 In sexual reproduction, we have the cells from two different individuals combining together to give us an offspring, which means that the DNA from two different individuals combine to give the DNA of the offspring. Now the big question we want to try and answer in this video is what happens to the DNA number in the offspring? Well, at least from this picture it seems as the offspring gets twice the amount of DNA compared to the parent. But that can't be true, right? If that were to happen, if the offspring kept on getting twice the number of DNA, the DNA number would just keep on exploding, right? That cannot be true. Somehow the DNA number needs to be maintained the same. The offspring needs to have the same number of DNA compared to the parents. So how do we do that? That's the big question we're going to try and answer in this video. Now before we proceed, let's back up a little bit. Why do we even have sexual reproduction in the first place? Well, the whole idea is if you combine DNA from two individuals, then the offspring will get a unique set of variations. And as a result, in this mode, variations can be accumulated much faster compared to asexual reproduction. And variations are great for a species to survive in our ever-changing environment. And of course, you've talked a lot about this in great detail in our previous videos, videos on why sexual reproduction and videos on variation. So if you need more detail on all of that, great idea to go back and check that out. But now let's concentrate more on the DNA number over here. Now what I've drawn over here is not very accurate. So you know what we'll do? We'll go back to the cells of the mother, father and the offspring and let's concentrate first on the DNA of the parents carefully. Let's start with the father. If you think about the cell of the father, the DNA of the cell of the father, then you can say that, you know, the father himself must have received one copy of DNA from his mother, right? One copy of DNA from his mother. It would be your grandmother in that case. And similarly, he must have also received one copy of DNA from his father because even he was born from sexual reproduction, right? And the same thing we can say about your mother as well. The mother must have also got one copy of her DNA from her mother and another copy of a DNA from her father. So from this, what we can see is even your parents must have two copies of DNA, a pair of DNA. And that means if you take any sexually reproducing organism, then all of them should have cells which have a pair of DNA, one that they got from each parent. Does that make sense? So now comes the question, how do we transfer this to the offspring cell without increasing the number? That's where a very special kind of cells come into picture called the germ cells. Germ cells are usually found in the gonads. Gonads are reproductive organs. In males, the gonads would be the testes. And in females, the gonads would be the ovaries. That's where you find these germ cells. But what are they? Well, they are the ones that can produce gametes. Wait, what? I'm just throwing names over here, right? So let me clarify. First, let's imagine that this was not a germ cell. Imagine this was any cell except these germ cells. Then when the cell is ready to divide, it'll basically split into two. And what you will now find is both of these cells will have the same amount of DNA content as the original cell, right? The DNA just gets copied, right? That's what happens in non germ cells, right? Now, in contrast, let's see what happens in germ cells. When a germ cell is ready to divide, it can divide in such a way that the resulting cells will get only half the amount of DNA content. And that's the important thing, okay? So for in this particular example, this particular cell only got his mother's DNA content. And this one got his father's DNA content, just as an example. And similarly over here, when this germ cell wants to divide, it can divide in such a way that again, the resulting cells only get half the amount of DNA content. And these resulting cells which only contain the half DNA content, these are what we call the gametes. And I've written half over here just to say that they contain only half the amount of DNA content over here. You might be familiar with these gametes. The male gametes are what we call as sperms. And the female gametes are what we call as eggs. But we'll come back to that those technical names a little bit later. But one thing to understand over here is I've just drawn an overview. In reality, it turns out that the germ cells, when they divide, they undergo multiple process and they divide into not two but four gametes, okay? But don't worry about the technical details over here. Just look at the overview. The whole idea is germ cells can produce cells called gametes which contain only half the amount of DNA content as the original germ cells. And your textbook, your NCRT textbook has done a mistake over here. They have taken the germ cells to be the same as gametes. They are not the same. Germ cells contain full set of DNA, like any other cells. But it's the germ cells that create gametes which contain half the set of DNA. Remember that. Anyways, now you can see where we're going with this. Now we can make one of the male gametes fuse with one of the female gametes. Let's say these two fuse together and then form a new cell. Now that new cell will contain this half, this half of the DNA and it'll also get this half of the DNA. And as a result, you will see the eventual offspring ends up with a full set of DNA just like the parents. And this is how the number of DNA gets maintained in the offsprings. And so in this whole process, you can see that the DNA content of the offspring is unique. It's not the same as the father or the mother. So it makes that offspring unique, but also maintains the same number as their parents. And of course, now if the parents decide to have another offspring, different gametes might end of fusing. Maybe this one will fuse with this one. And that will produce yet another unique DNA combination. So again, even the siblings will have different DNA content. And that's why usually even they don't look identical, even though they come from the same parents. Of course, identical twins are exceptional cases. We'll talk more about them when we talk about human reproduction later on. And there's another thing which I have oversimplified over here. You see over here I've drawn just one single strand of DNA of the mother and one strand of DNA of his father, right? In reality, that's not true. We don't have just one single strands. We have at least humans, we have 23 such pairs. All right? So if I have to show this, make this a little bit more accurate, we should look at it this way. So I've shown four pairs over here, four pairs of DNA. But in reality, we'll have 23 such pairs of DNA. And of course, the strand is highly coiled and we call them chromosomes now, but don't worry too much about that. So why I'm telling you this is because now we can understand what's really going to happen in gametes. So when the gametes are formed, it's not necessary that each gamete should get all of the content from just one of the parent. So maybe this particular gamete, you know what it can get? Maybe it can get the first from the mother, the second from the father, the third from the father, the fourth from the mother. Maybe the next one gets a little different. It might get the first from the father, the second from the mother, the third from the mother, the fourth from his father, and so on. Now just imagine, which when you have 23 such pairs, how many different varieties of gametes can be formed, right? If you do the math, which I encourage you to try and do that, you will find about 8 million varieties are possible, 8 million. And similarly, over here also, we can have 8 million female gametes varieties are possible. And so if you now consider the fusion of them, you will have 8 times 8, 64 million. Different varieties of offspring can be made. Yeah, let that number sink in. You don't have to remember the numbers of course, but I'm just trying to make you realize how amazing sexual reproduction is at making variants. Anyways, that's not really all that important for us. Let me, before winding up the video, tell you some technical names. So this process where the gametes fuse with each other, we technically call it fertilization, fertilization. And finally, after the fusion, the new cell that is formed, we don't call it as the offspring cell, the first cell, we don't call that that way. Again, there's a technical name for that. We call it the zygote. Zygote. You keep hearing these names over and over again in the future videos. And finally, to give some details on the gametes also turns out the male gametes and the female gametes also share some differences. For example, if you look at the male gametes, it turns out that usually these male gametes are small and they're small because they are motile. If they were bulky, they wouldn't be able to move. So it's usually the male gametes that are motile and they're called the sperms. The male gametes are called the sperms. And similarly, if you look at the female gametes, it turns out that they're big and bulky. So they're just big and they're not motile. And they are called usually called the egg cells, egg cells or just eggs. And another name for them would be ovum. Ovum would be singular. Owa would be plural. And why are they non motile and big? Well, because usually they are the ones that contain the food source. So they contain the food, the other source of food for the zygote initially when the zygote has to grow. Basically, it has to divide and multiply into number of cells as it grows into the new baby eventually, right? So that food is usually initially present in the egg itself. And that's why it's big and non motile. And that means that it's usually the sperms that have to make their way towards the egg to fuse and fertilize it. All right, that's pretty much it. So let's quickly summarize. Sexually reproducing organisms have structures called gonads, which are reproductive organs. In humans, male gonads are called testes and female gonads are ovaries. They contain germ cells. Germ cells are special cells which can divide and form gametes, which have only half the amount of DNA content. That's important. Usually the male gametes are called sperms, which are small and motile. The female gametes are usually called eggs or ovium, which are usually big and non motile. And so for reproductions, the male gametes and the female gametes have to fuse together to form a new cell, which now contains a full set of DNA. And this is how the DNA number gets maintained. And that new cell is now called the zygote. Think of it as the first cell of the baby or the offspring. And as a result of this process, it now contains a very unique set of DNA because of which it might be similar to the parents, but definitely not clones of the parents. And this is how sexual reproduction produces variations in the offsprings and maintains the DNA number.