M O LE C 1 LAB I ONFIGURATION OF NUCLEIC ACIDS 



tnond Gosling, who bad our only X-ray equipment (made from war-surplus 

 radiography parts) and who was using it to obtain diffraction photographs 

 from heads of ram spermatozoa. This research was directed by Randall, who 

 had been trained under W. L. Bragg and had worked with X-ray diffraction. 

 Almost immediately, ( rosling obtained very encouraging diffraction patterns 

 (see fig. i ). One reason for this success was that we kept the fibres moist. We 

 remembered that, to obtain detailed X-ray patterns from proteins, Bernal 

 had kept protein crystals m their mother liquor. It seemed likely that the 

 configuration of all kinds of water-soluble biological macromolecules would 

 depend on their aqueous environment. We obtained good diffraction pat- 

 terns with ON A made by Signer and Sch wander 4 which Singer brought to 

 London to a Faraday Society meeting on nucleic acids and which he gener- 

 ously distributed so that all workers, using their various techniques, could 

 studv it. 



Realization that the Genetic Material was a Pure Chemical Substance and 

 Signs that its Molecular Structure was Singularly Simple 



Between 1946 and 1950 many lines of evidence were uncovered indicating 

 that the genetic substance was DNA, not protein or nucleoprotcin. For 

 instance, it was found that the DNA content of a set of chromosomes was 

 constant, and that DNA from a given species had a constant composition 

 although the nucleotide sequence in DNA molecules was complex. It was 

 suggested that genetic information was carried in the polynucleotide chain in 

 a complicated sequence of the four nucleotides. The great significance of bac- 

 terial transformation now became generally recognized, and the demonstra- 

 tion by Hershcy and Chase 5 that bacteriophage DNA carried the viral genet- 

 ic information from parent to progeny helped to complete what was a 

 fairly considerable revolution in thought. 



The prospects of elucidating genetic function in terms of molecular struc- 

 ture were greatly improved when it was known that the genetic substance 

 was DNA, which had a well-defined chemical structure, rather than an ill— 

 defined nucleoprotcin. There were many indications of simplicity and reg- 

 ularity in DNA structure. The chemists had shown that DNA was a polymer 

 in which the phosphate and deoxyribose parts of the molecule were regularly 

 repeated in a polynucleotide chain with 3 '-5' linkages. Chargaff () discovered 

 an important regularity: although the sequence of bases along the poly- 



s-34 



