On 30 May 1953 James Watson and Francis Crick published in Nature a correct interpretation of the crystalline structure of deoxyribonucleic acid, DNA. It was a great discovery, one which went far beyond merely spelling out the spatial design of a large, complicated and important molecule. It explained how that molecule could serve genetic purposes -- that is to say, how DNA, within the framework of a single common structure, could exist in forms various enough to encode the messages of heredity. It explained how DNA could be stable in a crystalline sense and yet allow for mutability. Above all it explained in principle, at a molecular level, how DNA undergoes its primordial act of reproduction, the making of more DNA exactly like itself. The great thing about their discovery was its completeness, its air of finality. If Watson and Crick had been seen groping towards an answer; if they had published a partly right solution and had been obliged to follow it up with corrections and glosses; if the solution had come out piecemeal instead of in a blaze of understanding: then it would still have been a great episode in biological history, but something more in the common run of things; something splendidly well done, but not done in the grand romantic manner.
The work that ended by making biological sense of the nucleic acids began forty years ago in the shabby laboratories of the Ministry of Health in London. In 1928 Dr Fred Griffith, one of the Ministry's Medical Officers, published in the Journal of Hygiene a paper describing strange observations on the behaviour of pneumococci -- behaviour which suggested that they could un-