DNA Synthesis 139 



I. Synthesis of DNA 

 Method 



Roots were grown for periods of time from 2 to 48 hours in 

 water containing ^^p as NaH2P04, with 16 mg. per htre of 

 added carrier phosphate. The activity was such that the 

 product concentration X time was constant at 4-8: thus roots 

 grown for 24 hours were put into a solution containing 0-2 

 /xC. per cc, while roots treated for shorter periods received 

 relatively higher activity. An exception w^as made in the 

 case of roots treated for 36 and 48 hours: these received an 

 activity equal to that of the 2 4 -hour sample. 



After treatment the roots were fixed in alcohol-acetic acid 

 (3:1), washed in water, hydrolysed in N. HCl at 60°C. for 

 10 minutes, washed in water, and portions squashed in 45 

 per cent acetic acid. After removal of the cover slip and 

 further water washing, the slides were autoradiographed by 

 the stripping-film method (Doniach and Pelc, 1950). Exposure 

 was from 2 to 28 days. After development and fixation, 

 slides were mounted in chrom- jelly and observed with phase 

 contrast. 



Controls 



1. Autoradiograph. Material treated as above showed 

 definite autoradiograph over some nuclei (Fig. 1). Material 

 treated in an exactly similar manner but without ^^P showed 

 no autoradiograph. 



2. Growth Rate. Roots were treated for 48 hours with 0-2 

 jLtC. per cc. of ^^P, and then grown in carrier without further 

 ^^P for 10 days. These roots grew as fast throughout the 

 12-day period as did inactive controls. We may therefore 

 conclude that ^^P at the activities used does not cause radia- 

 tion damage sufficient to aff'ect the mitotic rate of the 

 meristem. 



3. Interpretation of Nuclear Autoradiograph. The phos- 

 phorus compounds which may be expected to remain in the 

 cell nuclei after fixation and hydrolysis as described above 



