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Video transcript

- [Instructor] Asexual reproduction is the art of making new babies, or offsprings, from just one parent. In contrast, sexual reproduction is where offsprings are made from two parents, one male and a female, like in human beings, a male and female unite to give babies. But in asexual reproduction, we get offsprings from just one parent. And a lot of animals and bacteria and plants undergo asexual reproduction. And therefore, let's talk about their types. So let me get rid of this. Asexual reproduction can be broadly classified into five types. And let's not worry about their names right now, because I wanna talk about them in great detail, and I can't do that in one single video. And so in this particular video, we will focus only on the first type called fission. So, let's talk about fission. Okay, so what exactly is fission? Fission is basically a cell splitting into two. So let me just write that down. Fission is cell splitting, that's important, a cell splitting into two or more. It doesn't have to be just two. It can be two or more cells. Now at first you might be saying, "Hey, this is just cell division, right? Why do we call this as a reproduction method?" Well, think about it, in multicellular organisms, like in you and me, cell division just makes more cells inside the body, so that's not reproduction for us. However, if you consider unicellular organisms, organisms that just have one cell, only one cell, now imagine if that splits into two, we will now have two individuals. Or in other words, reproduction has happened, right? And therefore, fission, or cell division is a kind of reproduction for unicellular organisms, not for multicellular, but for unicellular organisms. So let's take the example of amoeba, the most famous unicellular organism you might know. When amoeba matures, meaning when it's ready to reproduce, we will see some changes happening. First of all, we will see that its nucleus starts to elongate like this. Inside, the DNA starts getting copied. Eventually, as time passes by, we will see that the nucleus starts splitting into two. After the copy of the DNA has been made the nucleus starts spitting into two, as you can see over here. And eventually, that amoeba splits into two new amoebas. Now, since two brand new amoebas are born from a single parent, this qualifies as asexual reproduction. And a couple of interesting things to notice is in asexual reproduction, we will find that the offsprings are identical to the parents. We say they are clones of the parents, in all kinds of asexual reproduction, not just fission. And why is that? Well that's because if we just have one parent, only it's DNA gets copied. And so the DNA over here is exactly same as the DNA over here, unless there is some kind of mutation. But in sexual reproduction, there is a DNA mixing that happens. And that's why the offsprings will not be identical to their parents, like in humans. You are not exactly identical to your mother or father because you have a mix of their DNA. And the second thing to see about fission, especially, is that you can see that the pattern itself split into two new babies. We usually call them as daughter cells. So the pattern itself split into two daughter cells. All right, now based on how fission takes place, we can further classify fission itself into different kinds. So let's take the example of amoeba by itself. In this example, we see that the fission is happening along this axis, right? This axis, let me draw that over here also. Okay, now could this amoeba, could this amoeba split along some other axis, say, along this axis? The answer is yes. It could have elongated this way and finally split like this and this as well. That's also fine. It could have also split along this axis. The amoeba can also split along this axis. So as long, when it comes to amoeba and some other unicellular organisms, we will find that there is, they can split, or fission can happen, along any particular axis. Okay, so one kind of fission is this irregular kind of fission that can happen along any axis. But in some other unicellular organisms, we will find that fission happens in a fixed axis. So let me take an example of that. Let me just make this picture small. Okay, I think that's fine. Let me just keep that over here. And now let's talk about another organism called Leishmania. You can see it has, what your textbook calls it, a whip-like structure. If you're wondering what a whip is, whip is that leather thing, which when you hit, it does that (whip lashing) sound. Anyways, so if you're take an organism, which has that structure like this, this is also unicellular. It also undergoes reproduction by fission. But what's interesting over here, or what's different over here is, is that the fission can only happen along this plane, along this plane, as you can see. The fission cannot happen along this plane. So let me just put a cross over here. This can not happen or along any other plane. The fission can only happen along this plane. Now, can you pause for a while and think about why this might be the case? Well, if you guess it might have something to do with this whip, it's not called whip, the technical name for this is flagellum. Anyways, if you're wondering, it's got something to do with that, then you're absolutely right. You see, when this Leishmania divides, it has to make a copy of this flagellum also, right? And that can only happen if division happens along this axis. Just imagine if division happened along this axis, this bottom one, I will not get a copy of this whip because that's over here. And so in this one, in this particular case, the fission happens only along fixed axis. So one way we can divide fission, further classify fission, is we can say fission that happens along any axis or fission that can happen along a fixed axis. All right, now you can see that in these two cases, even though the fission is happening irregularly over here and in a fixed axis over here, what's common in both of them is that, in both of these cases, they are dividing into exactly two new offsprings, right? On the other hand, there are certain unicellular organisms that can divide into more number of offsprings. Let me give you an example of that. An example of that would be plasmodium. When plasmodium is ready to divide, what you will find is that it will split simultaneously into more than two. I've just shown five as an example over here. But it can split into more than two offsprings. And as a result, eventually more than two copies or more than two offsprings are made over here. So this is another way in which we can classify fission. We can say, one kind of fission is where two offsprings are formed. We call this binary fission, binary fission. And what do you think we will call this one? Well, since more than two offsprings are formed, we'll call this multiple fission, because multiple offsprings are formed. This is called multiple fission. And you can guess now, by multiple fission, that organism can spread rapidly. And in fact, plasmodium is the one that causes malaria inside us. So let me just write that down as well. Okay, I don't know. Let me use a different color. All right, so plasmodium is responsible for causing the disease malaria. And since we're talking about disease, Leishmania is responsible for causing the disease called kala-azar. And if you're wondering suddenly, why am I talking about diseases, that's because your exams will ask you these questions. They will ask you what causes malaria or what causes kala-azar. Or they'll ask you what disease Leishmania causes or what disease plasmodium causes. So just for that sake, I thought I should mention this. So that pretty much wraps up our fission. So to quickly summarize, fission is basically cell division. We call it as a reproduction for unicellular organisms because in unicellular organisms, as a cell divides, then we have brand new individuals. We can classify them into two kinds. Binary fission is the one where we get two offsprings. Multiple fission is the one where we get multiple offsprings. That's one way to classify fission. Another way to classify fission is based on which orientation, along which axis it undergoes fission. One way, some kinds, some unicellular organisms undergo fission in any axis. But as in some other cases, they will undergo fission along a fixed axis.