It may be hard to believe but in your distant past you were a small, single cell created by the fusion of two special cells: one from your father - the sperm - and the other from your mother - the egg. These cells fused together, fertilisation happened, and a unique person was created: you!

Over the next nine months the newly made single-celled zygote went through a lot of changes. Early on, the single cell divided and multiplied incessantly to produce a multi-cellular ball called a blast, then an embryo, and finally an adult. If you think about it, the fact that a single cell can grow into an adult with trillions of cells is pretty awesome.

What is allowing us to grow at such a fast and furious pace?

It’s a process called the cell cycle. Entire organisms grow, and develop by going through the cell cycle again and again.

The cell cycle also helps us maintain a healthy body. Every day, we need to replace skin that is constantly wearing away, repair cuts or wounds, and replace large numbers of red blood cells that die. In fact, look at any living organism, and you’ll find cells growing and dividing. The cell cycle is essential to life.

Launch it ... How much do you know about the Cell Cycle? See if you can identify the stages in this game.

But for the cell cycle to be a success, it must guarantee that the ‘daughter’ cells inherit all the tools they need to repeat the process themselves. To ensure this happens, whenever a cell grows and multiplies, it must pass on a full copy of the genome.

So how does a single cell go about delivering a full set of genetic instructions to its 'daughter' cells?

Since the stuff of heredity is carried by structures called chromosomes, the first thing a cell must do is make an extra set of them.

Chromosomes are tightly coiled DNA molecules wrapped around special proteins called histones. Such complex structures may seem tricky to duplicate but, in practice, the cell finds it simple. The structure of the DNA helix is so inspired that the two chains can easily unwind and unzip, each acting as a template to make a new strand of DNA.

Did You Know ...?

This is the first step in the cell cycle: duplicating the DNA. Biologists call this early stage the S phase (S for synthesis). What follows is a series of events that are known as mitosis. During mitosis the cell accomplishes its goal of dividing into two new cells, each with a full set of chromosomes.

Scientists have artificially divided mitosis into several stages to make it easier to study. But mitosis is really one continuous, dynamic process. If you stain the chromosomes with a dye and look under the microscope, you can see exactly what goes on in the dividing cell.

Did You Know ...?

Just before mitosis begins, there is little happening inside the nucleus and the duplicated chromosomes look like a jumbled, tangled mass of DNA. But as the cell enters the first stage of mitosis called the prophase, the chromosomes become more condensed and visible. It’s now possible to see that the chromosomes - all of which had made an identical copy of themselves back in the S phase - form an x-shaped double chromosome. These double chromosomes are held together at a central point called the centromere.

If you now look to one side of the nucleus, you will see a pair of tiny bodies called centrioles that start to move towards opposite sides of the cell. These centrioles form a new structure called a spindle, made of many fibres.

The spindle fibres attach to the centromeres and start to pull the chromosomes back and forth across the cell until they eventually settle down in the middle, along what could be called the equator. The cell has now entered metaphase, the second stage of mitosis.

Suddenly, as if someone had issued a command, the duplicated chromosomes are separated and are pulled to opposite sides of the cell, drawn along by the spindle fibres that begin to tighten up and shorten. These events are part of what is called anaphase. Later in this stage, the cell begins to divide in two by pinching in the cytoplasm around the middle.

Finally, in telophase, the chromosomes reach the poles. The spindle disperses, and new nuclear envelopes start to reform around the chromosomes as they lose their distinctness. The cell has now completed its mission: the division into two new cells, each with an identical set of chromosomes.

Like mother, like daughter - the beauty of this process is that these newly formed cells end up with exactly the same chromosome content as the original cell. In this way, mitosis conserves the chromosome content, passing the entire genetic information from cell generation to cell generation.

Egg and sperm cells arise by a different process called meiosis. As a result of meiosis, each new cell has only half the genetic material of the original. Halving the genetic material ensures that a full set of chromosomes will result when sperm and egg cells fuse.

Discuss it ... Is it right to attempt to clone humans?
Teachers' Notes ... For Teachers' Notes on this subject, click here.