So after prophase, the second phase of mitosis is prometaphase. It's important to note that in some of your textbooks, prometaphase is also sometimes referred to as late prophase. In some textbooks, prometaphase isn't even mentioned as a phase; instead, all of the events of prometaphase are batched into prophase because it is sometimes referred to as late prophase. This is important to keep in mind if your textbook does not have a prometaphase phase; that means your textbook is just batching the events of prometaphase with prophase, the phase that comes right before. Now, there are really two specific events that are going to take place during prometaphase that are important for you all to know. The first is that the nuclear envelope, or the nucleus itself, is going to degrade or, in other words, break down. When the nuclear envelope degrades or breaks down, it exposes the chromosomes to the cytoplasm, allowing the mitotic spindle, which has been forming, to link to the chromosomes by attaching to specific proteins called kinetochore proteins that are found within the centromere of the chromosomes. Recall the centromere, which is not to be confused with the centrosome. The centromere is the "waist" position of the chromosomes where the two sister chromatids meet.

Notice down below, again in our image, we are showing you all the stages of mitosis, but we are not focusing on these over here. We are specifically focusing on the second phase of mitosis, prometaphase. So notice on the left, we are showing you the first phase, prophase, and the image that we talked about in our last lesson video. In this video, we are talking about prometaphase, which is shown in this image here, and the events taking place are right here in the middle. The first event during prometaphase is the degradation or breakdown of the nuclear envelope. You can see over here in prophase, the nucleus is intact, but over here in prometaphase, the nuclear envelope has broken down and is degrading, which is why we're showing it in these small pieces. This exposes the chromosomes to the cytoplasm of the cell and allows the mitotic spindle to attach to the kinetochore proteins found in the centromeres of the chromosomes. Here in these positions of the chromosomes, you'll find the kinetochore proteins, shown as little green proteins where the microtubules of the mitotic spindle can attach. When these microtubules attach to the kinetochores, they can move the chromosomes around, and that's what we are going to see happening in the next phase.

This here concludes prometaphase, and in our next phase of mitosis, we'll talk about metaphase itself. I'll see you all in that video.