Mitosis is one type of cell division. There is another type called meiosis that you will learn about next week (called meiosis). Mitosis is the type of cell division where one cell (the parent cell) divides to give rise to two identical cells (the daughter cells), which are not only identical to each other, but are also identical to what had been their parent cell.
Why would you need to have mitosis? You need it for growth and repair. Think about the example at the start of this chapter, when they talk about starting off as a zygote. In order to grow into a human being we have to have enough cell division to make trillions of cells! It is important that every cell division yields daughter cells equivalent to the parent cell and not cells that have half the DNA in them.
The previous example was of growth. But mitosis is also important for repair. When your skin is cut, you certainly want more skin cells to come in and repair the cut. So, the nearby skin cells undergo mitosis (there are also other things that happen that are beyond the scope of this short discussion) and voila! You have repaired the cut.
Two other important terms are poles and equator. In the parent cell above, the poles of the cell and the equator of the cell, would be as shown here. That's because the parent cell drawn above is going to split from side to side, so the two poles of the cell (the future positions of the daughter cells) are to the sides. The equator is the midline between the two poles.
How does mitosis occur?
You already know a few things about this. To keep the genetic information intact during all the shuffling around of the DNA, we're going to have to condense our DNA. We are also going to have to give the chromosomes more room for movement than just being stuck within the nucleus-- so you are going to see that the nuclear envelope falls apart during mitosis. All the sorting of the DNA progresses through some steps, so each step is given a name-- there are 4 phases (steps) of mitosis. Below you will find descriptions of each step.
How does the cell physically split in two? Believe it or not, that is not mitosis. Keep in mind that both mitosis and meiosis are types of cell division. And in both cases, the cell physically splits-- so the cell itself being cut up into two cells is called cytokinesis, and it is considered a separate phenomenon from mitosis or meiosis. It is the topic of another page for this lesson.
The phases of mitosis
Here's a quick overview: The first thing that happens is that the DNA condenses; this will then obviously occur during the first mitotic phase, prophase. Then you know that for sorting, the chromosomes have to somehow organize (metaphase) and then split apart (anaphase). Finally, when all the DNA moving about is done, the chromosomes have to decondense (telophase). So, conceptually, you should already understand the four stages of mitosis. Your book shows you all of mitosis in Figure 5-2.
The four sections below are rather short, just giving you the minimal information necessary. I have links to mitosis web sites off our links page, and many of these are very good. There are even movies of mitosis occurring on those web sites. You can also look at your CD that came with the book to see mitosis movies.
Prophase marks the beginning of mitosis with DNA condensation. Throughout prophase, the DNA is condensing so that by the end of prophase, chromosomes are clearly seen in the cell. However, keep in mind that this occurs gradually, so that during most of prophase the chromosomes are not yet fully formed.
Along with DNA condensation, the nucleoli disappear. This is because the nucleoli are made up of DNA (and protein), and ALL the DNA of the cell must condense including the DNA of the nucleoli. As the chromosomes appear, then, the nucleoli must disappear.
In preparation for the task of organizing and sorting the chromosomes, the nuclear envelope breaks down during prophase. All that happens is that membrane pinches off the nuclear envelope as vesicles until the entire nuclear envelope is pinched away. Basically, the entire nuclear envelope is floating around in the cell as vesicles. In this manner, the vesicles are available for later, when they will have to re-fuse to reform the nuclear envelopes (in telophase).
In this phase, the chromosomes line up at the equator of the parent cell in a nice, neat row.
Why is it that the chromosomes line up? Well, during metaphase the chromosomes have to become organized, because each daughter cell is going to need one full set of chromosomes, not just any random grouping of them. Each daughter cell needs to get one copy of chromosome #1, one copy of chromosome #2, one copy of chromosome #3, etc. through #46. If the chromosomes do not somehow organize, a daughter cell may end up with two copies of chromosome #1, none of chromosome #2, none of chromosome #3, one of chromosome #4, two copies of chromosome #5, one of chromosome #6, etc. This daughter cell would die. The other daughter cell (with no chromosome #1, 2 chromosome #2, etc.) would also die. So, lining up at the equator is the way that mitosis solves this problem, as you will continue to understand as you read about anaphase.
It is during anaphase that the chromosomes split and move toward the poles of the cell. One half of the chromosome viewed in prophase heads toward each pole. Because each half (sister chromatid) of the chromosome is identical, identical molecules of DNA are moving toward each pole. This way, each pole ends up with a full set of DNA, not just a random assortment. Once the sister chromatids are no longer connected, though, we don't call them sister chromatids any more-- we now call them chromosomes again.
One more point: because the chromosomes are really moving around at this point, it is important to remember what allows that to happen. You should all be thinking now, "hmm... movement... that is the cytoskeleton." And, of course, you are correct! Microtubules organize themselves into something called a spindle apparatus. In this manner, the chromosomes move right along the microtubules toward the poles of the cell; I have described this for you on another page.
Everything that happened in prophase happens here, but in REVERSE. I'm just going to list the things that happen during telophase for you:
© 2006 STCC Foundation Press, content by Dawn A. Tamarkin, Ph.D.
Last changed: January 21, 2007