In this video, we're going to briefly introduce centrosomes and mitotic spindles. Recall from our last lesson video that during the S subphase of interphase, it's characterized by DNA synthesis where the cell's going to replicate their DNA, but they also replicate another structure called the centrosome. The centrosome is not to be confused with the centromere, which again is the waist position of a replicated chromosome where the two sister chromatids come together. The centrosome is different than the centromere. The centrosome is a cytoplasmic protein complex that forms or organizes what are known as the mitotic spindle.
And the mitotic spindle is very very important for mitosis, which occurs, of course during the M phase of the cell cycle. The mitotic spindle is really just microtubule proteins of the cytoskeleton that coordinates division of the chromosomes, essentially dividing the DNA into the two daughter cells. Here what we're seeing is that in our example image is that the centrosome is going to be replicated in the S phase and the centrosome is ultimately going to be responsible for forming the mitotic spindle during mitosis. Over here in this image down below, notice over here on the left we're showing you a single cell that has only one centrosome. Notice that a single centrosome actually consists of these two smaller proteins here, and so we have one single centrosome right here.
And, we have the nucleus of the cell and within the nucleus we have these unreplicated chromosomes. Notice that, after the S phase, remember the S phase is the subphase of interphase where DNA synthesis is going to occur, DNA replication. But in addition to DNA replication, the centrosome is also going to be replicated. Notice that the centrosome is replicated. Now there are two centrosomes. There was one over here and now there's a second centrosome over here. We have these centrosomes here, but also in addition to the centrosomes being replicated, of course, the DNA is going to also be replicated. So we convert the unreplicated chromosomes here into replicated chromosomes. Notice they each, we have this, these X-shaped chromosomes here indicating that they are replicated, whereas over here they are not X-shaped, they are more linear like what you see here. And so really that is the main point here, and, ultimately these centrosomes, again, are going to be very important during mitosis because during mitosis, they are going to form what are known as the mitotic spindle.
Here you can see the mitotic spindle are these structures that are projecting off of the centrosome and they are forming these microtubules that are going to be important for ultimately moving the DNA around and splitting the division of the chromosomes, dividing the chromosomes. Again, this will make, the centrosomes and the mitotic spindle are going to be more relevant once we start talking about mitosis a little bit later in our course. So this is a little bit of foreshadowing here for what you are going to see as we move forward. This here concludes our brief introduction to centrosomes and the mitotic spindles, and we'll be able to get some practice as we move forward in our course. So I'll see you all in our next video.