 Have you ever wondered that you started off in this planet as a tiny single cell inside your mother's body? And look at you today. You are a complex life form with billions of cells inside you. I mean, isn't this amazing? How can one cell over time become you? This is all possible because of the cell's unique property to divide. All sexually reproducing organisms started off as a single cell inside their mother's body. So let's explore cell division in this video. And in cell division, we mainly focus on the division of the genetic material that resides inside this nucleus. So this circle inside the bigger circle is the nucleus. And this is the cell we are discussing about. Now as I just said, we focus mostly on the division of the genetic material or we call it the DNA. Now when we talk about the genetic material, people often get confused with the terminologies. We sometimes call it the DNA, sometimes we hear people calling it the genes and sometimes they call it the chromosomes and sometimes they even call it chromatin. Now do they all mean the same thing or are they different? So let's get these terminologies together. Let's begin with the DNA. What is a DNA? So a DNA is a double helical structure. DNA or deoxyribonucleic acid can be compared to a recipe book. A recipe book that holds the recipe to form you. It is a very long strand that has a lot of different information. Now this huge book is divided into several chapters. So let's pick out one chapter that starts here and ends here. And this chapter has the recipe to form your eye color. And then maybe another chapter which lies here speaks about what your height will be. And the technical term for these chapters that I'm talking about is gene. So a gene is nothing. It is just a small part of the DNA strand that has the information of a particular characteristics of your body. Now as we already discussed that this DNA strand is huge. If we calculate the length of all the DNA strand inside the nucleus, it comes around 1.8 meters. It is that long and we know it resides in nucleus, right? And the diameter of the nucleus is somewhere around 6 micrometre. So to fit this long strand of DNA inside this tiny nucleus it has to be coiled and compacted, right? So to coil and condense this DNA strand it is wrapped around special kind of protein called the histone protein. Okay so these are histone proteins and around these histone proteins this DNA strands are wrapped several times. Finally giving it this bead on a string kind of appearance and we call this structure of the DNA as the chromatin. Now this bead on a string structure which we call chromatin can condense even further. It condenses to form something called the chromosome. So I hope I made the picture clear that chromosome and chromatin are the same thing. They all mean the DNA strand at their different level of condensation. Alright now that we got our terminologies clear we know what a chromosome is. Let me tell you that the chromosome number is species specific. That means when we say that the chromosome number of human is 46. It will stay for the sex for the entire species. That means for the whole of Homocipiens and it will never change. Or if we talk about some other species for example the chimes that has 48 chromosomes it will remain the same for all the chimpanzees. And let's talk about a plant now the pea plant has 14 chromosomes. And talking about my favorite pet the dog they have 78 chromosomes. So the point I want to drive home is that every species has their unique chromosome number. And even though a cell divides it has to maintain that chromosome number in the newly formed cells. Okay so all these years all we learned about division or dividing something is to equally divide something into two. I mean if we consider a cell or a nucleus of a pea plant that has 14 chromosomes and we are asked to divide this into two equal cells what will we do? We will divide it into two cells that will have seven chromosomes each right. And if these cells are further divided we will get another two cells from each of them with 3.5 chromosomes each. And this way the number of chromosomes will decline with each division. But this does not happen in cell division even though a cell divides it maintains the number of chromosomes it had in the initial dividing cell or we call this the parent cell. The cell that divides we call it the parent cell and the newly formed cells are called the daughter cells. Okay and even in the daughter cells the chromosome number is maintained. All the daughter cells will have the same number of chromosomes as that of the parent cell. But now the question is how is that even possible? If we are dividing it it should get reduced to half. But how is this number of chromosomes maintained? Now how about you pause the video and think about it for a moment. Alright so here is what happens. Before dividing into two cells this 14 chromosome duplicates inside the parent cell. This 14 chromosomes make a copy of itself and then when it divides give one copy to each daughter cells. And this process of cell division is given a special name it is called mytosis. Now let's go back in time and look at the cell that grew and developed into you. Let's see how mytosis took place in that cell. So this is that cell that finally made you. This is you okay. Now since you're a human being your cell will have obviously 46 chromosomes. Now showing the division of each of these chromosomes would be difficult. So we will pick just one chromosome pair out. This is one and this is another one. Now why do you think I have given them separate colors? Well that is because half of your chromosomes have come from your mom and half from your dad. And how did that happen? Well we will talk about that in a minute. But for now remember that we have taken one pair of chromosome or two chromosomes out from the very first cell of yours. Now let's see how these chromosomes divide. Each chromosome will form a copy of itself and initially they will stay attached at one point. Okay now both these chromosomes will slowly start to separate. Their point of attachment will break and they will go their separate ways. Well the picture will be more clear if we make the cell membrane around them. Okay so here you go. Each of these move their separate ways and then a cell membrane is formed around them forming two separate cells. So this is how even after cell division the number of chromosomes in the parent cell which was two chromosomes remain the same even in the daughter cells. Again in the daughter cells we get to see two chromosomes each and even the cell size remains the same. And this is how from one cell millions and millions of cells were formed and gave rise to what you look like today. Okay now we knew how this cell divided. Are you not curious to know how the cell came into existence in the first place? Well you would say that hey come on we know that the egg cell of our mother fused with the sperm cell of our dad and then it gave rise to this particular cell. We call it the zygote. Okay but then tell me one thing egg is a human cell. That means it would have 46 chromosomes right and same as the case with the sperm cell. It's a human cell and we learned that human cells have 46 chromosomes in it. Now when egg and sperm cell fused it would add up the chromosome number inside it. It would give rise to 92 chromosomes inside that cell inside the zygote we are talking about. But we just saw that the zygote has 46 chromosomes. How is that even possible? Well the answer lies in the type of cell division through which these special cells eggs and sperms are formed. They never undergo mitosis. Instead they undergo another type of cell division called meiosis. Now let's see what this meiosis is. And even here we will start with a pair of chromosome which just like mitosis will divide. But the unique thing here is that they share a part between them and this process is called crossing over. Now we don't need to go into the depth of it. Just remember that they cross section or share sections between them and why do they do that? So that there is mixing of genes so that variation takes place in nature. Now after crossing over it gets divided into two separate cells. Now these chromosomes they break apart and they are divided into four different cells. Now I want you to carefully look at the daughter cells that we got at the end of meiosis. We started off with a parent cell that had two chromosomes and look at the daughter cells now. They all have just one chromosome in them. This is the beauty of meiosis. In this process the chromosome number get reduced to half. And we need exactly this process to form eggs and sperms in the body. So that when the male and female gametes or the eggs and the sperms fuse they can form a cell, a zygote with the required number of chromosomes for that particular species. Alright so with this we have learned about two very important kind of cell divisions mitosis and meiosis. Now let's quickly talk about the very prominent differences between the two. In mitosis from one parent cell at the end we get two daughter cells. So at the end of mitosis we get two daughter cells. Whereas in case of meiosis at the end of meiosis we get to see four daughter cells. So at the end of meiosis we see four daughter cells. Now let's talk about the number of chromosomes. At the end of mitosis the daughter cells has same number of chromosomes or equal number of chromosomes as that of their parent cell. But in case of meiosis we saw that the chromosome number is reduced to half in case of daughter cells. So we write chromosome number is reduced to half. Now the third difference is that meiosis takes place only in cells that gives rise to sex cells. That means it takes place only in cells that forms gametes. Gametes are eggs in case of women and sperms in case of men. So meiosis is the process that takes place only for the formation of gametes that is only for the formation of eggs and sperms. Whereas mitosis is the process that takes place in all body cells except for the sex cells. All body cells. For the growth of your body, for the repair of your tissues, for replacing the old cells it is mitosis that is always in action. Okay now you can pause the video and look at the images of both mitosis and meiosis and try to recall what all happened in the different stages of cell division.