Cell Division

All animals and plants are made up of cells.

When an organism grows, its cells get bigger and divide to form two new cells.  This is called cell division.  This happens many times to produce many cells.

In a fully grown organism, cells still grow and divide, but at a much slower rate.  New cells need to be produced to replace dying cells and for repair of damaged tissues.

The nucleus of a cell contains chromosomes.  Each chromosome carries many genes.

Chromosomes are made of the chemical DNA.

A gene is a section of DNA.

Genes control the characteristics of the body, for example, eye colour.

Genes may have different forms, for example, there are genes for blue eyes, brown eyes, green eyes and hazel eyes.  The different forms of a gene are called alleles.

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Mitosis

In the body cells of an organism, the chromosomes are normally found in pairs.

Human body cells contain 23 pairs of chromosomes, making 46 chromosomes altogether.

Before each cell division, a copy of each chromosome is made.

The new cells produced by cell division are identical to the parent cell.  They contain 23 pairs of chromosomes carrying identical genes to those found in the parent cell.

This type of cell division is called mitosis.  Two identical cells are produced.

The diagram below shows a simplified explanation of what happens.

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This type of cell division is called meiosis and produces four daughter cells each with half the number of chromosomes.

The diagram below shows a simplified explanation of what happens.
 
The cell divides twice to form four gametes, each with a single set of chromosomes.

During fertilisation, gametes join together.  This produces a single body cell that has pairs of chromosomes.

This cell divides repeatedly by mitosis to produce a new individual.

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