154 J, SOSKA, L. BENES, V. DRASIL, Z. KARPFEL, E. PALECEK AND M. SKALKA 



secondary character and that the inhibition of synthesis of the DNA- 

 precursors was primary. (Soska et al., 1958; 1959). 



A^ 



<L) 



(a) 



(b) 



Fig. 1. — The mitotic index in the bone marrow of mice (total-body irradiated with a dose 

 of 600 r) on the sixth day after irradiation (a) and on tlie fifth day after the administra- 

 tion of nucleic acid derivatives (b). The lieight of the colunms corresponds to tlie mitotic 

 index, expressed as number of mitoses per 1,000 cells. Tlie horizontal line with the indi- 

 cation 100 per cent corresjDonds to the mean mitotic index in control irradiated mice, 



injected with saline only. 



The effect of some components of the micleic acids on the mitotic 

 index of the l)one marrow of mice on the sixth day after irradiation 

 with a dose of 500 r \^'as compared. The snbstances snbjected to trial 

 were injected intraperitoneally 24 hr after irradiation in a dose eqniva- 

 enttoO-3 mg of deoxycytidyhc acid. A statistically significant increase 

 of the mitotic index, as compared with the irradiated controls, was 

 found only after the administration of deoxycytidyhc, thymidylic and 

 deoxyadenylic acids. (Fig. 1). Deoxygnanylic acid had either no effect 

 at all, or, in one experiment, the mitotic index was decreased after its 

 administration. The other nucleotides and nucleosides had either a 

 shght, not significant effect (cytidylic acid) or no effect whatsoever. 



Some substances were then compared according to their effect on 

 DNA synthesis in irradiated l)one-marro^^ . Tlie guinea-pigs were 

 irradiated M'ith a dose of 600 r, a suspension of their bone-marrow was 

 prepared hr later, and incubated with 32p-phos])hate for 6 hr. Then, 



