This was the gateway to manipulating genes, says Lewis Wolpert, our science expert
THE most remarkable feature about the discovery of the structure of DNA by Watson and Crick lies at the end of their paper in what is probably one of the greatest understatements in the history of biology. "It has not escaped our notice that the specific pairing we have postulated immediately sug-gests a possible copying mechanism for the generic material," they write.

In fact they had solved the greatest of all biological problems - how life reproduces itself. Cells are the basic units of life and it is their ability to grow and divide, and so multiply, that is the characteristic of life. It was known that DNA was the genetic material that could control the character of cells but how could this material be duplicated each time the cell divides? The answer was now clear - DNA was a double helix made up of two strings each composed of four different bases in such a way that one base could only pair with one other. So if the helix separated into two separate strings the cell could make two new double helices by adding the right bases opposite their normal partners on each string. Thus life could copy itself.

Any doubts that DNA was the genetic material, what genes are made of, were now over and scientists concentrated on the hard problem of how it worked. The behaviour of cells is determined by what proteins they have; proteins are astonishing in their variety and enable cells to move, produce energy, conduct nervous impulses. They are the workers in the cell. What DNA does is to provide the code for making proteins and so to effect the behaviour of every cell in our body. DNA is thus, by comparison, a rather passive molecule and only has an effect when the code for a particular protein is read. Mutations alter the DNA so that a normal protein cannot be made, as in cystic fibrosis and thousands of other genetic diseases. It is now possible to take the DNA from cells and analyse it and so determine the sequence of bases in a particular gene to see if it is normal or not. This is the basis of pre-natal diagnosis of genetic diseases and also can be used to screen adults to see if they have a genetic abnormality that may, for example, predispose them to cancer.

DNA can be cut up and rejoined in myriad different ways and introduced into cells. This has been used to produce essential medical agents like human growth hormone. A related approach underlies current attempts at gene therapy. For cystic fibrosis the aim is to introduce into the cells of the child a normal copy of the gene that is the cause of their disability. Again new genes can be introduced into plants to make them, for example, more resistant to pests. In principle, modifying the DNA of an animal or plant can result in the development of quite new organisms which can be beneficial to humankind, but there are of course, risks.