454 



INACTIVATION OF BACTERIOPHAGES 



Crick (1953), of which a schematic diagram is presented in Fig. 5. This struc- 

 ture reveals DNA as a double helix composed of two intertwined polynucleotide 

 chains of opposite polarity held together laterally by specific hydrogen bonds 

 between purine and pyrimidine bases of opposite strands. The radioactive 

 P^2 atoms are located in the diester bonds responsible for the continuity of 



interru 

 ch 



on-lethol 

 decay 



nterrupted 

 ain 



Fig. 5. Schema of the Watson-Crick structure of DNA. The two ribbons sym- 

 bolize the two phosphate- sugar chains, and the horizontal rods represent the pairs 

 of bases holding the chains together through a pair of hydrogen bonds. The breaks 

 in the ribbons indicate the spontaneous interruptions of the polynucleotide chains 

 proposed by Dekker and Schachman. 



the polynucleotide chains. It appears almost inevitable that every ester linkage 

 is destroyed upon decay of its radioactive phosphorus atom. First of all, the 

 maximum recoil sustained by the phosphorus nucleus is of the order of 80 ev. 

 (the average value being somewhat lower owing to the random orientation 

 of neutrino and beta electron), whereas the energy of theP-0 bond holding the 

 atom in place is less than 5 ev. The ester bond is, therefore, probably broken 

 by the Szilard-Chalmers reaction {cf. Libby, 1947). Secondly, even if the 



293 



