Overview of Mitosis: Mitotic Spindle & Phases of Chromosome Segregation

Instructor: Jeremy Battista
Have you ever wondered how we grow? We start off as tiny embryos that eventually develop into adult humans, but how? The answer is through mitosis. We will look at mitosis and some specifics of it here.

What Is Mitosis?

Mitosis is the splitting of one cell into two similar daughter cells. Notice, I did not say an exact copy of the original cell because the two daughter cells are similar but not 100% the same. I will explain this in greater detail as I get to a later section.

We see here a brief and simplistic view of the start, process, and end of mitosis

Mitosis consists of five phases: prophase, prometaphase, metaphase, anaphase, and telophase. Each of these phases sees a number of different and specific things occur. For our purposes here, we will not go into each individual phase as we are focusing on mitotic spindle formation and chromosome segregation. It is in the prometaphase where we see the mitotic spindle begin to form.

Mitotic Spindle Formation

This mitotic spindle forms from microtubules, which are protein structures that are part of the cytoskeleton. These microtubules work together with the centrosome to form the mitotic spindle. The spindle will act as the control center for the kinetochores that soon form. These kinetochores are basically rope-like structures that attach to the spindle and the individual chromatids that make up the chromosomes.

This is metaphase. Notice the mitotic spindles at either side of the cell. Also, we can see how the chromosomes line up along the metaphase plate or equator

The spindles then help tug on the kinetochores which shorten in response to the next phase of mitosis, where the chromosomes begin to segregate or split apart.

Chromosome Segregation

Prior to the chromosomes separating or segregating, they first make a duplicate copy of themselves. Picture the chromosome as if two lines crossed to form the letter 'X.' Each line that makes up the X is a chromatid. Now, when it copies itself, there are four X's in the cell. This is very important since we want the two new cells that are forming to have the same number of chromosomes (but arranged differently).

This chromosome segregation occurs in the phase after the spindles form, metaphase or middle phase, and into anaphase. The chromosomes line up along an imaginary center 'equator' in the cell, during the end of prometaphase as the cell enters metaphase. After the spindles form during the same phase, they are attached via kinetochores to the chromosomes. As mitosis progresses and enters anaphase, the spindles begin to tug on the kinetochores, which in turn start to shorten their proteins in order to separate the chromosomes into their smaller chromatid makeup.

Here is a picture of anaphase. Notice, the chromosomes are torn apart into their chromatids

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