So after prophase, the second phase of mitosis is prometaphase. It's important to note that in some textbooks, prometaphase is also sometimes referred to as late prophase. In some textbooks, they won't even mention prometaphase as a phase; instead, they'll just batch all of the events of prometaphase into prophase. Because again, sometimes prometaphase is referred to as late prophase. So that's important to keep in mind if your textbook does not have a prometaphase phase in it, then that means that your textbook is just batching the events of prometaphase with prophase, the phase that comes right before.

Now, there are really 2 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. And so when the nuclear envelope degrades or breaks down, it's actually going to expose the chromosomes to the cytoplasm, and that means that the mitotic spindle that has been forming is capable of linking 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 2 sister chromatids are going to meet.

And so, notice down below again in our image we're showing you all the stages of mitosis, but we're not focusing on these over here. We're specifically focusing on the second phase of mitosis, prometaphase. And so, notice on the left we're showing you the first phase, prophase, and the image that we talked about in our last lesson video. And in this video, we're talking about prometaphase which is this image right here and the events that are taking place are right here in the middle. And so, the first thing that's going to happen is the nuclear envelope is going to degrade or break down in other words.

And so, you can see over here in prophase the nucleus is intact but over here in prometaphase notice that the nuclear envelope has been broken down, it's degrading which is why we're showing it in these small pieces. And that is going to expose the chromosomes to the cytoplasm of the cell and that allows the mitotic spindle to attach to these kinetochore proteins that are found in the centromeres of the chromosomes. And so again, the centromere is going to be the waist position, right here in these positions of the chromosomes. You'll find these little green proteins, that are the kinetochore proteins, and this is where the microtubules of the mitotic spindle can attach. And when these microtubules attach to the kinetochores, they can move the chromosomes around, and that's what we're going to see is going to happen in the next phase.

And so this here concludes prometaphase. In our next phase of mitosis, we're going to talk about metaphase itself. So I'll see you all in that video.