168 R. VENDRELY 



III. Can the DNA Content of the Nucleus be Considered as a "Constant"? 

 1. Interpretation of the Notion of Constancy in Biology 



The results quoted above suggest strongly that the DXA content of the 

 nucleus is a constant characteristic of the species and, within the species, 

 related to the number of chromosomes. From the theoretical point of view 

 is it possible to speak of "constancy" of the amount of DNA in the nucleus 

 in one animal as one speaks of constancy for the number of chromosomes? 

 This does not, of course, mean that the DNA content of the nucleus is 

 always the same during the whole life of the cell. Each substance in the cell 

 is involved in a cycle of anabolic and catabolic processes, and, especially 

 before mitosis, a certain quantity of DNA must be synthesized so that the 

 two daughter cells contain the same amount of DNA as did the mother cell. 

 Even during interphase, one can imagine that the DNA may undergo some 

 variation in relation to the physiology of the cell. In resting nuclei, however, 

 it seems that these variations in general are slight and do not appreciably 

 affect the total amount of DNA in the nucleus. Studies made with iso- 

 topes^'"^^ have shown that the turnover of DNA is very slow compared to 

 the turnover of PNA (Chapters 25 and 26), so that variations most likely 

 occur only in a small number of molecules at a time without any change in 

 the quantity of DNA in interphase nuclei. 



On the other hand, the physiological changes which sometimes act very 

 strongly upon PNA do not affect the DNA content of the nuclei. Fasting 

 or protein-deficient diets which produce a considerable decrease of PNA in 

 rat liver have no significant influence on the DNA content of the liver 

 nucleus, which remains unchanged even in extreme cases of prolonged fast- 

 ing and prolonged consumption of protein-free diets, as was shown by 

 Mirsky and Kurnick," Mclndoe and Davidson,* Campbell and Koster- 

 lits,** Villela,^^ Thomson et al," and Fukuda and Sibatani.^^ In young 

 animals which are still growing, it seems that a protein-deficient diet pro- 

 duces a slight increase of the DNA content of the nuclei.^*' ^°- ^^ This change 

 could be explained, as Thomson et al. point out,^^ by the fact that protein 

 deficiency in a still-growing animal might inhibit growth and mitosis to a 

 greater extent than it affects the premitotic synthesis of DNA, and this 



" A. M. Biues, M. M. Tracy, and W. E. Cohn, J. Biol. Chem. 155, 619 (1944). 



5^ E. Hammarsten and G. Hevesy, Acta Physiol. Scand. 11, 335 (1946). 



" G. B. Brown, M. L. Petermann, and S. S. Furst, J. Biol. Chem. 174, 1043 (1948). 



56 G. G. Villela, Rev. hrasil. biol. 12, 321 (1952). 



" A. E. Mirsky and N. B. Kurnick, quoted by A. E. Mirsky and H. Ris, J. Gen. 



Phxjsiol. 34, 451 (1951). 

 5» R. M. Campbell and H. W. Kosterlitz, Science 115, 84 (1952). 

 59 M. Fukuda and A. Sibatani, Exvtl. Cell Research 4, 236 (1953). 

 «« J. O. Ely and M. H. Ross, Science 114, 70 (1951). 

 " C. Lecomte and A. de Smul, Compt. rend. 234, 1400 (1952). 



