266 NUCLEIC ACIDS AND GROWTH 3 



the interpretation of the resuks: while most of the American authors (Swift, Alfert, 

 etc.) firmly believe that the DNA content of the nucleus is very closely related to the 

 degree of ploidy, a group of Belgian investigators (Pasteels and Lison, 1950; Fautrez 

 and Fautrez-Firlefyn, 1953; Roels, 1954; etc.) strongly disagree. According to 

 them, the DNA content of the nucleus can vary, not only in relation with the mitotic 

 cycle and chromosome reduplication (see below), but also with the physiological 

 activity of the tissue. For instance, according to Pasteels and Bullough (i953)> 

 great variations in the DNA content occur in the cells of the skin when the latter is 

 rubbed with croton oil. Most striking perhaps among these exceptions is Pisi and 

 Cavalli's (1955) recent finding, in Fautrez' laboratory, that hypertrophia of the 

 kidney produced by thyroxin injection does not modify the DNA content of resting 

 nuclei, while stimulation by nephrectomy increases this content. Also, according to 

 Roels (1954), the DNA content of the nuclei varies during secretory cycles in the 

 thyroid. 



All these variations in the DNA content of resting nuclei are considered by 

 American authors, like Swift and Alfert, to be due to technical errors or to an 

 increase of DNA content prior to mitosis or endomitosis. A more cautious position 

 has been recently taken by Allfrey et al. (1955), who report that, according to 

 recent experiments by Lowe and Foley (1955), the DNA content of individual 

 resting nuclei can vary under the influence of cortisone. 



The author has imfortunately no personal experience of the methods used in 

 these and similar experiments; as a matter of fact, he feels inclined to view most 

 of the work done on the DNA content of individual nuclei, when estimated by the 

 Feulgen reaction, with a certain scepticism, because there is no truly convincing 

 evidence that this reaction follows Beer's law closely enough for quantitative 

 purposes. On the whole, the present impression is that the DNA content of the 

 cells is really linked to their degree of ploidy, but that one should not be too dog- 

 matic about it: the possibility of changes in the DNA content of cells, related to 

 surrounding physiological conditions, is by no means eliminated. 



Finally, it should be added that, for general biochemical purposes, estimation 

 of the DNA content of a tissue, when the average content of the nuclei in this 

 tissue is known, remains a very useful method for the study of changes in cell 

 number (Davidson and Leslie, 1950; Mandel and Bieth, 1951): as a matter of 

 fact, such estimations are still the best way we have to decide, with biochemical 

 techniques, if growth is due to an increase in cell number or not. 



{b) Metabolic stability of DNA 



The fact that there is a definite tendency for a constancy in the DNA content of 

 the various somatic cells in the same species is insufficient to carry conviction that 

 DNA is of genetic importance: it is also to be expected, in view of the well-known 

 stability of the genes, that DNA is a very stable molecule. 



Considerable evidence, which has been recently reviewed by Brown and Roll 

 (1955), shows this to be the case; however, interesting work by Daly et al. (1952) 

 indicates that the metabolic stability of DNA is not complete and that the physio- 

 logical activity of the considered organ has an effect on its DNA metabolism. 

 Their main evidence is as follows: they have studied the uptake of glycine-^^N by 



