THE DEOXYRIBONUCLEIC ACID CONTENT OF THE NUCLEUS 157 



II. The DNA Content of the Nucleus 



1. The DNA Content of the Nucleus in the Various Tissues of the 



Same Animal 



a. Chemical Measurement of the DNA Content of the Nucleus in the Somatic 

 Cells and Gametes 



Various authors studying the DNA content of isolated nuclei of animal 

 tissues by chemical analysis have used different methods for the determina- 

 tion of DNA. The two most commonly employed techniques are the method 

 of Schmidt and Thannhauser^ and that of Schneider/ which have been dis- 

 cussed in Chapter 16. They involve the estimation of DNA either by the 

 determination of a nonspecific element (phosphorus, purine nitrogen) or by 

 the estimation of a specific molecular fraction, the sugar (deoxyribose). 



The estimation of DNA phosphorus is convenient and is used by many 

 authors, but the total elimination of interfering phosphorus compounds 

 (phosphoproteins, organic phosphoric esters)* is essential and is not always 

 adequately carried out. The estimation of the purine bases by chemical 

 methods of precipitation or by physical methods (ultraviolet absorption) 

 is open to criticism to a certain extent on account of the variations of these 

 purine bases in nucleic acids of different origins (Chapter 10). Colorimetric 

 methods for the estimation of sugars are liable to interference by ill-defined 

 impurities (Chapter 9). Consequently, when we compare the results ob- 

 tained by different authors, each of them working with the technique of his 

 choice, we must keep in mind that the absolute values of one author may 

 not be strictly comparable with those of another author; the discrepancies 

 between the results are significant only when the same method has been 

 used. 



The first results published by Boivin et al} and by Vendrely and Ven- 

 drely,^ which are reported in Table I, were obtained with beef tissues and 

 show two features of interest. First, the DNA content of the nucleus in the 

 different tissues studied seems to be the same; second, the DNA content of 

 the sperm is very approximately half of that in the diploid somatic nuclei 

 in the same species. Mirsky and Ris,"* working with several animal species, 

 confirmed and extended the view that the sperm cells contain half the DNA 

 content of the somatic cell nuclei (Table II). In the ox, the bull, and the 

 calf, their results were initially not in agreement with those of the French 

 authors, but subsequently Mirsky and Ris revised their early results on 

 cattle^" and declared themselves in full agreement with the conception of 



« G. Schmidt and S. J. Thannhauser, J. Biol. Chem. 161, 83 (1945). 

 ' W. C. Schneider, J. Biol. Chem. 161, 293 (1945). 

 8 W. M. Mclndoe and J. N. Davidson, Brit. J. Cancer 6, 200 (1952). 

 ' R. Vendrely and C. Vendrely, Experientia 4, 434 (1948). 

 "> A. E. Mirsky and H. Ris, J. Gen. Physiol. 34, 475 (1951). 



