122 ALEXANDER L. DOUNCE 



part of reasonable accuracy, since a gross transfer of lipid from cytoplasm 

 to nuclei during the isolation is unlikely. The nuclei of the writer* and those 

 of WiUiams ct al.^^ which were isolated at pH 6.0 must have adsorbed some 

 broken mitochondria, but the quantitative error thus produced may have 

 been rather small. The nuclei isolated in strong citric acid or at pH 4 prob- 

 ably adsorbed less of the broken mitochondria. The nuclei prepared by 

 Stoneburg*- had been subjected to digestion by pepsin-HCL in addition to 

 treatment with strong citric acid, but this should not have changed the 

 lipid content appreciably. Further careful studies of the various nuclear 

 lipid fractions are obviously needed. 



c. Intranuclear Distribution of Lipid 



Little is known of the intranuclear distribution of lipids. Isolated chro- 

 mosomes contain a small amount of lipid (about 2% based on dry weight)*^ 

 and nucleoli may contain some lipid since they tend to blacken in osmic 

 acid. It is not yet certain whether there is generally appreciable lipid in 

 the nuclear membrane, but Callan and Tomlin*" believe that some is pres- 

 ent in one of the layers of the nuclear membrane in the egg cells of certain 

 amphibia. Since most of the nuclear lipid appears to be phospholipid, it is 

 likely that this lipid is located in structural elements such as those just 

 mentioned. 



d. Turnover of Nuclear Lipid 



Marshak,^" Barnum and Huseby,*' and Davidson et a/.***^ have studied 

 the turnover of nuclear phospolipid by means of radioactive phosphorus. 

 As judged by this criterion, nuclear phospholipid has a very considerable 

 rate of turnover, even in the resting cell, and in this respect is like all other 

 nuclear constituents except DNA, which probably has a very low or neg- 

 ligible rate of turnover in the resting cell (see Chapter 26). 



3. Composition of Nuclei with Respect to Nucleic Acid 

 a. Deoxyribonucleic Acid 



It is well known that in normal somatic cells all the detectable deoxyribo- 

 nucleic acid is located within the cell nucleus, and furthermore that it is 

 very definitely a constituent of the chromosomes (see Chapter 20). Ribo- 

 nucleic acid also occurs in the cell nucleus. 



(1) Percentage of DNA in Cell Nuclei. Early work on the percentage 



" A. Claude and J. S. Potter, J. Exptl. Med. 77, 345 (1943). 

 " H. G. Callan and S. G. Tomlin, Proc. Roy. Soc. (London) B137, .367 (1950). 

 «6 C. P. Barnum and R. A. Huseby, Arch. Biochem. 27, 7 (1950). 

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

 " R. M. S. Smellie, W. M. Mclndoe, R. Logan, J. N. Davidson, and I. M. Dawson, 

 Biochem. J. 54, 280 (1953). 



