Mitosis I: The Mitotic Spindle

Microtubules

We know that if we're going to accurately segregate or separate the chromosomes during mitosis, we're going to need to condense the chromosomes because a small, compact object is going to be easier to move than a long, stringy, floppy one. Now, we need a way to move those objects around in the cell. Alignment of the chromosomes is an important step in mitosis, and it's accomplished by microtubules, which are rope-like components of the cytoskeleton.

Now, microtubules organize into what is known as the mitotic spindle. Before we can understand how the mitotic spindle works, we need to know a little bit more about the components that make it up. Microtubules are the major component of the mitotic spindle. As far as mitosis is concerned, you can think of them as molecular rope. During mitosis, they're used to position the chromosomes at a specific position inside of the cell. If we attach microtubules to both sides of the chromosome, we can increase or decrease the tension on each side to position the chromosomes in the middle of the cell.

Centrosomes

Let's see how the cell accomplishes this. First off, if we're going to use ropes to tether the chromosome, we should probably anchor at least one end of the rope to something that's not going to move, and the centrosome can serve this function. The centrosome is an organelle that serves as a microtubule organizing center during division. It's duplicated during S phase, and the two copies move to opposite sides of the cell. Now, once the mitotic spindle has been assembled, the centrosomes are also referred to as spindle poles. The centrosomes initiate the assembly of several types of microtubules, but let's just consider two of them.

Two Types of Microtubules

The first microtubule is short, and it radiates out from the center of the centrosome in a star-shaped pattern, so we call them astral microtubules. Now, astral microtubules anchor the centrosomes in the cells, and, in the process, they position the mitotic spindle apparatus. You can think of them as tent spikes that are going to stabilize the entire spindle apparatus.

The second type of microtubule is called a kinetochore microtubule. To understand this type of microtubule, we'll need to figure out what a kinetochore is first. The kinetochore is a protein structure that assembles in the centromere during mitosis. The kinetochore assembles on each side of the chromosome or essentially on the outermost side of each of the sister chromatids. Not surprisingly, the kinetochore microtubules attach to the kinetochore.

Chromosome Movement

Now that we've established all of the players in the mitotic spindle, we can figure out how these are going to help align chromosomes in the cell. The astral and kinetochore microtubules are two of the principal components of the mitotic spindle, and the mitotic spindle apparatus provides the means to position the chromosomes during mitosis.

So why is chromosome movement so important to mitosis? The goal of mitosis is to move the chromosomal copies, or chromatids, to opposite sides of the cell. This ensures that each daughter cell gets a full set of chromosomes. Now, there's so much information stored on a chromosome that loss of even a single chromosome is a catastrophic event that can lead to disease for the organism or death for the cell.

It would be most efficient if we divvied up the chromatids all at the same time. Positioning them in the middle of the cell in the same orientation makes that possible. The only question is how're we going to get them there?

Recall that we said that each chromosome is bound by microtubules on either side and that the microtubules are sort of like molecular rope, which can exert tension on each side of the chromosome. What that means is that by increasing tension on the right side of the chromosome while simultaneously decreasing tension on the left side, we could move the chromosome toward the right. But how are we going to control the tension? Well, the microtubules are already nailed down at the centrosome side, so the kinetochore proteins are pretty much in the driver's seat.

Let's think of the situation on either side of the chromosome as a guy with a winch sitting on either side of the chromosome. The kinetochore on the right can decrease the length of its microtubule while the kinetochore on the left can simultaneously increase its length on the left. In this way, the chromosomes can move back and forth until they've reached the center of the cell.

Oh, one more thing before we go. Has there been something that's been nagging you about the mitotic spindle, like why the heck it's called a spindle? Well, if you look at the shape of the kinetochore or spindle microtubules, they actually form the shape of a spindle, like the ones that are used in the textile industry to form wool or other fibers into thread.

Lesson Summary

The mitotic spindle is a structure composed of microtubules which segregates chromosomes into the daughter cells during mitosis. A microtubule is a rope-like component of the cytoskeleton.

The centrosome is an organelle that serves as a microtubule organizing center during cell division. The spindle pole is the region of the cell where the centrosome is located and toward which the chromosomes will move.

An astral microtubule is a short microtubule which emanates from the centrosome and serves to accurately position the mitotic spindle within the cell. A kinetochore microtubule is a microtubule which attaches to the kinetochore and positions the chromosomes during mitosis.

Wait, wait, wait! Don't leave yet. Here's one more way to remember the mitotic spindle:

Condensation makes DNA ready

And microtubules hold it steady

A microtubule's a fancy rope

To move the DNA is the hope

From spindle poles they do extend

To align each chromosome in the end.