774 ADVENTURES IN RADIOISOTOPE RESEARCH 



The yield of pure nucleic acid was 31 mgm whereas the weight of the 

 crude amounted to 160.8 mgm (No. I). 



The purification of nucleic acid with respect to added active phospha- 

 tides was checked in a similar way. An emulsion of radioactively labelled 

 phosphatides was added to the sarcoma extract, and the activity of the 



Table 1. — Purification of Crude 

 Nucleic Acid with respect to Added 

 Acid-Soluble Phosphorus Compounds 



nucleic acid obtained after three purifications with methanol and hydro- 

 chloric acid was investigated. About 10^ units of activity were added; 

 the 15 mgm of nucleic acid isolated in this experiment had an activity not 

 exceeding 1 unit. The radioactively labelled phosphatides were prepared 

 from various organs of rats which had been injected 4 hr previously 

 with radioactive phosphate. 



DETERMINATION OF THE SPECIFIC ACTIVITY OF THE FREE 



PHOSPHATE 



If the free phosphate takes a direct part in the synthesis of radio- 

 active nucleic acid molecules, the ratio of the specific activity in the 

 nucleic acid P to that in the free P is a measure of the nucleic acid mole- 

 cules newly formed during the course of the experiment. If a phosphate- 

 containing intermediate product, rather than free phosphate, enters the 

 nucleic acid molecule, it makes no difference to the above inference. 

 The ratio quoted above would not be an exact measure of the newly 

 formed molecules of nucleic acid if the intermediate product were syn- 

 thesized more slowly than the nucleic acid. In this event, the synthesis 

 of nucleic acid molecules containing no active phosphate would take 

 place for some time in a medium containing active phosphate. There is, 

 however, no evidence in support of this last-mentioned case. 



