 Home kids, we're back! Our next set of topics in this class are going to build on the foundation of DNA and energy that we set up in the last section. In this section, we're going to start out by talking about nuclear division, mitosis and meiosis. When I say nuclear division, you should automatically go, oh, if we're talking about nuclei, we're probably going to have something to do with DNA. I hope, at this point in the class, we're halfway through this course. I hope at this point in the class, if you hear DNA, you go, oh, important, magical, amazing molecule that's very important because it codes for proteins. And if you don't yet have an appreciation of proteins, follow along, friends, follow along, because proteins are phenomenal! And the capacity to build proteins accurately allows you to be alive and allows you to do some unbelievable things like talk, understand, communicate, hug, all eat ice cream. All those good things are because of proteins. In the last set of content, we actually started our discussion of DNA with the human life cycle. Now I'm going to do it again and we probably are going to do it again because not only are we going to talk about nuclear division in this section, we're going to talk about heredity and how you, why you look the way that you do based on sperm parent and egg parents' qualities and characteristics, how you end up with the characteristics that you have. And that's heredity. That's one of our topics in this next chunk. All of it has to do with DNA. And the fact that DNA is a molecule that codes has instructions, carries instructions for building proteins. Okay, I could go on forever, right? You push pause and you do the task that I'm about to do, which is to draw out the human life cycle. Based on the last time that we did this, I'm going to hold to approximately the same level of detail as I did in the last one, which means we're not going to go into adolescence and puberty and all those exciting old age, whatever pregnant. No, we're not going to go into all those different stages. We're just going to do our cellular processes. So you push pause and go see what you can remember. And then when you come back, we'll start your back. Good job. You did a great job on your drawing. And here I am. I'm about to draw draw mine. Okay, we know that you came from an egg and a sperm. You know that each of those cells, that is chromosome has 23 chromosomes. You actually know, I don't know if you remember this, but you know that 23 chromosomes in a human, that is haploid. And that means that you have one copy of each chromosome. If you're going like, I know this, I don't remember knowing this. It's okay because we will talk about it over and over and over like this whole like all six lectures in this section. We will be talking about haploid things and diploid things and how many chromosomes we have in various cells that are going through division, combining, whatever. So you will get cozy with us. You also know that if an egg and a sperm combine, they combine their DNA into one cell with 46 chromosomes. That cell is called a zygote. That cell is no longer haploid. It is diploid. All your cells in your body are diploid, which means there are two copies of each chromosome. That diploid cell somehow has to turn into the grown-up you, and we know you're a grown-up because you're wearing a top hat and look how happy you are in your top hat. What was the process? Mitosis is the process of division, cell division, that results in identical daughter cells. That zygote, that one cell went through mitosis and divided and divided and divided whatever 100 trillion times to get you as you are standing right now. Now the adult grown-up you has gonads, ovaries or testes, and in the ovaries and testes a different kind of cell division takes place. A fancy cell division called meiosis. And I remember we put a little box around the happy parts that go through meiosis to produce sperm and eggs. You can imagine that in the process of meiosis we actually halved the number of chromosomes that we started with. So we started with 46 in all your cells and in sperm and eggs we only have 23. So meiosis is a reduction division. We reduce the number of chromosomes by half, which if you think about it, thank you for reducing the number of chromosomes by half because otherwise we would have too much DNA in all of our cells. That matters. If you end up with too much DNA, even one extra chromosome, even one extra part of one extra chromosome, it changes the your phenotype. It changes who you are and it can be fatal. So it is it's important that the process takes place in a very careful manner. Okay so today we're going to talk about this part. Tomorrow we're going to talk about that part. So today we're going to focus on mitosis. As is often the case, these processes are really complicated. Like what? And there's lots of vocabulary. We are at a cellular level of organization so it's a little bit less biochemical than what we've been dealing with and that's kind of nice. So we're kind of zooming out a little bit but what you do, how you learn mitosis, will determine how it is for you to learn meiosis. If you buckle down and go okay I'm going to make sure that I really understand mitosis. I'm going to maybe put a little extra time into this lecture. It's worth it because then meiosis, we're definitely going to tweak some stuff. There's differences between the two processes but there's a lot of similarities and you won't feel like you're having to learn two whole giant processes. You'll learn one mitosis and then you'll learn some tweaks when we do meiosis in the next time. That was me telling you to take a deep breath and really focus on this lecture and you will be glad that you did. Okay are you ready to get started? Of course you are but first before we start talking about mitosis we have to have some chromosome anatomy review.