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Germ cells, gametes & sexual reproduction

Let's see how the DNA number is maintained in sexual reproduction. Created by Mahesh Shenoy.

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Video transcript

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 it 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 offsprings 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 gonna 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 more variations can be accumulated much faster compared to asexual reproduction and variations are great for species to survive in our ever-changing environment and of course you've talked a lot about this in credited in our previous videos videos on why sexual reproduction and a 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 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 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 and all of them should have cells which have a pair of DNA one that they got from each parent that makes 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 our reproductive organs in males the gonads would be the testes and in females the gonads would be the ovaries that's where you'll 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 if these resulting cells which only contain in 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 a 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 there undergo multiple process and they divided 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 NCERT 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 and it is now you can see where we are 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 up 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 where 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 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 in humans we have 23 such pairs alright 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 thus the 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 water 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'll find about 8 million varieties are possible eight million and similarly over here also we can have 8 million female gamete varieties are possible and so if you now consider the fusion of them you have 8 times 864 million different varieties of offsprings can be made yeah let that number sink in you don't have to know 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 James 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 zài good you keep hearing these names over and over again now in the future videos and finally to give some details on the gametes also turns out the male gamete 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 turns out that they're big and bulky so they're just big and they're non motile and they are called usually called the egg cells egg cells are just eggs and another name for them would be ovum home would be singular 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 they're the source of food for the zygote initially when the zygote has to grow means 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 for in in the egg itself and that's why it's big and non motile and as 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 our 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 ovum which are usually big and non motile and so for reproduction 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