116 CELL HEREDITY 



is present as fibers not encased in a protein coat. It is clear that these 

 fibers, which contain phage genes, multiply exponentially as though each 

 newly formed one acts as a template for the formation of another. We 

 draw this conclusion from the fact that the number of phage mutants 

 formed in different cells is distributed according to the calculations of 

 Lea and Coulson, which assume the production in series of two particles 

 from one (see Table 2.1). During the last part of the latent period, 

 mature infectious phages are assembled from the separately multiplying 

 DNA, protein coats and tails. By prematurely disrupting the cell, these 

 intracellular phages can be found to increase arithmetically as though 

 during maturation the pool of exponentially multiplying DNA were 

 tapped by some process occurring at a constant rate. The DNA that gets 

 into the newly formed phages cannot replicate and, as a consequence, 

 the over-all rate of increase of DNA in an infected cell becomes arith- 

 metic. 



It is most interesting that the bulk of this phage DNA seems to pass 

 intact from generation to generation. Levinthal used an autoradio- 

 graphic technique to demonstrate this. Phage DNA was labeled with 

 P'^"^ (by growth on P'^^-grown bacteria) and separated by osmotic shock 

 from the phage protein fractions. Both intact and disrupted prepara- 

 tions were plated in a photographic emulsion where the P was allowed 

 to disintegrate before the film was developed. Each piece of radioactive 

 DNA was a focus for the emission of /3 rays, detected by the microscopic 

 observation of tracks of exposed silver grains caused by the passage of 

 fast electrons. The more P'^^ in a DNA particle, the more disintegra- 

 tions during the exposure period, and for every disintegration there is one 

 ionization track. Therefore, if 25 disintegrations occurred in a piece of 

 DNA, a star pattern with 25 arms would result in the film (Figure 5.2). 



It was observed that the DNA released from the phage coat formed 

 stars with about 40 per cent of the number of arms found when intact 

 P'^^-labeled phages were plated in the emulsion (Table 5.1). Therefore, 



TABLE 5.1 



Number of P'^'^ Atoms in the Star-Forming Particles Relative to the Number in 



the Intact T2 Phages which is Corrected to 100 



(From Levinthal, 1957, in McElroy, and Glass The Chemical Basis of Heredity. 

 Baltimore: The Johns Hopkins Press, p. 737). 



Intact Phage Particles After Osmotic Shock 



