DNA AND Effects of Radiation and Peroxides 293 



This dualism is not due to genetic heterogeneity, since DNA 

 extracted from bacteria transformed by the resistant units 

 gives the same broken curve. It is not due to aggregation, as 

 will be seen below, since urea, which breaks the aggregates, 

 does not change this curve. 



Dr. Ephrussi-Taylor suggests that an explanation might be 

 looked for in two different mechanisms for inactivation. The 

 sensitive one would damage some structure of the molecule- 

 fibre which is needed in one step of the transformation process, 

 such as a successful integration in the bacterium. The 

 resistant one would damage the fundamental structure actually 

 endowed with the biological specificity. This structure should 

 thus be exceedingly small. 



(2) Aggregation and energy transfer. We used two samples 

 prepared by Dr. Simmons, in which DNA was aggregated. 

 The first one (Bl) gave an inactivation curve (Fig. 6, curve 1) 

 with a long plateau and a slope similar to the initial one of 

 "normal" TP (curve 2). We can consider in this case that 

 each aggregate, although containing a number (10-20) of units, 

 acts as one transforming particle when brought into contact 

 with a sensitive bacterium. The type of aggregation is such 

 that each unit is individually inactivated by radiation, i.e., that 

 energy transfer, if it exists, is insufficient to spread throughout 

 the aggregate. When this preparation was treated with 5 

 M urea, its titre rose, its viscosity increased, and its inactiva- 

 tion curve became similar to curve 2 of normal TP. 



The second sample (B2) was actually the centrifugation 

 residue of the preparation, the supernatant of which had 

 formerly been used as normal TP. It gave the inactivation 

 curve 3 of Fig. 6, similar in shape to that of normal TP, but 

 with a two- to threefold increased sensitivity. When treated 

 with urea, this sample too gave the normal curve 2. We may 

 consider that there exists a second type or degree of aggrega- 

 tion, of lower multiplicity than the first one (2-3 normal 

 units), which is also dissociated by urea, but within which 

 radiant energy migrates in such a fashion that the whole 

 cluster may be inactivated by a single hit occurring within 



