In this paper we shall survey the"standard"history of molecular genetics with the aim of discovering important conceptual transformations. We shall identify the most striking of these with assumptions about the direction in which "information" travels. What do we gather from Phage and the Origins of Molecular Biology I and The Double Helix? 2 We learn the central importance of the phage group led by Delbriick and Luria. This group focused attention on information as opposed to mere structure. Its members worked with a system -the phage-infected cell and from their work it emerged that DNA rather than protein might be the genetic substance. J. D. Watson was therefore sent to Europe to learn nucleic acid chemistry; he saw M. H. F. Wilkins' X-ray pictures; he decided to go to Cambridge to learn the structural crystallography of biologically important molecules and met F. H. C. Crick. They collaborated on the structure of DNA and at the second attempt they got out the structure. This suggested that the gene replicated, not by long-range forces, not by curiously resonating structures, not by stereochemical fit between like amino acids, but by hydrogen-bonding between purines and pyrimidines. The Watson-Crick model showed that the specificities of the gene could be described in terms of the mere one-dimensional sequence of bases in a giant DNA molecule. The stage was thus set for the analysis of the fine structure of the gene down to the level of the bases strung 3.4 A apart. This was many orders of magnitude smaller than the minimum target volume calculated from radiation genetics in the 1930's. In truth molecular genetics was born.
Is this picture broadly correct? Has a revolution taken place around the pivotal point of the structure of DNA and the methodology of the phage system? Or does the history of molecular genetics represent a smooth linear development in which the divisibility of the gene fight down to its molecular sub-units was progressively approached? To ex-