Mitotic Spindle: Definition, Formation & Function

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  • 0:31 Microtubules
  • 1:29 Centrosomes
  • 2:36 Kinetochores
  • 3:29 Chromosome Movement
  • 5:16 Lesson Summary
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Lesson Transcript
Instructor: Greg Chin
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


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.

The mitotic spindle
Microtubules Illustration

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.


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.

Illustration of the mitotic spindle highlighting the centrosomes and astral microtubules
Centrosomes Illustration

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.

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