THE DEOXYRIBONUCLEIC ACID CONTENT OF THE NUCLEUS 177 



ferred to the single average cell and give a more realistic and significant ex- 

 pression of the phenomena. Davidson and his colleagues have carried out 

 an exhaustive study of the variations of PNA, phospholipids, and proteins 

 in embryos at different stages of development and in animals submitted to 

 nutritional changes (Chapter 16). 



We have also seen that the DNA value per nucleus can be used in prob- 

 lems related to the evolution of closely related species and in the examina- 

 tion of the possibility of polyploidy in the evolutionary process. Finally, 

 DNA can be used in a comparative chemical study of the other components 

 of the nucleoprotein of the nucleus and their possible variations. 



The DNA content of the nucleus therefore appears at present to be a 

 very convenient tool for biologists in different fields, since it allows the 

 study of biochemical processes at the level of the individual cell. 



V. Addendum 



The fact that the DNA content of the cell nucleus is constant per set of 

 chromosomes in the rat was confirmed recently by Thomson and Frazer,^^ 

 who found that the coefficient of variation in the DNA content of individual 

 nuclei of the same class probably does not exceed 5 to 15%. Alfert and 

 Swift^^ attributed the discrepancies reported by Pasteels and Lison''^ in 

 rat liver to a different degree of fixation in the blocks of tissue from the 

 edge to the center. In rat liver nuclei isolated from sucrose homogenates, 

 the diminution reported by Pasteels and Lison is no longer found. Pataui"^" 

 also reached the conclusion that there is no variation in DNA between nu- 

 clei vnth equal chromosome complements. 



That, the DNA content per nucleus is not affected by physiological 

 changes has been confirmed by Laird, ^"^ who reported that thioacetamide — 

 which provokes considerable changes in the rat liver nucleus (an increase 

 in protein and PNA) — has no effect on the DNA content. Phillips et aU'^^ 

 showed that the treatment of sexually immature pullets with gonadal hor- 

 mones increases the number of liver cells but does not affect significantly the 

 DNA per nucleus. However, Fautrez and Moerman,^"' studying the DNA 

 content of the fiver nuclei in the fish Lebistes reticulatus, concluded that the 

 variations from the normal diploid value were related to the changes in 

 the physiological activity of the organ, and Govaert,^"^ using the vitellogen 



58 R. Y. Thomson and S. C. Frazer, Exptl. Cell Research 6, 367 (1954). 



33 M. Alfert and H. Swift, Exptl. Cell Research 5, 455 (1953). 

 >»» K. Patau, Records Genet. Soc. Amer. 22, 90 (1953). 

 "" A. K. Laird, Arch. Biochem. and Biophys. 46, 119 (1953). 



102 W. E. J. Phillips, W. A. Maw, and R. H. Common, Can. J. Zool. 31, 167 (1953). 

 1"^ J. Fautrez and J. Moerman, Compt. rend, assoc. anat. 80, 554, (1954). 

 i"^ J. Govaert, Compt. rend. soc. biol. 147, 1494 (1953). 



