156 R. VENDRELY 



out that the best sources of DNA are organs which are rich in nuclei^ (thy- 

 mus gland, pus cells, sperm, etc.)- Examination of the tables in Chapter 16 

 show clearly that those tissues which contain large numbers of nuclei in a 

 given mass are richest in DNA. There appears, therefore, to be an approxi- 

 mate relationship between the DNA content of a tissue and the number of 

 its nuclei. 



Techniques for the isolation of nuclei have allowed a more precise study 

 of the question, since they have made possible the study of the chemical 

 composition of nuclei and of the amount of DNA as a percentage of the dry 

 weight. But the results of analyses of isolated nuclei expressed in this way 

 do not give an exact picture of the real composition of the nuclei in living 

 tissues since, during the isolation process, nuclei may lose part of their sub- 

 stance, especially protein.^ The most satisfactory manner of expressing the 

 DNA content of nuclei is to calculate the average amount of DNA for a 

 single nucleus, as was done for the first time by Boivin, et al.^ in 1948. The 

 results of directed mutations in bacteria, which showed the important 

 part played by DNA in the chemical constitution of the genes, led these 

 authors to put forward the hypothesis that the DNA, a permanent compo- 

 nent of the nucleus and the chief component of the hereditary material in 

 the chromosomes, must be present in the same amount in all the cells of 

 an animal with the exception of the haploid cells which would contain half 

 this amount. Boivin et al.^ studied the amount of DNA in individual nuclei 

 by determining DNA in a suspension containing a known number of iso- 

 lated nuclei and by calculating the average DNA content of a single nucleus. 

 The results were in good agreement with their hypothesis. Similar results 

 have been obtained by other authors (Mirsky and Ris,* Davidson etal^). 



The method of quantitative photometry in visible light (Chapter 17) 

 allows a quantitative estimation in arbitrary units of the DNA content of 

 one particular nucleus in a tissue section colored by the Feulgen procedure. 

 It has been applied to this problem with results which are, in general, in 

 good agreement with those from chemical studies. 



Finally, photometry in ultraviolet light with the apparatus of Caspersson 

 has been used to measure the DNA content of nuclei in absolute units. The 

 problem of the DNA content of the nucleus has therefore been attacked with 

 different tools, and, although it is a rather controversial topic at the present 

 time, a considerable amount of data has been collected and will be discussed 

 in this chapter. 



1 W. C. Schneider and H. L. Klug, Cancer Research 6, 691 (1946). 

 ^ A. L. Dounce, Exptl. Cell Research Suppl. 2, 103 (1952). 

 ' A. Boivin, R. Vendrely, and C. Vendrely, Com-pt. rend. 226, 1061 (1948). 

 •• A. E. Mirsky and H. Ris, Nature 163, 666 (1949). 



* J. N. Davidson, I. Leslie, R. M. S. Smellie, and R. Y. Thomson, Biochem. J. 46, 

 Proc. xl (1950). 



