you're probably wondering, well how do I eventually get these haploid number cells? How do I eventually get, if I'm talking about a male, how do I eventually get these haploid sex cells, these gametes, these sperm cells? I'm talking about a female, how do I eventually get these ova, these egg cells that have a haploid number? And the way that happens is some of these cells up here are going to differentiate into germ cells. So they're going to differentiate into germ cells. In the case of, and they're going to differentiate when I say into germ cells, they're going to differentiate into your gonads. In the case of a female, the gonads are the ovaries. And in the case of a male, the gonads are the testes. The gonads are the testes. And the germ cells in the gonads or the cells that have differentiated into being part of the testes and ovaries, those germ cells. So we differentiate them from somatic cells. So there are germ cells there. Germ cells in your ovaries and testes. They, through the process of meiosis, they through the process of meiosis, can produce the gametes. So if you're female you're going to produce eggs. If you're male you're going to produce sperm. But this is through the process of meiosis. Meiosis you're going to produce sperm in the case of a man, and you're going to produce ova in the case of a female. And this brings up a really interesting thing, because throughout biology we talk about mutations and natural selection and whatever else. And it's important to realize how mutations may affect you and your offspring. So if you have a mutation in one of the somatic cells here, let's say in a skin cell, or in you brain, or in the heart, that may affect your ability to, you know, especially if God forbid it's a really dangerous thing like cancer, and it happens when you're young, before you've had a chance to reproduce and you're not able to survive, that might affect your ability to reproduce. But if this is happening in a somatic cell, it's not going to affect the DNA make-up of what you pass on. The DNA make-up of what you pass on, that's determined by what's going on in the gametes. So a mutation, if on the way to differentiating into gametes a mutation happens, so if one of these mutate and then keep replicating, so let's say there's a mutation here, and they keep replicating and they differentiate into the germ cells the mutation is right over there, then through meiosis that produces some mutated sperm. Then that would pass on to your, well, that has a chance of passing on to your children. Because once again, it might not be all of the sperm cells that have that mutation. It could be only a handful of the two to three hundred million of the sperm cells, and so if that mutation somehow makes it harder for some of the sperm cells to kind of function, either fuse with an egg or even potentailly develop and kind of swim through fluid, then it still might not be the thing that makes it. So mutations only affect your offspring in the situation where those, the cells in which they happen are eventually differentiated into things, into gametes that you will pass on to your children.