136 BARBARA E. HOLMES 



We have not nearly enongh data to make any statement abont the 

 radiosensitivity of the formation of this ]iarticular protein. We can, 

 however, say that in many individnal animals, the X-rays have been 

 sufficient to cause a sharp inhilntion of nucleic acid synthesis without 

 causing any inhibition of synthesis of this particular protein fraction, 

 which in normal cells apparently occurs at the same time as the nucleic 

 acid synthesis. The effect of X-rays appears therefore, to cause some 

 dislocation in the usual relationship between the two. 



The work of Richards (personal communication), who has made 

 measurements on individual cells, is in agreement with the idea that 

 the rate of ])rotein formation is not depressed together with the rate 

 of deoxy nucleic acid formation. 



After these diversions one must, however, return to a considera- 

 tion of the effect of X-rays on various nuclear structures, since there 

 is no doubt of the great importance of these effects in dividing cells or 

 resting cells that may later come into division. 



The nucleolus may, perhaps be considered as the prime target for the 

 inhibition of synthesis in the nucleus at some stages of the mitotic 

 cycle. Seed, (1900), in our laboratory, irradiated the three nucleoli in the 

 nuclei of mouse heart fibroblasts with about 500 rad X-rays. The 

 X-rays were collimated into a thin pencil 1// in diameter. Two daughter 

 cells were chosen from a previous division; one of them was used as a 

 control and the other irradiated about 2 hr after the division. The pro- 

 gress of DNA synthesis in these nuclei was followed by taking an ultra- 

 violet photograph after the chosen period of time had elapsed. It was 

 found that synthesis had occurred in the control nucleus but not in the 

 irradiated nucleus at times of 3 to 7 hr after the irradiation. 



If we wish to speak of an initial effect we should obviously have 

 grounds for considering it to occur immediately after irradiation ; in a 

 technique such as Dr. Seed's this cannot be demonstrated. However, 

 Gaulden and Perry (1958), working at Oak Ridge, have been able to 

 demonstrate immediate cessation of mitotic progress after nucleolar 

 irradiation with u.v. light in very early pro])hase. Later stages of 

 mitosis were not so sensitive to nuclear irradiation so that here we 

 may consider oui- effective target to have changed. 



The nature of the X-ray injury to the nucleoprotein complex of the 

 cell has not proved easy to detect. The extraction of nucleoproteins 

 from irradiated and unirradiated cells for comparison of their physico- 

 chemical properties is not al)solutely satisfactory because such small 

 changes in technique cause variations in the extracted products. The 

 control sam])le may show different viscosities, different abilities to 

 form gels and so forth if the speed or duration of homogenization of the 



