DNA Synthesis 



145 



it after 20 hours in ^sp (Howard and Pelc, 19516). This would 

 be expected if the ^^S is present in protein attached to DNA 

 and if the two parts of the nucleoprotein entity are syn- 

 thesized at the same time in the nuclear cycle. 



lOO 



80 



7 



/o 

 ARG 



60- 



40- 



20- 



III. Effects of X-Radiation 



Roots which w^ere treated as described above were given 

 140 r. of X-rays (190 kV., | mm. Cu +1 mm. Al) immediately 

 before a 24-hour period in ^^P. 

 These roots showed a marked 

 reduction in DNA synthesis 

 when compared with unirradi- 

 ated controls (Fig. 9). The 

 number of positive cells was re- 

 duced in all classes, and also 

 the strength of autoradiograph 

 which appeared over positive 

 nuclei (about 24 grains per nuc- 

 leus in irradiated roots compared 

 with about 65 in unirradiated). 



In contrast with this obser- 

 vation, we have not, in pre- 

 liminary experiments, been able 

 to detect any effect of a similar 

 dose of X-rays on the nuclear 

 uptake of ^^S during 20 hours. 

 If nuclear uptake of ^^S is due 

 to synthesis of nucleoprotein, this means that this protein 

 can be synthesized in the absence of DNA synthesis. 



n 



/ 

 / 



/ 



; 



/ 

 / 

 / 



/ 



/ 



/ 



/ 

 / 



/ 



/ 

 / 

 / 

 / 

 / 

 / 

 / 



i 



/ 

 / 



/ 



/-, 



/ 

 / 



AC MR D 



Fig. 9. Percentage of cells showing 

 autoradiograph after 140 r. X-rays 

 followed by 24 hrs. ^^p. Hatched 

 columns: unirradiated controls. 

 Empty columns: irradiated. 



AC. All cells in the final 2 mm. 

 of the root. 



Meristematic resting nuclei. 



Dividing cells. 



MR 

 D. 



Discussion 

 1. Time of Synthesis of DNA 



While earlier work (Caspersson, 1939; Ris, 1947) suggested 

 that DNA is synthesized by cell nuclei during prophase, three 

 authors have recently put forward evidence to the contrary. 

 All have used photometric analysis of Feulgen stained 

 material to provide relative estimates of the amount of DNA 



ISOTOPES 



11 



