374 B. MAGASANIK 



The development of modern methods of cytochemistry allowed the 

 demonstration that both types of nucleic acid are present in all cells, ani- 

 mal, plant, and microbial, deoxypentose nucleic acid in the nucleus, and 

 pentose nucleic acid in the cytoplasm as well as in the nucleus.^ While 

 early preparations of PNA usually were badly degraded, the lability of 

 PNA was later recognized, and consequently milder methods of isolation 

 came into use which resulted in PNA preparations of considerably higher 

 molecular weight.^'* Finally, the application of modern methods to the 

 analysis of nucleic acids, such as paper chromatography,^ ion-exchange 

 chromatography,^ and paper ionophoresis,^ demonstrated that nucleic 

 acid preparations isolated from different tissues differed in composition 

 and did not in general contain the four constituent nucleotides in equimolar 

 quantities. 



The description of these newer methods of isolation and the results 

 obtained by the modern methods of analysis are the subject of this chap- 

 ter. 



II. Isolation of Pentose Nucleoproteins 

 1. General 



Pentose nucleic acids are found in tissue extracts in combination with 

 proteins as nucleoproteins, the "nucleins" of the early workers. Generally, 

 isolation of nucleoprotein is the first step in the isolation of PNA. 



A short description of the distribution of PNA in the cell, a subject 

 discussed in detail in Chapters 16-21, is helpful for the understanding of 

 the experimental approach used for the isolation of nucleoproteins. PNA 

 is found in both nucleus and cytoplasm. The nuclear PNA accounts for 

 about one-tenth of the total PNA of the cell.^ The isolation of nucleoli and 

 the composition of their PNA constituent has been described.^ The major 

 portion of the PNA of the cytoplasm is contained in the particulate frac- 

 tions, and most of it is found in the microsomes. Chemical analysis has 

 shown microsomes to be complex macromolecular structures composed of 

 lipid, protein, and PNA.^ The fractionation of the components of the mi- 

 crosomes without the denaturation of the protein has not been accom- 



1 T. Caspersson and B. Thorell, Chrovwsoma 2, 132 (1941). 



« H. S. Loring, /. Biol. Chem. 130, 251 (1939). 



3W. E. Fletcher, J. M. Gulland, D. O. Jordan, and H. E. Dribben, /. Chem. Soc. 



1944, 30. 

 * E. Vischer and E. Chargaff, J. Biol. Chem. 176, 715 (1948). 



6 W. E. Cohn, /. Am. Chem. Soc. 72, 1471 (1950). 



« R. Markham and J. D. Smith, Biochem. J. 52, 552 (1952). 



7 W. C. Schneider, J. Biol. Chem. 165, 585 (1946). 



8 W. S. Vincent, Proc. Natl. Acad. Set. U. S. 38, 139 (1952). 



9 A. Claude, Science 97, 451 (1943). 



