The discovery of the Double Helix

By playing with pieces of cardboard cut into the shapes of the 4 chemical components that make up DNA, 2 scientists called Francis Crick and James Watson were able to work out how the pieces fitted together in pairs. From this discovery in 1953 it only took them a day or two to make a Meccano like model, perhaps the most famous piece of work in modern science, the model was made up of metal plates bolted together in a spiral. It was without question a brilliant piece of detective work, for it they were awarded the nobel prize.

Crick and Watson decided that if you could find out the shape of a DNA molecule you'd be able to see how it did what it did. The shape is rather like a spiral staircase:the famous double helix:

Double helix DNA is actually very simple. It has just 4 basic components- which is like having a alphabet of only 4 letters. 

The components pair up in  particular ways to form rungs and the order in which they do this as you move up or down the ladder forms the DNA code. Because you can combine them in different ways, like you do with the dots and dashes of morse code, you end up with 3.2 billion letters of coding, enough to provide a number of possible combinations that is almost impossible to imagine (10 x 10 1,920,000,000 times if you really want to know).

DNA is not itself alive, no molecule is, but DNA is especially 'unalive'. Thats why it can be recovered from patches of long-dried blood in murder investigations, and coaxed from ancient bones to date prehistoric people.

DNA exists for just one reason: to create more DNA-and you have a lot of it inside you, nearly 2 metres squeezed into almost every cell. In fact you may have as much as 20 billion kilometres of DNA.