200 Alma Howard 



years. It is now clear that synthesis of DNA is a function of the 

 mitotic cycle. This is inherent in the fact that in any given 

 species, a fixed amount of DNA is associated with each chromo- 

 some set, so that, with due allowance for differences in ploidy 

 and for periods of synthesis, each nucleus contains a constant 

 amount of DNA. This means that each cell must double, but 

 no more than double, its content of chromosomal DNA during 

 every interphase that is to be followed by a mitosis. Further- 

 more, the time period in interphase occupied by this synthesis 

 appears to be fixed for any given cell type. As far as we 

 know, no other component of the cell behaves in this fashion 

 as regards amount per cell or dependence on the mitotic 

 cycle, so that DNA synthesis might be expected to be unique 

 in its response to radiation-induced changes in that cycle. 



Radiation -Induced Changes in Cell Populations 



The changes in the cell population that result from irradia- 

 tion of growing tissues arise in the following ways : 



(1) Delay in entry of cells into and progress through mitosis, 

 expressing itself as a shift in the proportion of cells in various 

 stages of the mitotic cycle. Larger doses cause longer delays. 

 The sensitive period for delay is just before visible prophase 

 (in the grasshopper neuroblast, during prophase). Recovery 

 after moderate doses is characterized by a temporary increase 

 in the number of cells in mitosis due to the release of those 

 delayed. In the most favourable material, the delaying 

 effect of 4 r can be observed (Carlson, 1948). In many other 

 tissues, delays are known to result from very moderate 

 doses. In some such cases, protein synthesis, RNA turnover, 

 and increase in cell volume and dry weight all appear to be 

 unaffected (Klein and Forssberg, 1954). 



(2) Death of cells. 



(a) Due to physiological or morphological changes in the 

 chromosomes. Such changes may result in death of cells at 

 metaphase or anaphase of the mitosis following irradiation or 

 later, usually during the following interphase, due presumably 

 to loss of genetic material. The rate at which cells die in this 



