Mitosis
Joanna George-Johnson, Contributor

Students from St. Jago High School in performance at the ceremony for the presentation of the Prime Minister's Medal of Appreciation for Service to Education, hosted by Prime Minister Portia Simpson Miller. The event was held at Jamaica House, Hope Road, on Wednesday, May 24, 2006. - Winston Sill/Freelance Photographer

In today's lesson, we will discuss mitosis and cytokinesis.

Mitosis

In most classes, and in some textbooks, mitosis is described as a form of cell division where the number of chromosomes is the same as in the original cell.

Today, I intend to say that mitosis is a process of nuclear and cell division. The duplication and division of the nucleus are the key factors of mitosis. Cells such as red blood cells do not undergo mitosis as they do not have a nucleus. A cell division section is actually called cytokinesis, which normally follows mitosis, but not always.

Mitosis results in the production of two daughter cells from a single parent cell. The daughter cells are identical to one another and to the original parent cell. There are two main reasons why cells divide rather than continue to grow indefinitely.

• The larger a cell becomes, the more demands it places on its DNA.
• The cell would have more trouble moving enough nutrients and wastes across the cell membrane.

This is the form of division that organisms undergo that reproduce asexually.

Mitosis is also important for organisms that reproduce sexually because growth is achieved by the division of the organisms' cells. This is a very complex process, so for simplicity the process is normally studied in four phases. (This is an area where students sometimes make mistakes. There are only four phases, not five.)

In most books on the topic, interphase is normally mentioned first. However, interphase is not a part of mitosis. Interphase is the stage of the cell's life where it grows and matures for reproduction. Near the end of this stage, the chromosomes replicate in preparation for the start of mitosis. (I normally tell my students that interphase is like the 'childhood' stage of the cell.)

Prophase

The first stage of mitosis is called prophase. During this stage, the following things occur:

• Each chromosome has been duplicated and now consists of two sister chromatids.
  
  
• The chromosomes shorten and become fatter, and are therefore visible with the light microscope.
  
  
• The nuclear envelope breaks down into vesicles.

Metaphase

The next stage is called metaphase. In this phase:

• The duplicated chromosomes (still connected by a centromere) line up on the middle of the cell.
  
  
• Spindle fibres emanating from the centrioles connect to the centromeres from either side.

Anaphase

The next stage is called anaphase. During this stage:

• The centromeres that join the sister chromatids split.
• The sister chromatids separate and become individual chromosomes.
• The chromosomes move to opposite sides of the cell.
• They move until they have been separated into two groups near the poles of the spindle.
• Anaphase ends when the chromosomes stop moving.

Telophase

The final stage is called telophase. During this stage:

• The chromosomes, which were distinct and condensed, begin to disperse and become less visible under the microscope.
  
  
• A nuclear envelope re-forms around each cluster of chromosomes.
  
  
• The spindle begins to break apart, and a nucleolus becomes visible in each daughter nucleus.

Mitosis is complete!

However, the process of cell division is not complete at telophase.

Cytokinesis

• As a result of mitosis, two nuclei, each with a complete set of chromosomes, are formed, usually within the cytoplasm of a single cell. All that remains to complete the process of cell division is cytokinesis, the division of the cytoplasm itself. Cytokinesis usually occurs at the same time as telophase.
  
  
• Cytokinesis can take place in a number of ways.

In most animal cells, the cell membrane is drawn inward until the cytoplasm is pinched into two nearly equal parts. Each part contains its own nucleus and cytoplasmic organelles.

In plants, a structure known as the cell plate forms midway between the divided nuclei, as shown below. The cell plate gradually develops into a separating membrane. A cell wall then begins to appear in the cell plate.

Note Well:

• Do not confuse chromosomes with DNA.
• Chromosomes are cell organelles that consist of DNA and protein.
• DNA is a type of macromolecule known as nucleic acids.

Exercises

1. Give two reasons why cells divide.

2. Between its surface area and its volume, which increases in size more rapidly as the cells increase in size.

3.In certain species of flies, the full complement of chromosomes in their cells is four. Draw diagrams showing the four stages of mitosis that will take place in a typical cell of this species of fly.

Joanna George-Johnson teaches at Ardenne High School.
Email: Masterbio@gmail.com.