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Mitosis

To grow and develop, organisms need to make more cells. But how do we do this? We do this using mitosis! Mitosis happens when one parent cell divides into 2 daughter cells. The daughter cells are genetically identical to the parent cell. This means they have exactly the same sets of chromosomes!

Illustration of the entire mitosis process.

The entire mitosis process.

Mitosis is very important for our bodies. It helps us grow bigger by adding new cells. It also helps us heal and repair our bodies. If a cell is too old and tired, mitosis steps in to create a strong new cell that can take over the old one’s work.

Mitosis is necessary for asexual reproduction. Asexual means that there is only one parent cell. Another type of cell division, meiosis, happens during sexual reproduction. In meiosis, two parent cells are involved!

Phases of Mitosis

Mitosis happens in 5 phases: prophase, prometaphase, metaphase, anaphase, and telophase with cytokinesis. There is also usually an interphase at the very beginning of each cell cycle. During the interphase, the cell begins preparing itself for mitosis.

Illustration of the 5 phases of mitosis in order.

Here are the 5 phases of mitosis. We will be talking about these below.

Interphase

Cells spend a lot of time preparing for mitosis. In fact, around 80% of their cell cycle happens in the interphase! The interphase is split up further into 3 smaller phases: G1, S, and G2.
 
In the G1 phase, which stands for the first gap phase, the cell grows bigger in size. It also begins to copy its cell organelles and starts making the proteins that it needs to build its DNA.
 

In S phase, short for the synthesis phase, the cell makes a copy of the DNA in its nucleus. It also copies the centrosome. The centrosome is an organelle near the nucleus that organizes microtubules. The centrosome plays an important role in separating DNA from the daughter cells!

An illustration of an organelle near the nucleus called the centrosome

An illustration of the centrosome

In the final G2 phase or the second gap phase, the cell continues to grow and make more organelles and proteins. The cell also starts reorganizing its parts to finally begin mitosis.

Here’s a helpful YouTube video on the interphase!

Prophase

 
The prophase is the very first phase of mitosis. In this stage, the cell already has a copy of its DNA. The chromosomes in its nucleus all have 2 connected copies called sister chromatids. The cell also has 2 centrosomes, made in the S phase.
 
3 main things happen during the prophase:
 
  • The chromosomes change from long strings into tight coils. Think how an extended spring snaps back into shape when you let it go! The smaller the chromosomes become, the easier it will be to divide them.
 
  • Microtubules combine to form the mitotic spindle. This will help separate the chromosomes later on. The mitotic spindle grows between the 2 centrosomes and pushes them farther apart. The mitotic spindle is named that because it looks like an actual weaving spindle. It has a bunch of strings extending between two end points!
 
  • The part of the nucleus that makes ribosomes, the nucleolus, also begins to break down!

    An image of a spindle used with a loom as a stand-in for the mitotic spindle.

    An example of a spindle – a bigger model for the mitotic spindle

An example of a spindle – a bigger model for the mitotic spindle
 

Prometaphase

 
  • During the prometaphase, the chromosomes finish coiling up. As a result, they become very small and compact.
 
  • The nuclear membrane breaks down and releases the chromosomes.
 
  • The mitotic spindle continues to grow. As the nucleus breaks down, microtubules in the mitotic spindle catch escaping chromosomes.
 
  • Microtubules grab onto the chromosomes at their kinetochores. Kinetochores are where the two sister chromatids are the closest to each other. One microtubule will attach itself to one chromosome in a sister chromatid pair.

 

Metaphase

 
  • During the metaphase, the mitotic spindle catches all the chromosomes. The microtubules then put all the chromosomes in one neat line in the middle of the cell. This line is called the metaphase plate.
 
  • Microtubules on each end of the mitotic spindle should be attached to the kinetochore of one chromosome in a sister chromatid pair.
 
  • The cell checks to see that all the chromosomes are in a straight line. This is the spindle checkpoint. If they aren’t in a line and the microtubules aren’t attached correctly, the cell won’t be able to divide evenly. The cell will not begin dividing unless all the chromosomes are on the metaphase plate.

 

Anaphase

 
  • In the anaphase, the sister chromatid pairs separate down the middle. The microtubules then pull them apart to opposite ends of the spindle.
 
  • Free microtubules push the ends of the spindle even further apart to make the cell longer.

 

Telophase and Cytokinesis

In the telophase, the cell finishes up the dividing process.
 
  • The mitotic spindle breaks down.
 
  • Two nuclei appear, one for each set of chromosomes.
 
  • Chromosomes stretch out and become long again.
 

Cytokinesis is the process in which the cell’s cytoplasm divides into two new cells. The daughter cells each have one set of chromosomes that is exactly the same as the parent cell!

An image of two complete daughter cells after Mitosis.

The two complete daughter cells

You can watch this YouTube video to better remember the mitosis phases!

Some fun, bonus content:

Here’s a funny science joke that you can ask your biology teacher!

Question: What did one cell say to his sister after she stepped on his toe?

Answer: Ow! My-toe-sis!

Other Great Resources

https://www.khanacademy.org/science/ap-biology/ap-cellular-molecular-biology/ap-mitosis/a/phases-of-mitosis

 

Written by: Minh Nguyen