184 KARL G. LARK 



initiated by means of previous thymine starvation. Tliis material was 

 characterized by a greater susceptibility to DNase than normal DNA. 

 The kinetics of DNase action were interpreted as evidence that the 

 molecule contained half of the number of component strands usually 

 present (Cavalieri and Rosenberg, 1961a). In all of its characteristics 

 it resembled DNA obtained at other times from the same organism but 

 heated in cesium chloride. On the basis of their findings these workers 

 suggested that there exist two types of DNA molecules during the 

 rejilication cycle: before DNA synthesis the molecule is cleaved and 

 subsecjuently rebuilt during the replication process. Further studies of 

 this type would be desirable, since no evidence was obtained to indicate 

 whether or not the observed change in state of the DNA was reversed 

 following DNA synthesis. Evidence was presented to show that DNA 

 existed in a different state in non-proliferating as compared to dividing 

 cells (Cavalieri and Rosenberg, 1961b). 



Indirect evidence for a change in the state of DNA during its repli- 

 cation has been obtained by a study of the action spectrum for the 

 inactivation of bacteriophages by ultraviolet light. Studies on the 

 inactivation of viruses believed to contain a single- as opposed to 

 double-stranded DNA molecules have demonstrated that those with 

 single-stranded DNA have a different action spectrum from those with 

 two-stranded DNA. A shift in the action spectrum of two-stranded 

 DNA virus to that characteristic of a single-stranded DNA virus occurs 

 following host infection and initiation of virus replication. As virus 

 multiplication proceeds this spectrum shift is reversed (Setlow, 1960; 

 Setlow and Boyce, 1960; Setlow and Setlow, 1960). These results have 

 been interpreted as evidence that during replication viral DNA passes 

 through a single-stranded or "denatured" stage. 



A change in the physical state of intracellular bacterial DNA im- 

 mediately following replication has been observed by Goldstein and 

 Brown (1962). They found that newly synthesized DNA is more 

 resistant to sonic disintegration than the bulk of cellular DNA. The 

 nature of their experiment excluded a determination of whether this 

 state was achieved prior to replication. 



Recent analysis of DNA isolated at intervals during the division 

 cycle of synchronized bacteria (.4. faecalis) has revealed the presence 

 of small amounts of material whose density corresponds to that of 

 heated or denatured DNA. The amount of material observed (which is 

 sensitive to DNase and can be heat renatured) is usually less than 10% 

 of the total DNA content of the culture (Lark, Cavalieri, and Rosenberg, 

 unpublished results) . 



Much less, or no such material, is found in cultures of randomly 

 dividing cells. The "denatured DNA" arises when the cells are lysed with 



