Genetic engineering today means moving genes -- molecules that contain biological instructions -- from one species to another -- something that is virtually impossible in nature. The very definition of the word 'species' means it is impossible for organisms of different species to interact sexually -- the basic mechanism of gene transfer in nature -- and produce viable offspring.

But now this is possible, thanks to path-breaking work in the 1950s of young Cambridge scientists James Watson and Francis Crick. They identified the structure of DNA (deoxyribonucleic acid) and scientists now know the instructions for making all living things are written in the language of DNA, apart from some viruses that use RNA (ribonucleic acid).

This means that when a gene, a bit of DNA that contains the instructions for making a certain protein, is taken out of one cell and put into another of a different species, it will continue its job in the new home telling the cell to make the protein that it codes. This technology has come to be known as recombinant DNA technology, as new strands of DNA are formed where they never existed before.