STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 111 



e.g., irradiated cells incapable of division may be induced to synthesize a 

 new enzyme, RNA viruses and the infectious RNA from these viruses can 

 also be inactivated by irradiation. It must be asked, therefore, whether 

 damage of this component does not play an unsuspected role in cellular 

 damage. Kelly (1952) has reported that in X-irradiated animals, there is a 

 depression of P^^ uptake into nuclear RNA, without a similar effect on 

 cytoplasmic RNA. One would evidently wish to know the relative sensi- 

 tivities of nuclear RNA and DNA and the sensitivity of DNA and RNA as 

 a whole. 



E. Unbalanced Growth 



In addition to the instances of radiation damage noted above, a number 

 of other pathological situations have been provoked in experimental systems 

 which have been instructive in revealing the extent to which cell functions 

 can be separated. AVe ■\^^I1 briefly consider four of these. 



1. Thymineless Death 



It was discovered initially that when a thymineless strain of E. coli strain 

 15t" was grown at 37°C. in an aerated glucose-mineral salts medium in 

 limiting amomits of thymine, it was not possible to establish a simple relation 

 between cell number and mass of the culture (Earner and Cohen, 1954). When 

 incubated in the presence of a carbon source and essential salts and in the 

 absence of thymine, the ceUs lost the ability to produce colonies ("died") on 

 thynune-containing medium at the rate of about 90 % per division time. The 

 ceUs did not die in the absence of a carbon source, and appeared to have 

 to metabolize and grow in order to produce this irreversible change, the 

 pattern of which is presented in Fig. 20. Synchrony may be produced in such 

 a culture if thymine is added just before the cells begin to lose the ability to 

 multiply; at this moment they are maximally phased (Earner and Cohen, 1956). 



Such ceUs made RNA and protein, but only traces of DNA, and became 

 quite enlarged. After many hours they finally lysed. If thymine is added to a 

 "dead," unlysed culture, the cells make considerable amounts of DNA but 

 are not resuscitated (Earner and Cohen, 1956). Thymine-deficient cells can be 

 induced by xylose and other substrates to produce new enzymes (Cohen and 

 Earner, 1955), mdicating that, as with irradiated cells, DNA synthesis is not 

 essential to this type of enzyme synthesis. Although it may be suggested from 

 this type of result that such synthetic functions are localized in the cytoplasm, 

 this problem should be more cleanly answered by studies with enucleate 

 Acetahularia or Amoeba} 



^ Although Brachet has stated that enucleate Acetahularia did on occasion produce 

 catalase in response to the presence of H2O2 in the medium, he has more recently re- 

 marked that these results have been difficult to reproduce, 



VOL. I — 9 



