 Now, we have the words that we needed in order to describe our DNA. Now we need to know, okay, when does all those stuff happen? And we can use the DNA. It's going to give us clues and hints. Every cell goes through a cell cycle. And the amount of time they spend in the different stages of the cell cycle depends on the kind of cell it is, and probably on the stage of the critter's life cycle, like overall life cycle. So you can imagine our 100 trillion cells are all in various stages of the cell cycle. There are two main parts, and the main parts of the cell cycle are, I'm going to write this out even though it's on this little diagram that I have here. The two main parts are interphase, and I'm going to say M, but I'm saying M because that's like nuclear division, and it can like be mitosis or meiosis, all cells. If you got in and you grabbed some of your cells, every single one is in one part of the cell cycle. Pick out a cell and go, oh, that one is in the S phase of the cell cycle. Pick out another one. Oh, gee, it's in the first gap. We call that G1. Pick out another one. Oh my gosh, this one's in mitosis. It's going through one of the stages of mitosis. Oh, there's one. It's in G2, the gap, the second gap. We'll talk about what each one of those stages is, but the thing that I want you to, the big picture I want you to hang on to is that every cell just is constantly in one of these stages, and the stages lead to the next one, and then you go to the next one. Most of the time a cell spends, okay, most cells spend most of their lives in G1. Most of the time is here in G1. G1 is a cell living its best life. It's doing its thing. If it's a muscle cell, it's contracting. If it's a brain cell, it's firing messages. If it's an endocrine cell, it's making hormones. It's just doing its job and living its life. Some cells stay in G1 forever. In fact, they have a name for G1 if you never leave it. If you just stay there until you die and you never go to the next stage in the cell cycle, they call it G0 or G0, little sub-zero, not. Most cells, even neurons, brain cells, do divide. They do go through this ultimate cell cycle, and at the end of it here, at the end of mitosis, we have two cells. So we start out with one cell. It goes through the process. When it gets to the end, we have two cells that are then going to go through the cell cycle, both of them. So it's kind of like this constant process. The cell's doing its thing in G1 somehow. This is chemistry, which is amazing. Somehow, at some point, for some reason, a cell might get a message that it's time to divide. So it's not okay for that cell to stay one. It needs to become two. Maybe you're working out, and so the cell is like, dude, we need to get bigger. So we need more. That's a really bad example because the cells don't actually, muscle cells don't increase in number. They actually increase in size, but not in numbers. So that was a bad example. Sorry. But adipose tissue cells, those increase themselves. Bone tissue cells, whatever, we've got lots of cells that reproduce themselves. Okay. So we get the message to divide. The cell that was in G1 is now going to move to S. Guess what S stands for? Synthesis of what DNA? During the S phase, all the DNA is replicated. So the whole genome goes through DNA replication. Who's the hero? Who's making that happen? DNA polymerase. You already know this stuff. So synthesis, DNA polymerase gets their moment in the sun and goes through the S phase and doubles all the DNA. Now I'm just going to show you a picture. I'm going to draw one chromosome. The fact is that during interphase, all DNA is in loose chromatin form. During the whole time, all of interphase, which if you stay in G1 for a long time, that's a long time that you have chromatin, that you don't even ever condense into those cute little chromosomes that are buried over here. You don't ever get to this stage most of the time. But it's a mess. Can you imagine where we had 46 chromosomes, 23 pairs? We now have 46 chromosomes, each one with twice as much DNA. So the chromosome that started out looking like this, if we condensed it, after S, that same chromosome has a copy, an identical copy, they stay attached. So this is something that we didn't really do in our chromosome anatomy piece because I wanted to make sure that you knew when this replication happens. These two butterfly wings are called sister chromatids. They are still connected at the centromere. So here's one chromosome because there's one centromere. Here's one chromosome because there's one centromere. But the second chromosome has sister chromatids twice as much DNA. Too much DNA. You don't want to leave that like that. But, oh my gosh, if I had two things of DNA or four things of yarn DNA in this pile, would you be able to tell the difference? Dude, it's a tangle. It's a tangle that got a little more dense because there's more tangles in there. But now we have sister chromatids and the identical copies are attached to each other. They stay connected even though it's a mad tangle. They stay connected. This is important. Once they get through the S phase, all the DNA is replicated and that takes between eight to 10 hours in a human for all the DNA to get replicated. Then the cell goes into G2. In G2, we're replicating everything else. It's not just DNA that we're going to need two copies of if we're going to divide our cell. We're also going to need two copies of the ribosomes and two copies of all the mitochondria and two copies of all double the cytoplasm. Okay, two copies is a bad example, but we want to double all the stuff so that we have enough stuff for both cells to have an equal amount and all that doubling happens in G2. That's the end of interphase. G2, when we get to the end of G2, we head into mitosis. Mitosis has, I don't like it. I always learned it that it has five phases, but they've started adding a sixth phase. There's four phases that I learned and they started adding a fifth one. That's cool. We're going to go through each one of the phases of mitosis, big picture. We divide the DNA, that crazy doubled tangle. We're going to divide it up and I'm going to walk you through each of these phases. We're going to talk about prophase and prometaphase together. Then we're going to talk about metaphase, then anaphase, and then telophase. And cytokinesis, telophase is the last stage of mitosis. Cytokinesis is division of the cytoplasm. Cytokinesis happens in order to end up with two cells. We're going to talk about each one of those parts and list out the things that we will see. The other thing that we're going to do, and this is interesting, we can actually look at slides of roots, root tips, onion root tips, that are going through mitosis. A root, think about what the roots are doing. They're growing. They're growing out to get more nutrients and more water and we can ask our botanist hero to tell us more about root dynamics if we want to learn more about that. But you can take a slide of the root and you can see mitosis, all the stages of mitosis you can find. We're going to look at a root and see what the different stages look like in the actual microscope. Okay, let's get started with prophase and prometaphase.