175 



THE DEOXYRIBONUCLEIC ACID CONTENT OF THE NUCLEUS 



P M T P M T 



4N ' ' ' 

 2N 



4N 

 2N 



4N 



2N 



Fig. 1. Development of the DNA content of the cell in the course of mitosis: (a) 

 theory of Caspersson*^ and Ris'"; (6) the generally accepted theory (Swift", Alfert'*', 

 Walker and Yates"; (c) theory of Pasteels and Lison.^^ 



tion of the synthesis of DXA in late interphase seems to be accepted by- 

 most authors at the present time. 



b. DNA of the Nuclei in Embryonic Tissues and Tissue Cultures, and the 

 Problem of Differentiation 



Whatever be the moment when the synthesis of DNA occurs, cell division 

 is associated with a doubling of the DNA content, so that the DNA of the 

 nuclei in actively growing tissues (embryos, tissue cultures, regenerating 

 tissues, etc.) must be higher than in resting tissues. This is evident from 

 the gross chemical analysis of Thomson et al." on eleven different organs 

 of the rat. Salivary gland and intestine, where the rate of mitosis is higher 

 than in other tissues, show a higher average amount of DNA per nucleus. 

 In liver regenerating after partial hepatectomy the average amount of DNA 

 per cell is increased markedly during the first days when the rate of growth 

 and multiplication is extremely high.^^' ^" Lison and Pasteels^^ studied cyto- 

 photometrically the DNA content of the nuclei in developing sea urchin 

 embryos and noted particularly high values in the actively dividing parts 

 of the embryo. Swift^^ also reported in dividing cells amounts of DNA 

 higher than the values found in resting nuclei and attributed these varia- 

 tions to the building up of DNA for future mitosis. This interpretation 

 seems to be quite logical, but Lison and Pasteels^^ advanced the view that 

 the increase of DNA in dividing cells is not exactly related to variations in 

 the mitotic rhythm. They consider that some changes in the DNA per cell 

 could be caused by the cytoplasmic factors acting on morphogenesis. The 

 results of Moore^^ on embryos of Rana pipiens seem also to indicate a cor- 

 relation between differentiation and a wide range of DNA values in a tissue. 



S5 B. C. Moore, Chromosoma 4, 563 (1952). 



