Dr Andrew Hart: My name is Dr Andrew Hart and I'm a senior forensic scientist.

DNA stands for deoxyribonucleic acid, it's a complex chemical. It's found in the nucleus of cells.

DNA is fundamental to our job. And what it is, it codes for your sort of genetic makeup, such as things like your eye colour and your hair colour, sort of your physical and chemical characteristics. Everyone's DNA is almost unique apart from that of identical twins.

With the police, we're using forensic science to help with their criminal investigations, using things like bloodstain pattern analysis, we're looking at body fluids.

DNA can transfer in many different ways, my saliva could be deposited on a surface or I might have cut myself and left my blood or a hair might have fallen out.

DNA can survive thousands of years, so we can go back to old crime scenes.

If you're thinking of applying for this job, is to get a good grounding in science, chemistry, physics, biology. The job I do as a forensic scientist, you know, it's different every day, and that one particular case, it could be that, you know, the breakthrough that the police need. That gives you such a buzz and a rush and you just feel that, you know, you've really made a difference.

Professor Brian Cox: It's here in Cambridge that Francis Crick and James Watson discovered the structure of DNA, the molecule that passes biological information from generation to generation.

Crick and Watson's approach to finding that structure was to build physical models of the molecule. But it was proving unsuccessful. They desperately needed more and better data. And it came from a branch of physics called 'X-ray crystallography.'

This is a very famous photograph. It's called Photograph 51. It was actually taken by another scientist, Rosalind Franklin and it's what's called an X ray diffraction photograph, so Franklin shone X rays through a sample of DNA molecules and the way they scatter or diffract off the molecules, the pattern they leave on the photographic plate, allows you to deduce the structure of those molecules.

The key piece of evidence is the X that allowed Franklin to suggest that the molecule must be helical, and in fact, must have that famous double helix. So this photograph along with Franklin's suggestions, her interpretation of the pattern, allowed Watson and Crick to go away and build their model of DNA.

When they published the structure of DNA in 1953, Crick said "we have discovered the secret of life." Crick was right. The discovery of the structure of DNA was one of the great moments in modern scientific history.

By the early 1970s, the genetic code had been translated, making it possible to identify individual genes and study their function. We now had access to the workings of life itself.