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Cell division: mitosis and meiosis

Multi cellular organisms, like humans, are made up of billions of cells. These cells need to divide and copy themselves for a variety of reasons. For example:

cells wear out and need to be replaced
new cells allow the body to repair damaged tissue
new cells allow the body to grow (cells themselves don't grow).

Mitosis

The most common form of cell division is called mitosis. This happens in all the body cells and is used for growth and repair. During mitosis, a cell makes an exact copy of itself and splits into two new cells. Each cell contains an exact copy of the original cell's chromosomes in their 23 matching pairs. This is the reason why all the cells in an organism are genetically identical.

Picture 6. The 23 pairs of human chromosomes. These have been arranged using photographic software - they never look like this in a cell.

Use the animation to see what happens during mitosis.

Picture 7. Chromosomes are stained green in the nucleus. The cytoplasm is red.
To begin with, the chromosomes are bundled in the nucleus. Then they copy and line up and finally, the cell splits into two genetically identical cells. And the process can repeat.

Mitosis animation. Open in a new window.

Meiosis

There is a second type of cell division called meiosis. This is a specialised process that produces the sex cells: eggs and sperm. It produces four cells that are genetically different. These cells contain chromosomes that are single, rather than in their matched pairs.

Human eggs and sperm have 23 single chromosomes so that when they come together a fertilised egg containing 23 pairs of chromosomes is formed. Meiosis also produces genetically varied cells. This helps to give the variation in the offspring that result from sexual reproduction.

Sometimes meiosis can go wrong and this can lead to an inherited disorder. Down's syndrome is caused when the fertilised egg contains three copies of chromosome number 21. During meiosis, when the cells split, the chromosomes do not pull apart equally. An extra chromosome is carried into one of the sex cells and this gives the fertilised egg the additional chromosome 21 that causes Down's syndrome.

Question 3

Mitosis is involved in the growth, repair and replacement of cells. Not all cells go through mitosis at the same speed.

Look at the types of cell below and decide how often they are replaced by mitosis. In each case, choose an answer using the radio buttons.

Constantly Every few months When damaged Only once in a lifetime

Hair cells

Muscle cells

Cells lining the small intestine

Liver cells

Sensory and motor nerves (neurones)

Skin cells

Red blood cells