biosynthesis of nucleic acids 379 



2. Role of Hormones 



It has not been demonstrated that any hormone functions specifically 

 in the control of reactions leading to nucleic acid production (see Chapter 

 16). A small amount of evidence has been accumulated, however, which 

 indicates that hormones may influence nucleic acid biosynthesis. 



An increase in the amount of PNA in the uterus and vagina of castrated 

 mice^" and in the uterus of castrated rats^^* is induced by injections of 

 estradiol. In both of these experiments these changes were associated with 

 increased PNA:DNA ratios, demonstrating that the treatment mth the 

 hormone resulted in an increase of PNA per cell. It was also found that the 

 nucleic acid content and the rate of incorporation of P^^ into the nucleic 

 acids of the uterus of castrated rabbits was increased by treatment with 

 estradiol. '^^ In these experiments it was variously noted that the estrogen 

 also caused increases in the amounts of protein, phosphohpid, and enzymes, 

 so that it is impossible to assign to estradiol a specific role in nucleic acid 

 synthesis. 



It has already been stated that progesterone causes an increase in the rate 

 of incorporation of P^- into the nucleic acids of castrated rabbits and it also 

 causes an increase in the nucleic acid content of the endometrium.^^® In the 

 mucosa of the gizzards of pigeons treated with prolactin, there is an in- 

 creased synthesis of PNA as well as an increased protein synthesis.^" In all 

 of the above experiments, the changes in nucleic acids induced by hormones 

 have been accompanied by changes in other cellular constituents. However, 

 in a study of the growth of chick heart explants, it was observed that in 

 insulin-treated cultures the PNA content per cell was higher than in un- 

 treated cultures, whereas there was no significant difference between 

 treated and untreated cultures in respect to hpid and protein content per 

 cell,'^^ which would suggest that insulin may have a specific effect on nu- 

 cleic acid biosynthesis. 



In the animal the biosynthesis of nucleic acids can be influenced by other 

 factors such as irradiation, diet, pregnancy, and the presence of tumors, and 

 these are discussed in Chapters 16 and 26. 



V. Present Possibilities for Pathways 

 of Assembly of Polynucleotides 



In considering the mechanisms which may be involved in the biosynthesis 

 of the polynucleotides, it must be recognized that our knowledge is very 

 fragmentary indeed. Discussion at this time can at best serve only as a 



1" R. Jeener, Biochim. et Biophys. Ada 2, 439 (1948). 



158 M. A. Telfer, Arch. Biochem. and Biophys. 44, 111 (1953). 



159 I. Leslie and J. N. Davidson, Biochem. J. 49, xli (1951). 



