858 ADVENTURES IN RADIOISOTOPE RESEARCH 



having no opportunity, cell division being obstructed, to be distributed 

 between the mother and a daughter cell, the cell swells. When the cell 

 recovers its DNA synthesizing and dividing capacity it may divide, 

 but the cell being abnormal this process often has lethal consequences. 

 That exposed cells often die when trying to divide has been known for 

 many years and was emphasised, for example, by Lea in his classical 

 book Actions of Radiations on Living Cells. 



As shown by Howard and Pelc (1953) exposure to radiation can also 

 interfere with mitosis in a stage of the mitotic cycle in which synthesis 

 of DNA is already terminated. Thus we meet a second type of mitotic 

 interference which may lead to cell death or to the formation of abnor- 

 mall cells. 



In a recent investigation of Forssberg (1956), during the first two 

 post-irradiation hours the incorporation of glycine- 2-^*0 into DNA of 

 ascites tumour cells was found to be reduced to 70 per cent of that into 

 non-irradiated controls. In the following amitotic period no further 

 reduction in the specific activity of DNA took place. A slight increase 

 in these values was in fact observed. It is well-known that after moderate 

 doses the division of such cells as are in a sensitive stage gets retarded 

 while those in a less sensitive stage proceed through their cycle at a 

 normal rate. This may result in a piling-up of cells which were to enter 

 mitosis. When at a later stage the cycle of the last-mentioned cells sets 

 in again, the mitotic index increases as compared with unirradiated 

 material. No indications of an increased mitotic activity were found in 

 Forssberg's experiments, but steps preceding such an activity may have 

 taken place involving slightly increased DNA formation. It is the radi- 

 ation effect mentioned above which does not involve interference with 

 DNA formation that may have prevented the manifestation of the 

 mitotic process. 



Autoradiographic studies 



r Our knowledge of the part of the mitotic cycle in which synthesis of 

 DNA takes place and of the radiation sensitivity of various parts of 

 this cycle was very much enlarged through the work of Howard and 

 Pelc (1951, 1953 and 1956) who introduced autoradiographic methods 

 in the study of ^^P incorporation into DNA present in the meristem cells 

 of main roots of Vicia faba seedlings. They found the normal mitotic 

 cycle in the meristem to take 30 hours, of which four are spent in division, 

 1 2 between the end of division and the beginning of ^^P uptake into new 

 DNA, six in DNA synthesis as judged by ^^p uptake, and eight between 

 the end of synthesis and the beginning of division. The number of cells 

 synthesizing DNA is reduced by exposure to 50 r to about 60 per cent 

 of the normal value during the subsequent 12 hours. Increase of the 



