Cell reproduction: mitosis
By Joanna George-Johnson & company, Contributor

Principal of the newly-constructed Hopewell High School in Hanover, Mrs. Joyce Irving (seated at right), waves for the camera during a planning meeting. The school has opened its doors to students for the first time and already has over 300 students enrolled. It is among 17 high schools being built in western Jamaica. - Photo by Noel Thompson

The cell as we know is the basic unit of life. It therefore can exhibit all the characteristics of living things. So among other things, a cell can also reproduce. Cells of higher order organisms are reproduced by a process called mitosis. Unlike humans, these cells do not require a partner to reproduce. Therefore, the result of their reproduction is a cell identical to themselves.

Although quite a complicated process, mitosis may be split up into four distinct stages for simplicity.

The stages of mitosis

1. Prophase: This is the stage where the now duplicated chromosomes become shorter and fatter in appearance. (The reason for this is not important for this level of biology. However, it will become important as you go further into biology.)

• The sister strands now called chromatids are connected by what is called a centromere.
  
  
• The nucleolus and nuclear membrane also disappears.
  
  
• The spindle fibres emanating from the centrioles (only in animals) begin to form (plants have spindle fibres, but no centrioles).
  
  
• The centrioles separate and begin to move to opposite ends of the cell.

2. Metaphase: the chromosomes (still made up of sister chromatids) move towards the equator of the cell.

• The spindle fibres radiate from the opposite ends of the cell and attach themselves to each centromere at opposite sides. (Thus, each centromere has two attachments of spindle fibres.)

3. Anaphase: The two chromatids of each chromosome begin to separate from each other.

• Each sister chromatid moves toward opposite ends of the cell. They split at the centromere.
  
  
• This action is what allows for each of the resulting new cell having a copy of each chromosome in them.
  
  
• The action of the movement of the chromatids is guided by the spindle fibres.

4. Telophase: a new nuclear membrane forms around each set of chromatids (now called chromosomes).

• New nucleoli form within each nucleus.
  
  
• The chromosomes begin to get thinner in appearance, therefore less visible under the microscope.

Telophase is the last stage of mitosis. However, in most cases, cells go on to the process of cytokinesis. This is the process where the cell separates into two new daughter cells.

NOTE WELL !!!!

• Do not mix up the many Cs that have to do with mitosis - centromere, centrioles, chromatids and chromosomes. (Hint: memorise the definition of each.)
  
  
• Chromatids and chromosomes are basically the same things; they are just at different stages of development.
  
  
• When the chromosomes duplicate, each strand of the identical copy is called a chromatid.
  
  
• After the chromatids separate at anaphase, they are then called chromosomes by telophase.
  
  
• Remember that at the end of mitosis, a copy of each chromosome ends up at both ends of the cell.
  
  
• That is how we end up with identical cells at the end of mitosis.

Questions:

1. How many chromosomes are in a typical human cell?

2. How many chromosomes are found in human sperm or egg cells?

3. Is it possible for red blood cells to carry out mitosis? Explain.

4. What takes place during interphase in preparation for mitosis?

Joanna George-Johnson & company from Ardenne High School