1. Cell Division

What you need to know...

Why is cell division important to organisms?
What happens in cell division?
What is cancer?

Notes

Living things are made of cells

All living things are made of cells. The cell is the basic unit of living things.

Some living things like bacteria are only made up of one cell. Other living things like us are made up of lots and lots and lots of cells.

Making new cells

So why do living things make new cells?

Basically, an organism (living thing) needs to make new cells so that they can grow and repair damaged parts of themselves e.g. cuts, broken bones.

To make new cells, cells undergo a very special process called cell division. In this process a single cell which we call the parent cell, divides in half to produce two identical daughter cells.

Cell division

The following table shows the sequence of events of cell division stage by stage.

To keep things simple this cell has four chromosomes - two matching pairs.

*Diagram*	*Description*
	Before cell division the DNA in the nucleus is long, thin and unwound.
	The DNA is copied. The DNA winds up into duplicated chromosomes.
	The duplicated chromosomes line up along the middle of the cell.
	The duplicated chromosomes are pulled apart.
	Two nuclei are formed containing a full set of single chromosomes.
	Two daughter cells have been formed. They have the same number of chromosoems as the parent cell.

The key point to remember is that during cell division: the parent cell divides to produce two identical cells, which contain the same number of chromosomes in their nuclei as the parent cell.

Cancer is a disease of cells.

You have probably heard about cancer but may not know that cancer is a disease of cells. It develops when normal cell division goes out of control. Unlike normal cells, cancer cells make too many new cells and they don’t die when they are supposed to. They can form a group of cells called a tumour which grows into and damages the healthy cells around them. This can makes a person very sick.

So why do healthy cells start to divide uncontrollably? It is all down to the DNA. You already know that DNA contains the genetic instructions for a living thing. Well if the DNA in a cell gets damaged these instructions get broken and a cell can start to behave in a strange way.

Usually cancer is a disease of old age. But there are some things that can damage the DNA in a cell and lead to cancer. Things like smoking, unhealthy diet, lack of exercise, overexposure to sun, infection with a certain viruses (HPV) can all increase our risk of cancer. It is important to stress that many causes of cancer are simply unknown.