So here I've got a picture of an average person. They're a little more than 5 feet tall. So if we were to convert that into meters, the average person is a little less than about two meters tall. Now, a person is the largest unit of life we consider. When we talk about the smallest unit of life, we're talking about a cell that relatively looks like that. And while a human being is about two meters tall, a cell has a diameter of about 100 micrometers, which, to put into perspective relative to a human being is about one one-millionth of the size. And yet there's so much that goes on here. Just as human beings grow and maybe will have babies, cells do the exact same thing. They grow and have babies, as well, or they undergo cell division. So, in the next couple of videos, we're going to talk about how a cell grows and divides. So let's zoom in on the cell right here and spend the next couple of videos talking about how a cell grows and divides. The life span of a cell can be described by what's called the cell cycle. The cell cycle can be thought of as seasons in a year. Just as we have seasons such as the spring or the summer where things grow, versus fall and winter where they don't, the cell has times when it grows and divides and other times when it doesn't divide. There are two main overarching seasons, or types of seasons, that we can talk about here. There's this period here that's more like the fall or the winter, where you don't have as much cell division, but you have more growth of the cell. This period is where the cell spends most of its time, and it's called interphase. Interphase: where we primarily have cell growth occur, but not cell division. Interphase is where cells spend most of the time, so most cells live here. But there is one key exception. What do you think that one exception might be? I think I heard you correctly if you said, "Cancer." You are absolutely right. Cancer cells have some defect in them that causes them to want to divide more so than grow, and we'll talk more in detail about how that occurs in a minute. The other main phase of the cell cycle here is where you have active cell division, and it's called mitosis. Mitosis, or sometimes it's abbreviated with just an "M." Mitosis is the time where you have active cell division. Now, there are a few other phases that occur within interphase. The first part of interphase is a growth phase. It's usually abbreviated, "G1." As you can see here, G1 is the longest phase of the cell cycle, so most of a cell's life is spent here, and it's in this phase that we produce extra organelles, such as ribosomes and proteins. So we make proteins that will be useful when we get to the point of cell division. >From here, the cell has a choice. If it wants to continue growing and move towards the direction of cell division, it will move forward this way to the next phase that's called the, "S phase." The S phase just stands for synthesis, more specifically DNA synthesis, because here we're going to have DNA replication. That's where we take 23 pairs of chromosomes, so 23 pairs. We call them pairs because half of them are from your mom, half of them are from your dad, and we duplicate them, we replicate them, and we end up with 46 pairs. and most cells go in this direction as I mentioned over here. Some cells however, instead of going forward, from G1 they'll go in another direction here to a phase that's called, "G not," or G 0," where you have no more division. No more cell division. Because there are certain cells in the body that don't like to divide or don't tend to divide, and you can think of a quick example like neurons in the brain. Once your brain is formed it doesn't necessarily need to divide any more. You just have cells grow. So that means it's sort of the end point for these types of cells. They won't usually come back and enter the cell cycle in this way. But let's continue as if it had. The next step, or the next phase of the cell cycle, is called, "G 2," which is another growth phase where we are more directly preparing for mitosis. So, we prepare for mitosis in a couple of ways. Perhaps a good example is we make microtubules, which will be used to pull our chromatids apart when it comes time for anaphase, and we'll talk about what that means in a separate video. And finally, to be complete, we have our last phase right here, which is just mitosis, which is our final season of the cell cycle where our cell will divide. And once it's divided and turned into two cells, each of the cells will next enter the G1 phase, where they will grow and produce extra organelles and proteins that will eventually allow them to divide again. So, as you can see, just like seasons in the year, the cell cycle goes around and around as we divide cells further and further, unless it's a cell that goes off to this G not or G0 phase where we don't need any more cell division.