Mitosis

Key definition

Mitosis is nuclear division giving rise to genetically identical cells.
- Textbook definition: Mitosis is the type of nuclear division that occurs during growth and asexual reproduction. The daughter cells are genetically identical.

Before mitosis, an exact replication of chromosomes occur. This is a very reliable process in which exact copies are made.
While this copying process goes on, the DNA in the chromosomes is uncoiled and arranged very loosely in the nucleus.
- You won’t be able to see chromosomes in the nuclei as this happens.
The new copy of DNA of each chromosome is attached to the original copy.
As mitosis begins, the DNA coils up so that each chromosome becomes thicker.
- They become visible with the microscope when a suitable staining technique is used.
During mitosis, the copies of chromosomes separate, maintaining the chromosome number in each daughter cell.
- As the chromosomes have been copied by a reliable system, they are genetically identical to each other and to the parent cell. If they were genetically different, the body’s immune system would reject it.

In animals, this happens all over the body.
In plants, it happens in special growing areas such as the tips of stems and roots.
- In plants, group is confined to certain areas called meristems. They are plant regions made of unspecialised (not yet developed the particular structure and job of a mature plant cell), actively dividing cells.
As multicellular organisms grow, the number of cells making up their tissues increases.
- The new cells must be identical to the existing ones.

Mitosis is crucial for replacement of cells that wear out and die.
- For example, red blood cells, which only live for a short time.
- Skin cells and the cells in the gut are constantly dying and being replaced by identical cells.

This occurs in fungi and in plants, but is rare in the animal kingdom.
Asexual reproduction results in offspring that are identical to the parent. Mitosis occurs when unicellular organisms reproduce.
- For example, yeast and potato tubers.

Stem cells are unspecialised cells that divide by mitosis to produce daughter cells that can become specialised for specific functions.
Some of the cells that form an embryo remain as unspecialised cells. They do not develop into any type of unspecialised cell.
- These cells keep dividing by mitosis to provide the body with a constant supply of new cells. These cells are needed to replace cells that get worn out and die.
There is a layer of stem cells just under the outer layer of skin cells.
- These stem cells continually get rubbed off and need to be constantly replaced.
- When skin is wounded, these stem cells produce more cells to repair the damage.
There are many stem cells in the bone marrow to produce new red blood cells and phagocytes as replacements.