 If we were sharing space right now, I would stop and ask you to do this task before I said any words. This is the task I would ask you to do. So after I explain it to you, push pause and do that task and let's see what you come up with, except I won't be able to see it, but you know what I mean. What is the human life cycle? If you had to draw it out, what would you include? And how many stages would you include and how do we get here? I think it is super interesting to take stock of what you already know, especially when going into a topic that's pretty complicated and really molecular. So looking at the human life cycle is gonna let us frame the molecular details in context that makes sense to us. Okay, so push pause and go do that. And I'm assuming that you did what I asked and you thought about the human life cycle. And now let me show you the context that we're gonna talk about. This is very, oh, what do you know, simplified. And it's simplified because we're focusing in on only certain aspects of the human life cycle. The first thing that we wanna focus in on is where did we come from? I remember, I remember being, I think, in kindergarten and my mom reading me a book called Where Did I Come From or Where Did You Come From? So it started early. And so I learned as a kindergartner that I came from a sperm combining with an egg. I'm gonna go ahead and put titles here. Sperm are, both of these are gametes. Gametes are sex cells. So sperm are the gametes or the sex cells that are produced in testes. And eggs are the gametes or sex cells that are produced in ovaries. So folks who have testes produce sperm. Folks who have ovaries produce eggs. They're both gametes. And we're gonna see that they have a lot in common. When a sperm combines with an egg during the process of fertilization. Let's just draw that in, because I think that is also an interesting and important part. So we have fertilization. You get, there's a new cell that gets produced when sperm combine with eggs. And that new cell is called a zygote. One cell. So we started out with two cells, sperm and egg. They combine, we now have one cell and that cell has a name and it's called a zygote. A zygote goes through a process. Aw, it goes through a process called mitosis. And we are going to learn all the details of mitosis after we talk about DNA. So this picture, as many of our pictures do, it's gonna come back. We're gonna keep looking at this and reminding ourselves of the big picture. We'll learn how mitosis takes place and how that zygote goes from one cell to let's just draw what we turn into. We turn into a little top hat wearing happy person. A grown up. Did I skip any steps? Did I skip any steps that you included in your drawing? Probably because you probably did like a more detailed drawing than I did. But again, all that stuff in between being a zygote and being a grown up, I'm not worried about that. I'm worried about that grown up. It's gonna do some things to produce sperm and eggs. I do wanna make a note that this process of mitosis is also called oops, cell division. And the result is identical cells. This is weird. This should be like, wait a minute, how do we go from one single cell and divide into what I'm telling you right now is 100 trillion identical cells? Well, they have identical DNA. They have identical nuclei. Remember, that's where the DNA is held. So mitosis, that process of cell division results in 100 trillion cells that are you and every single one of those cells with an exception that I'm about to say right now, every single one of them has the exact same DNA in the nucleus. Super wild, like that should be mind blowing. Let's zoom in on this area. This is the area of the gonad. Gonads, it's not a skirt. It's a gonad, it's the gonad zone. We're focusing in on the gonads because the gonads are what are producing our gametes. And as soon as you roll into puberty home kids, you start producing gametes and you go through a different process. This process is called, this is gonna be really similar word, mitosis. Mitosis results in identical daughter cells. Mitosis is how you go from being a single cell zygote to being the fabulous specimen that you are now. Mitosis is also a process of cell division that results in unique cells with half the DNA. I gotta write that down for you. It's also cell division, but the daughters are unique and half the DNA. And those cells that result are guess what? Sperm and eggs, home kids, sperm and eggs. Mitosis gives us sperm and eggs. If you have ovaries, you get eggs. If you have testes, you get sperm. If you combine sperm and eggs, the nuclei combine and what started as half the DNA from the egg and half the DNA from the sperm, you now have the full amount of DNA in the zygote that goes through mitosis to form a grownup. What? I have some more words for you. So many colors. What color do I make? I'm gonna put this in black. I said that meiosis results in cells with half the DNA and I wanna zoom in on that a little bit. That is referring to the number of chromosomes that are in there. And there's two words that I wanna look at. First of all, cells that have half the DNA are called haploid. And I'm gonna add another definition. They have one copy of each chromosome. And yes, I will tell you what a chromosome is. Can you see how this is a vocabulary dense section? But we're learning the vocabulary now so that the whole next section when we talk about meiosis and mitosis and heredity will have been exposed to these words, they'll be familiar to us. It also gives us context for our DNA molecular stuff. Okay, so haploid cells have one copy of the DNA. Mitosis results in diploid cells. How many copies do you think they have of the DNA? Two copies of the DNA. Where did two copies come from? One copy from the sperm, one copy from the egg. So yeah, sperm brings one copy of DNA. Egg brings one copy of all your chromosomes and now you have two copies, which means you've gone from two haploid cells to one diploid cell. All the cells in your body, home kids, are diploid with the exception of sperm and eggs found in your gametes. There's something else, humans. I said this, I wanna focus, I want you to focus on chromosome. I said that you have one copy of each chromosome. Well, humans have 23 pairs. Of chromosomes. That's what humans have. So how many chromosomes are in a diploid cell? We have a total of 46 chromosomes, right? That makes sense. And how many chromosomes are in my gametes? We have a total of 23 chromosomes. That's how we go from one copy, 23 chromosomes per gamete to two copies, 46 chromosomes in the zygote. I wanna show you a picture of that. Okay, here's our life cycle. This provides us context. The whole purpose of mitosis and meiosis is to generate a copy of the DNA so that everybody has the instructions that they need and why do we need DNA? Because it builds protein. It's how we know what proteins we need to be the fabulous specimens that we are. This is called a karyotype. And this is a picture of all the chromosomes in your giddyup. We have 46 chromosomes total and this is a human karyotype. And you can see there actually, our chromosomes are numbered from one to 22. And then we have these sex chromosomes that x and a y or two x's, not two y's. We can't survive with two y's. We have to have at least one x. They're sorted by size. So chromosome number one is the biggest. And all it is, a chromosome, chromosome is one strand of DNA. One strand of DNA. And I wanna call out the fact that I just used the word strand, maybe molecule is a better molecule. Is a better word to use than strand. And we'll see why because DNA is a double-stranded molecule. So a chromosome is one molecule of DNA, one double strand. And we'll look at the structure of DNA next. This means that right here, chromosome number one, one copy of chromosome number one, like we could unravel it. It looks like a little log right now and it does go through a log phase, which is how we get this. But you can unravel it into like a tangle of one string of DNA. If you unraveled all the DNA in one of yourselves and hung it end to end, it would be six feet tall of this molecular substance that fits in one single nucleus in a cell. It's wild stuff. You'll notice since we have two copies of our chromosomes, every one of them, we have two copies of every one of these chromosomes, again with the exception of the sex chromosomes. And we'll talk about those in more detail later. These pairs of chromosomes are called homologous pairs. And if you think about it, one of those pairs, they're the same. They actually contain the same instructions for the proteins that they are going to build. So if they have a hair color protein coated, both of those chromosomes contain that hair color gene and they can have different forms. But one of those homologous pairs comes from your mom and the other one comes from your dad. One of them comes from the sperm and one of them comes from the egg. All right, that's a lot of background. That's a lot of big picture. It's a lot of vocabulary. But hang with me because I am confident, very confident that when we do, as we work through these lectures, we'll revisit these words and you'll be happy that you've seen them before and that you have some context for them. Okay, now we're heading back into DNA. Now we're gonna look at those chromosomes and go, what are they actually made out of and how did they figure it out? That's actually what we're gonna start with. We're gonna look at the race to discover the structure of DNA.