THE MECHANISM OF INHIBITION OF CELL DIVISION 213 



activity may be brought about most easily by irradiation througliout 

 the interkinetic period reveals the ini])ortauce of damaged synthetic 

 processes in the radiation inhibition of mitosis. In mentioning the 

 synthetic processes we should indicate first J3NA synthesis. Thus, 

 ionization hi the biochemical volume may lead to the appearance of a 

 retardation in the rate of the synthetic process, this fact may be viewed 

 as one cause of radiation damage to mitosis. At present it is hard to 

 identify the damage to the genetic apparatus \\ith any other definite 

 phenomena. In any event one may observe the blocking of mitosis 

 under conditions of sufficient DNA supply, it may also then depend 

 upon damage to the chromosomal apparatus. 



At any rate, the blocking of the processes of biochemical synthesis or 

 damage to the genetic mechanism leads to a decrease in the intensity of 

 the cell division process K; this fact is clearly observed post radiation. 

 The decrease of ii is not always easily deciphered. However, suitable ex- 

 perimental conditions may be produced and they may provide con- 

 ditions for the restriction of the reaction to definite cell areas. 



Similar experimental conditions are produced w hen exponential cell 

 populations are exposed to irradiation i.e. when there is not a limiting 

 factor effecting cell propagation (insufficient nutrient material and ex- 

 cessive metabolic processes). In this case all phenomena relating to 

 reaction to radiation may he considered from a practical standpoint, as 

 inti^acellular effects. 



The study of intracellular effects of radiation is a matter of first rate 

 importance. It may be solved by means of statistical analysis, having 

 estimated the possible changes of K during the process of inhibiting 

 mitotic activity and its restoration to its unchanged form. The action 

 of radiation may be also studied via pathological mitosis. To simplify 

 our reasoning we supposed that there are only two reasons for in- 

 hibition of mitotic activity : (i) blocking of the synthesis of biochemical 

 complexes required for mitotic activity and (ii) blocking of genetic 

 mechanisms. Some of the cells in the irradiated cell population may be 

 unchanged whereas in others the processes of biochemical synthesis 

 are blocked; in a third group both synthetic processes and genetic 

 activity are blocked. The blockade of genetic mechanisms occurs 

 immediately and is of irreversible nature, and a cell with a damaged 

 genetic mechanism perishes after a single and sole division. As has al- 

 ready been indicated these assumptions enable us to determine the 

 course of inhibition of mitotic activity and its restoration. The curves 

 obtained may be compared with the experimental data, and if they 

 coincide they testify to the reality of the initial assumptions i.e. they 

 may serve as material for the analysis of experimental data. 



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