[Music] so we all started life as one single cell believe it or not a zygote formed from the fertilization of the sperm and the egg but how then did we go from one single cell to absolutely billions of cells well that first single celled a zygote divided over and over and over again by a process called mitosis so one cell became 2 2 became 4 4 became 8 8 became 16 16 came 30 to accept to accept etc to get to the billions of cells that you are today so mitosis is a type of what we call cell division and what happens is is that you take a cell which has 46 chromosomes in remember all human body cells have 46 chromosomes in apart from gametes like sperm and egg and also red blood cells that have a nucleus at all but apart from those all cells have our 46 in and we call them diploid cells when you've got this full number 46 and when you do mitosis it simply copies that one cell into two genetically identical diploid daughter cells so there we go at once our 46 becomes two cells with 46 in now the only issue you may have spotted with that diagram is that you start off with a cell with 46 chromosomes in knowing that were two cells have 46 chromosomes in so we doubled the number of chromosomes one from 46 chromosomes in one cell to 92 so where do all these extra chromosomes come from well actually before the cell splits into two all of its DNA has to copy so every single chromosome becomes a double chromosome then a double chromosome can be pulled apart one copy goes into one of the cells and the other copy goes into the other cell so we can show that on this diagram here we've got four chromosomes we we could do all 46 but it's make the diagram too complicated we just show it with four and then we show here we go they've all copy themselves and then they sort of double chromosomes and then they line up down the middle of the cell and we then pull one copy of each chromosome to each side and then the cell can split and you can see there that we've got two identical cells which are copies of that initial cell that we had we had four chromosome and we've got two cells with four chromosomes in mitosis doesn't stop though when you finish growing because it occurs all the time to replace cells that we lose so yes it happened to happen has to happen really fast while you're growing but it also has to keep happening because you're losing sales of time you lose 40,000 skin cells a minute you replace yourself an entire stomach lining every month you replace cells all the time so you need to constantly do mitosis in order to keep regenerating cells so mitosis is actually used for growth repair it's also asexual reproduction and essentially because you've got one parent cell becoming two daughter cells identical so this is how there's like bacterial things that do egg sex reproduction they divide their cells by mitosis and it's also technically cloning because you're making genetically identical copies of a cell now if that cell was an organism like a bacteria then it's being cloned by making two identical copies of it now some simple organisms can just reproduce by splitting into by mitosis like I've been talked about things like bacteria and fungi and some plants can do asexual reproduction in a simple way but humans are more complicated we combine sex cells gametes in fertilization to form this original a zygote so we take an egg cell and we take a sperm cell now these gametes because they're going to combine they need to have a total between the two of them or 46 chromosomes so actually they those cells sperm and egg cells gametes they need to have 23 chromosomes each we call these haploid cells when they've got 23 chromosomes in each but to make haploid cells you need to do a different type of cell division you can't do mitosis you remember mitosis just makes more diploid cells if you want to make haploid cells you're gonna need to do a different type of cell division and this type of cell division is called meiosis so let's start off with the same cell that we did before so here's a set of cell with four chromosomes in and again we're going to copy all the DNA just like we did before to make a double chromosomes but the difference here is that actually that is going to divide twice it divides once and then it divides again and you end up with only two chromosomes in each so if hard and then we went from four to two so we've actually made four haploid cells instead of two diploid cells so that's difference between mitosis and meiosis now the offspring from sex reproduction very genetically and this because each sperm and egg are genetically different due to the mothers and fathers chromosomes getting mixed up when the cells are created by meiosis so this process of meiosis is really really important for creating variation between you know you and your brothers and sisters so when fertilization of gametes happen that's also completely random any sperm can fertilize any one of the eggs and each sperm is slightly different from each other sperm and all the eggs I even from the other eggs so you get all this random variation that means that you will end up being different from your brother or your sister so to summarize the two methods of cell division mitosis and meiosis you can show them in this diagram here mitosis one diploid cell force its chromosomes becoming two diploid daughter cells meiosis one diploid cell force its krepo becoming four haploid cells with 23 chromosomes in it as usual one have a go at these ten true or false questions to see if you understand this topic well you