ACTION OF IONIZING RADIATIONS ON CELL CONSTITUENTS 



after 24 hours. These experiments indicate a possible effect on DNA or on 

 the nucleoproteins. 



However Euler and Hahn^^ found no akeration of physical-chemical 

 properties of deoxyribonucleoproteins extracted from irradiated thymus 

 nuclei ; but unfortunately no quantitative recovery was attempted ; also 

 at the time of the experiment the methods of extraction were perhaps not 

 so gentle as they are now. Anderson^ has recently shown that the high 

 radio-sensitivity of DNA was a test for its degree of depolymerization, which 

 decreases very rapidly in homogenates. More recently, Kaufman^^ found 

 no decrease in the affinity for methyl green of the nuclei of irradiated onion 

 root tips but there is an increase in affinity for fast green and the author 

 thinks this is probably the reason for the smaller degree of swelling of irradi- 

 ated cell nviclei when they are placed in trypsin solution. 



It must be emphasized that in all these data, tending to show that DNA 

 or nucleoproteins may be affected a very short time after irradiation, most 

 of the evidence so far obtained is very indirect. One method which might 

 give interesting results would be to compare the radio-sensitivity (based on 

 chemical or physical tests) of DNA irradiated dry and at low temperature 

 where indirect effects are reduced to a minimum, in vitro and in organisms 

 like bacteria. One must not forget that at least for DNA, biological tests 

 are much more sensitive to minute alterations of the molecule than physical- 

 chemical ones. This appears clearly from the work of Zamenhof^^ on the 

 stability of the transforming factor ; the study of the transforming activity 

 of DNA extracted immediately after irradiation of the bacteria would indi- 

 cate perhaps if an immediate alteration of DNA is to be found. Actually the 

 results presented in this symposium by Ephrussi-Taylor and Latarjet indicate 

 that such an immediate alteration does occur. 



Some more direct evidence can be extrapolated from work done with 

 bacteriophage. The inhibition of phage multiplication when infected 

 bacteria are irradiated does seem to indicate that the phage itself is altered 

 because normal phage is capable of multiplying in heavily irradiated 

 bacteria. That unmodified DNA is necessary for phage multiplication is 

 proved by an experiment of Hershey where phage is inactivated by the 

 transmutation of ^^P atoms of its nucleotides^^. 



Biological effects become apparent only where some cellular constituents 

 are ' limiting ' : biochemical mutations in micro-organisms can be observed 

 only when some important metabolic pathway is interrupted ; this occurs 

 if some building block, normally synthesized by the enzyme lost as a con- 

 sequence of the mutation, is not available for the cell. In more complex 

 organisms it is possible that a chain of reactions may be blocked by radiations 

 in one area but that the products continue to be supplied from another site 

 which is less radio-sensitive. In this case no radiation effect may be 

 observed. To produce observably biological, biochemical, or biophysical 

 lesions the conditions must be such that the factor studied is ' limiting '. The 

 experiments on glycolysis of Sherman, mentioned previously, support this view. 



The influence of the ' ploidy ' of a cell on its radio-sensitivity is another 

 illustration of this concept. Latarjet and Ephrussi^^ have observed that 

 haploid yeasts are killed exponentially whereas if they are diploid the killing 

 curve becomes sigmoid. A similar effect was found by Zirkle and Tobias''^ : 



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