BIOLOGICAL ROLE OF PENTOSE NUCLEIC ACIDS 481 



results (Brachet^") have been obtained with acriflavine, which precipitates 

 nucleic acids in vitro; in the latter case, reversibility of development is im- 

 proved by the addition of PNA or adenylic acid to the blocked embryos. 



Bieber et al.^^ have recently studied the effects of 25 chemical analogues 

 of purines or pyrimidines on the development of frog's eggs: all inhibited 

 development, but at various stages characteristic for each compound. In- 

 hibitory effects on the development of chick embryos by unnatural syn- 

 thetic nucleosides have also been reported by Fox and Goodman. ^^ 



b. Other Toxic Substances 



Among the many toxic substances which have been tried on amphibian 

 eggs, only dinitrophenol, usnic acid, and female steroid hormones have 

 been studied from the viewpoint of PNA metabolism. 



Dinitrophenol and usnic acid, both of which inhibit the coupling between 

 oxidation and phosphorylation, stop embryonic development completely 

 and reversibly; cytochemically, an increase in the PNA content of the 

 nuclei in these blocked embryos (Brachet^^) is found; however, chemical 

 estimations show that PNA synthesis is almost entirely blocked while the 

 eggs are in the presence of the poisons and that it is resumed as soon as 

 development is allowed to proceed (Steinert^^) . 



Regarding the female sex hormones, Cagianut^^ has reported that stil- 

 bestrol and estradiol in appropriate concentrations affect the distribution 

 of PNA in amphibian eggs: the typical gradients are altered and develop- 

 ment is highly abnormal. Curiously enough, the addition of yeast PNA 

 works antagonistically and improves development. 



c, Phi^sical Agents 



Comparable results can be obtained by centrifuging amphibian eggs 

 (Pasteels and Brachet, in Brachet'^). Centrifugation of eggs just after ferti- 

 lization leads to marked alterations in the PNA initial polarity gradient 

 and to very abnormal development or even failure of development. The 

 morphological and cytochemical abnormalities induced by such treatment 

 show a close correlation. Centrifugation of blastulae, on the other hand, 

 leads to the formation of double embryos. Here again, the duplication is 

 linked to the abnormal distribution and local synthesis of PNA. 



But the effects of moderate heat shock in developing amphibian embryos 



30 J. Brachet, Cornpt. rend. soc. biol. 140, 1123 (1946). 



31 S. Bieber, R. F. Nigrelli, and G. H. Hitchings, Proc. Soc. Exptl. Biol. Med. 79, 430 

 (1952). 



32 J. J. Fox and I. Goodman, Biochim. et Biophys. Acta 10, 77 (1953). 



33 J. Brachet, Experientia 7, 344 (1951). 



34 M. Steinert, Biochim. et Biophys. Acta 10, 427 (1953). 

 36 B. Cagianut, Z. Zellforsch. 34, 471 (1949). 



