THE MECHANISM OF INHIBITION OF CELL DIVISION 



217 



doses of 1 ,400 r and 1,920 r the decrease of specific activity in DMA was 

 less pronounced. 



It is of interest that a dose of 030 r caused a marlvcd rethiction in the 

 mnnber of cell nuclei (19 per cent). The next exposure leads to the 

 restoration or even increase in nuclear number. 



2-4 



^ 



'm. 



■s. 



(a) 



(b) 



Fig. 3. — The number of dividing E. coli cells in irradiated cultures against the background 

 of a blocking of protein synthesis and inhibition of nucleic acid sj'nthesis (in percentage 

 of unirradiated control). 



(a) irradiation under normal growth conditions; 



(b) irradiation against the background of a block in protein synthesis and inhibition 

 of nucleic acid synthesis. 



Very similar data were obtained in studies of nucleic acid changes in 

 bone-marrow. At first, repeated exposui"es to doses of 30 r failed to 

 produce a pronounced change in the nucleic acid content of bone- 

 marrow. The total dose of 900 r led to considerable decrease of PNA-P 

 and DNA-P, the latter increased when the dose was 2,500 r. These 

 data, of course, do not deny the prime importance of DNA synthesis 

 for mitotic activity. 



One of the reasons for the absence of relationship between the change 

 of the DNA content and the exposure dose may be the fact that the 

 exposure of the entire organism involves rather powerful extracellular 

 factors. Indeed, the present day science has accumulated a sufficient 

 number of facts demonstrating the controlling influences of nervous 

 and humoral effects upon DNA synthesis and these materials require 

 certain corrections to the cited scheme of distribution. 



There are several experiments in favour of this point. We shall cite 

 only one taken from Pozdniakov's work who showed the increase of 

 DNA decomposition in connective tissue under the effect of humoral 

 and nervous influences (Lebedinsky et al., 1960). 



Plate I shows the normal fluorescence of rabbit conjunctiva (acridine 



