40. RADIATION AND NUCLEIC ACID METABOLISM 531 



irradiation were in their S period will have completed it (assuming that the 

 dose of radiation did not hold up or delay cells in the S period). In the sub- 

 sequent short labeling period (1-2 hours labeling) the cells which will col- 

 lect the label will have been in their Gi period at the time of irradiation, 

 and such experiments can measure the radiation effect on the presynthetic 

 period (Gi effect). 



In such experiments, however, it is vitally important that there should 

 be no cell death, and/or cell removal, especially selective cell removal. No 

 in vivo experiments would satisfy the necessary conditions, as emigration 

 and immigration of cells may occur (apart from efficient local destruction 

 of cells) without being detected. Under such in vitro conditions as presented 

 by cell suspensions, however, sufficiently accurate total cell counts and 

 absolute differential counts can be made to exclude such changes. 



5. Pool Changes 



Irradiations may produce cell death within minutes. Cells in mitosis, and 

 probably in interphase too, may be destroyed (especially with doses of a 

 few thousand roentgens); it is known that even after small doses of whole 

 body irradiation the lymphocyte count drops quickly (which may be the 

 result of trapping or destruction of these cells). Also it is known that nu- 

 cleases (deoxyribonuclease, ribonuclease) are liberated by irradiation. Al- 

 though the exact fate of DNA and RNA breakdown products is not known, 

 there is good evidence that there is an efficient recirculation of the purine 

 components 5 ; consequently, there is a good possibility for a temporary in- 

 crease of precursor pool size whenever cell destruction takes place. Espe- 

 cially experiments in which very high specific activity label is used (such 

 as thymidine-H 3 with specific activities ranging from 360 hc//jM to 1.9 

 mc.//xM) the possibility of pool size changes must be considered. The point 

 may be illustrated by the following example: If thymidine-H 3 (360 mc/juM) 



5 4N 



o 2N 



lO 20 



A/ours 



Fig. 1. Scheme of cell cycle with respect to DNA synthesis. S represents the period 

 of DNA synthesis; d , the first (presynthetic) gap; G 2 , the second (postsynthetic) 

 gap; M, mitosis. 



