This GCSE-level video explains how multicellular organisms develop from a single fertilized cell (zygote) through repeated cell division. It describes mitosis (one diploid cell divides once to make two genetically identical diploid daughter cells used for growth, repair and asexual reproduction) and meiosis (one diploid cell undergoes two divisions to produce four genetically varied haploid gametes), and highlights why meiosis creates genetic variation. The video also gives illustrative numbers (46→92 during DNA copying, 46→23 in gametes) and quick factual examples (e.g., skin cell turnover).
Chapters
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 cell, the zygote, divided over and over and over again by a process called mitosis. So 1 cell became 2, 2 became 4, 4 became 8, 8 became 16, 16 became 32, etc, etc, 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, because remember, all human body cells have 46 chromosomes in. Apart from gametes like sperm and egg, and also red blood cells, they don't have a nucleus at all. But apart from those, all cells have R46 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. We've got one cell with 46 becomes two cells with 46. Now, the only issue you may have spotted with that diagram is that you start off with a cell with 46 chromosomes in, then you end up with two cells of 46 chromosomes in. So we've doubled the number of chromosomes. We've gone 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 that 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, where we've got four chromosomes. We could do all 46, but it would make the diagram too complicated. We just show it with four and then we show. Here we go, they've all copied themselves and then they're 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 chromosomes in, 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 has to happen really fast while you're growing, but it also has to keep happening because you're losing cells all the time. You lose 40,000 skin cells a minute and you replace your 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, essentially, because you've got one parent cell becoming two daughter cells identical. So this is how things like bacteria and things that do asex reproduction, they divide their cells by mitosis. And it's also technically cloning because you're making genetically identical copies 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 in two by mitosis. Like, I've just been talking about things like bacteria and fungi. And some plants can do asex reproduction in a simple way, but humans are more complicated. We combine sex cells, gametes, in fertilization to form this original 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 of 46 chromosomes. So actually, 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, remember, mitosis just makes more diploid cells. If you want to make haploid cells, you're going to 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 cell with four chromosomes in. And again, we're going to copy all the DNA just like we did before, to make double chromosomes. But the difference here is they'll 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 we've halved and then we went from four to two. So we've actually made four haploid cells instead of two diploid cells. So that's the difference between mitosis and meiosis. Now, the offspring from sex reproduction vary genetically. This is because each sperm and egg are genetically different due to the mother's and father's chromosomes getting mixed up with when the cells are created by meiosis. So this process of meiosis is really, really important for creating variation between 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 of the other sperm, and all the eggs are slightly different 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 summarise the two methods of cell division, mitosis and meiosis, you can show them in this diagram here. Mitosis 1 diploid cell, 46 chromosomes becoming 2 diploid daughter cells. Meiosis 1 diploid cell, 46 Chromosomes becoming 4 haploid cells with 23 chromosomes in it. As usual, why not have a go at these 10 true or false questions to see if you understand this? SA.
