156 J. SOSKA, L. BENES, V. DRASIL, Z. KARPFEL, R. PALECEK AND M. SKALKA 



the specific activity of bone -marrow DNA-phosphorus was determined. 

 Under these conditions, DNA synthesis is reduced to 30 per cent of the 

 vahie obtained in non-irradiated bone-marrow. The substances to be 

 tested were added to bone-marrow suspensions in concentrations of 10 

 to 90ju.g/ml. Positive results were achieved with deoxycytidyhc acid as 

 well as with thymidylic acid, while deoxycytidine and thymidine, in 

 contradistinction to experiments i7i vivo were also effective, though to 

 a lesser degree than the respective nucleotides (Drasil et al., 1959). The 

 effect of all the above-mentioned substances on non-irradiated marrow 

 was considerably smaller than the effect on-irradiated marrow, but, 

 on the other hand, DNA synthesis in non irradiated marrow was in- 

 creased in the presence of adenosine-triphosphate or cytidylic acid. 



Using the same procedures, we concurrently tested the effectiveness 

 of chicken embryo extract. Fresh eml^ryo -extract increased DNA 

 synthesis as much as 86 per cent, while the effect of the extract heated 

 for 3 min to a temperature of 70°C was only slightly lower, no effect 

 being obtained with a 3-week old extract. An acid extract prepared 

 with 0-6 M perchloric acid was also effective. If such an extract had 

 been filtered through charcoal, the filtrate was not effective (Table I). 

 On the other hand, a pyridine eluate from this charcoal was effective 

 too. A fraction of the acid-soluble extract passing through the column 

 of Dowex 50 in H*-form was effective, while the effectiveness of the 

 fraction passing through Dowex 2, acetate, at pH 7.0, was reduced. 

 When this anion -exchanger was eluted by HCl the eluate was effective. 

 In the next experiment, the acid embryo extract was subjected to 

 fractionation on Dowex 2 — chloride according to Cohn and Carter 

 (1950). The most effective fraction proved to be that corresponding to 

 triphosphates, then to mono- and "tetraphosphates". In this experi- 

 ment, the effectiveness of the whole acid-soluble extract did not differ 

 from that of the extract in Ringer solution. 



Several experiments A\'ith bone-marrow were carried out by auto- 

 radiography. After inculcation with 32X>-phosphate or i4(^;-formate and 

 eventually with a nucleotide, the bone-marrow suspension was washed 

 with Hanks' solution, then with acetic acid and smears were prepared. 

 After exposure with a sensitive emulsion and staining with MGG-stain, 

 the number of cells that had significantly more grains than the corres- 

 ponding area of the background was determined. In the presence of 

 deoxycytidylic acid, the incorporation of ^ap.phosphate into the 

 reticular cells increased 2-5 times and 3 times in non-ii'radiated and 

 irradiated marrow res])ectively (Table II). Control experiments carried 

 out concurrently showed that under the same conditions two thirds of 

 the incorijoration of 32p into the bone-marrow could be accounted for 



