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A cell spends most of its life in interphase, which has three phases: G1, S, and G2. In the G1 phase, the cell grows and takes in nutrients. In the S phase, the cell's DNA is replicated. Each replicated chromosome consists of two sister chromatids connected at the centromere. The G2 phase is another growth phase, after which the cell is ready for mitosis. Created by Sal Khan.

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Video transcript

- [Voiceover] Let's talk a little bit about the life cycle of a cell. In particular, we're gonna talk about interphase. The interphase part of the life cycle of a cell. And as we'll see, interphase is where a cell spends most of its life. Let's draw a timeline for a cell. So let's say this is a new cell and it will go through interphase. So I'm gonna make it like a cycle so it's gonna go back on itself. At some point, so all of that is interphase. At some point it will be ready to divide and it will undergo mitosis. Mitosis is, more formally, it's the process by which the nucleus turns into two nuclei, but then that's obviously needed for cell division. So this is mitosis right here in green. So as you see, and this isn't precise, a cell spends most of its life in interphase and that's where it's just kind of living as a cell. It's living, growing, producing proteins, whatever other functions it has and mitosis, it's a shorter part of its life, a small fraction, a very interesting part. It's necessary in order for the cell to replicate, but you see it's a much smaller fraction. But what I wanna focus on in this video is interphase. To do that, let's draw ourselves a cell. So let me copy and paste. So this right over here, actually let me, I did that just to save time. So let's say this is a cell, so green. I have it's nuclear membrane, or not nuclear membrane, I have its cell membrane. Inside of that, of course, you have all of the, all of the cytosol, and then this, in this orangeish color, I have the nuclear membrane that defines the nucleus. And then inside of that I have the DNA. And you might be used to seeing DNA all tightly bound, or chromosomes all tightly bound like that and like that or like this, this would be another chromosome right over here in magenta. But during interphase, the chromosomes aren't tightly bound like that so that they're easy to see from a traditional or a simple light microscope. For most of a cell's life, the chromosomes are completely unwound. They are in their chromatin form. So they are in their chromatin form. It's actually hard to see if you have just a simple microphone (laughing) a simple microscope. It's all unwound, you just have the proteins and the DNA, it's all tangled together. Now there's one other thing that I drew here. You might say, why am I drawing it when I haven't drawn most of the other organelles? But I'm drawing this thing, which is called a centrosome, 'cause it's going to be important for, it's going to be important for when we go into mitosis. Now, this drawing as well, you might say, wait, doesn't a cell, at least a human cell that has a diploid number of chromosomes, and once again, if we're not talking about sex cells, we're talking about just our somatic cells, doesn't it have to have 46 chromosomes? It looks like you only drew two. And it is true, I only drew two chromosomes for the sake of simplicity, we're just going to assume that this is the cell of some organism that's much simpler, that it only has two chromosomes. So anyway, this is the new cell right over here. It is going to grow. So it is going to grow, it's going to take in nutrients from its environment, and it's going to grow as we would expect it to. So that's that right over there. And then let me give it its nucleus and its centrosome just like that. And this phase, this phase, where it is just growing from this new cell, this is, this phase right over here, is the G1 phase, the G1, actually I'm gonna do that in a different color since I'm already using that green so much. This is the G1 phase and so that might look something like this, different cells are going to do this for different periods of time, the G1 phase. But then you can imagine, well look, it's going to need to replicate some of the, or, it's gonna replicate the information inside of, or that's coded by the DNA at some point, and actually, this happens before mitosis. So let's depict that. So let me draw, let me draw the nucleus and the centrosome again. Let me draw that again. Let me draw the cellular membrane. This nice healthy growing cell. And now, its DNA is actually going to replicate. So instead of having one copy of its DNA, it's essentially going to go to two copies. But I wanna be very very careful now. So if I draw that magenta chromosome up here, so once again it's all unwound like that. When it replicates, it's going to create a copy of its DNA, and once again, I'm not doing justice for how much DNA, how much DNA there actually is. But it was one chromosome before, it was one chromosome when it was just like this, and it's still one chromosome, even though it's copied its genetic material. Let me draw this a little bit neater. So this is one chromosome right over here. And that one chromosome, after it's copied all of its genetic material, it is still one chromosome. Now how do we, but there's two copies over here, what do we call these two copies? Well, each of these two copies are called a chromatid and these two right over here, these are sister chromatids. Sister, sister chromatids. But either way, this is one chromosome right over here. Chromosome, chromosome. And this is also, so this is one chromosome right over there, and that is also one chromosome. This whole thing right over here is also one chromosome. Later on, when we go through mitosis, we'll see that these two sister chromatids get split apart, they're no longer connected. At that point, we refer to each of them as an individual chromosome. Now you might be wondering, is there a word for this place where these two sister chromatids are connected? And the answer is, yes, there is a word, and that word is centromere, not to be confused with centrosome. Let me give myself some space here. So that right over here, that is a centro-, centromere, right over that. So we had one one magenta, or we had this magenta chromosome right over here, and now it replicates. It's still one chromosome, although it has twice the genetic material right now. You have these two sister chromatids connected at the, say the centromere's right over there. And that's also going to happen for the blue chromosome. All this genetic material is going to replicate, you're gonna have two copies of it. And so now it's gonna be made up of two sister chromatids that are maybe connected once again at a centromere. And also while all of this replication is happening inside the nucleus, the centrosome also duplicates. The centrosome also duplicates. And this process, the part of the life cycle where all of this genetic information is duplicating, we call that the S-phase, S-phase for synthesis. So this is the synthesis phase. So that is the S-phase. And then before going into mitosis, there is one more growth phase. There is one more growth phase, and we call that G2. So we have one more growth phase, which we call G2. And then we are ready, so let me just copy and paste this. Let me just do this, so that's what we had before. Now we need to remember that our DNA has replicated. Our DNA has replicated, so let me draw that. Let me draw the two centromeres, one for each of the chromosomes. Let me draw the replicated, the duplicated centrosome, not to be confused with centromere. Now the cell has grown even more. The cell has grown, the cell has grown even more. And once again, going from this to this, we call that the G2 phase. At this point, at the end of the G2 phase, this is now when we are ready, this is now, what if we do this in a different color? This is now when we are ready for mitosis.