ISOLATION AND COMPOSITION OF NUCLEI AND NUCLEOLI 131 



nuclei. This result is reasonable, since the sum of the DNA, PNA, histone, 

 and globulin could easily add up to 40% or more. Judging from the method 

 of isolation of the lipoprotein and the total amount present, it would appear 

 that the protein component of this material is very probably the same as 

 the protein material found in the residual chromosomes of Mirsky.^" 



Mayer et al}^^ have isolated the lipoprotein fraction in question by ex- 

 haustively extracting the nuclei with 1 M sodium chloride, and then dis- 

 solving the lipoprotein in 0.1 N alkali. The lipoprotein is subsequently 

 recovered by precipitation through lowering of the pH to 5.7 to 6.0. The 

 NaCl extraction removes DNA as well as globulin from the nuclei, since 

 the DNA is not firmly bound in the pH 6 nuclei. The method presumably 

 would fail if applied to nuclei isolated at pH 4 or lower, since in such nuclei 

 the DNA is firmly bound, apparently to the residual protein, and will not 

 dissolve in 1 M NaCl. However, the method of Laskowski using deoxy- 

 ribonuclease^"^ should be applicable in this case. 



h. Turnover Studies. Hammarsten et aZ.^^** found a very appreciable rate 

 of uptake of radioactive glycine by the total protein of liver cell nuclei. 

 Miller et al. showed that all of the protein fractions of liver cell nuclei ap- 

 pear to show a considerable turnover, as measured by uptake of radioactive 

 lysine,^ although there is disagreement in regard to the histone fraction. '^^ 

 Mirsky et a/."- have demonstrated a considerable rate of incorporation of 

 radioactive glycine into the histone and "residual protein" fractions of 

 nuclei of liver, kidney, and pancreas. The turnover values for the nuclear 

 proteins are comparable with those for cytoplasmic proteins."-"-^ This 

 statement of course cannot be taken to mean that all individual proteins of 

 the cell nucleus necessarily have an appreciable turnover in the resting 

 cell, although there is no convincing evidence to the contrary at the pres- 

 ent time. 



The apparently negligible turnover of DNA as compared with the rela- 

 tively high rate of turnover for nuclear PNA, nuclear phospholipid, and 

 nuclear protein, is one line of evidence that has led to the choosing of DNA 

 as the substance of which genes are probably composed. Logically it would 

 seem that gene substance should have little or no turnover, since otherwise 

 genie material would presumably fluctuate in concentration and might 

 easily be lost. If a nuclear protein with no turnover in the resting state 

 should ever be found, this might also become a possible constituent of gene 

 material. 



'1" A. Bergstrand, N. A. Eliasson, E. Hammarsten, B. Norberg, P. Reichard, and 

 H. von Ubisch, Cold Spring Harbor Symposia Quant. Biol. 13, 22 (1948). 



'11 M. D. Hoberman and P. M. Peralta, Federation Proc. 11, 232 (1952). 



i'2 M. M. Daly, V. G. Allfrey, and A. E. Mirsky, /. Gen. Physiol. 36, 173 (1952). 



"2» R. M. S. Smellie, W. M. Mclndoe, and J. N. Davidson, Biochim. et Biophys. Acta 

 11, 559 (1953). 



