How cells divide

"How does a cut heal, and how does hair grow back?" Cells grow and divide to replace old, dead or damaged cells. A cell can only grow to a certain size, so a body grows mainly by its cells dividing to make more cells.

You can see this in an onion root tip, where cells divide all the time. A stained, squashed root tip under the microscope shows cells at different stages of division.

This happens on a huge scale. Every day, hundreds of billions of your cells are replaced: almost 1 per cent of all your cells.

There are two kinds of cell division:

"Why halve the chromosomes?" Each human body cell has 46. If an egg and a sperm each brought 46, the child would have 92, and their child 184. Meiosis halves them to 23 each, so fertilisation gives 23 + 23 = 46 again. Meiosis also mixes the instructions, which is why children look like their parents but are not identical to them.

A fertilised egg becomes a body of trillions of cells, all by mitosis.

When division goes wrong:

• An error in mitosis can cause cells to divide without control, forming a tumour.
• An error in meiosis can cause genetic disorders, early loss of a pregnancy, or difficulty having children.

Scientists can also grow cells outside the body in a clean (sterile) liquid full of nutrients. This is cell culture, used for research and to make medicines and vaccines.