ISOLATION AND COMPOSITION OF NUCLEI AND NUCLEOLI 123 



composition of the nucleus in regard to DNA was erroneous, largely because 

 of failure to realize that isolation of nuclei in 5 % citric acid involves removal 

 of considerable protein with an apparent increase in the percentage of DNA, 

 since the latter is not removed to any measurable extent. In the reviewer's 

 laboratory it has been found as the result of much effort and several mis- 

 takes, that the DNA concentration in nuclei isolated from the livers of rats 

 fed ad libitum on a fox chow diet is about 11 % (based on dry weight) .^-^^ 

 This statement applies to nuclei isolated in dilute citric acid at pH 6 or 

 pH 4, and to nuclei isolated by the calcium chloride method. Mirsky et alP 

 have found about 12% DNA in calf and horse liver nuclei isolated by the 

 Behrens procedure, whereas the reviewer and collaborators found only 

 about 4 to 5% DNA in rat liver nuclei isolated by this method. The ap- 

 parently obvious inference that the latter nuclei were only about 40 % pure 

 is not necessarily well founded. Since the work of Mirsky et alP has demon- 

 strated that in starvation the DNA concentration in horse liver nuclei could 

 rise to a value as high as 18 %, it seems clear that diet must be taken into 

 account in measuring DNA concentration. It has been demonstrated that 

 amount of DNA in liver cell nuclei does not change materially during starva- 

 tion,*^-^" although a few contrary claims can be found in the literature, such 

 as that of Ely and Ross.^' But nuclear protein, which is the material pres- 

 ent in cell nuclei in highest concentration, may vary widely in starvation, 

 so that a high concentration of nuclear DNA in starvation indicates mainly 

 a loss in protein from the nuclei. Thus it can be seen that the percentage 

 composition of the cell nucleus with respect to DNA is highly variable, even 

 within a given organ of a single species. The percentage of DNA may also 

 vary from one kind of nucleus to another in mammalian tissues, as can be 

 seen from the analyses of calf thymus cell nuclei isolated by Mirsky et al. 

 using a modification of the Behrens technique.^' These nuclei contained 

 about 26 % DNA. Nuclei of tissues other than liver and thymus were found 

 by Mirsky et al. to have percentages of DNA falling between the value for 

 liver and the value for thymus. 



(2) Amount of DNA -per Nucleus. One of the most important basic dis- 

 coveries concerning the DNA of cell nuclei was made by Boivin, Vendrely, 

 and Vendrely^' •''^ •''' and was confirmed by Mirsky and Ris^*'^® and David- 



«8H. W. Kosterlitz, /. Gen. Physiol. 106, 194 (1947). 



69 R. M. Campbell and H. W. Kosterlitz, /. Biol. Chem. 175, 989 (1948). 



'" R. Y. Thomson, F. C. Heagy, W. C. Hutchison, and J. N. Davidson, Biochem. J. 



53, 460 (1953). 

 '' J. O. Ely and M. H. Ross, Science 114, 70 (1951). 



'2 A. Boivin, R. Vendrely, and C. Vendrely, Compt. rend. 226, 106 (1948). 

 " R. Vendrely and C. Vendrely, Experientia 4, 434 (1948); 5, 327 (1949). 

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

 " H. Ris and A. E. Mirsky, J. Gen. Physiol. 32, 489 (1949). 

 '« A. E. Mirsky and H. Ris, J. Gen. Physiol. 34, 451 (1951). 



