BIOLOGICAL ROLE OF PENTOSE NUCLEIC ACIDS 495 



cytoplasmic PNA and its relationship to nuclear PNA, a problem directly 

 bearing on Caspersson's theory. We shall subsequently discuss another 

 aspect of the same theory, viz., the role played by the nucleus in protein 

 synthesis. Finally, the significance of the microsomes for protein synthesis 

 will be examined in more detail. 



b. New Facts since Caspersson's^^ Theory 



(1) Presence of PNA in Different Cytoplasmic Particles. The chemical 

 composition of the various cytoplasmic particles has already been dealt 

 with in Chapter 21. It is enough for our present purpose to recall that 

 PNA is present in all cellular fractions which can be obtained by differential 

 centrifugation (nuclei, mitochondria, microsomes, and cell sap). 



In the nuclei, PNA is probably associated mainly with the nucleolus 

 (see Chapter 18). While in most cases the PNA content of isolated nuclei 

 is rather low {ca. 1.5% according to Allfrey et al}^°), it can reach the same 

 value as in the cytoplasm in rapidly growing tissues, e.g., wheat germ 

 (Stern and Mirsky"'). While mitochondria are rather poor in PNA, the 

 amount present in the microsomes can exceed 30 % in embryonic material : 

 there is little doubt that most of the basophilic or ultraviolet-absorbing 

 substances which have been studied by the cytochemists are in fact micro- 

 somes or aggregates of microsomes. Nonsedimentable (cell sap) PNA is 

 especially abundant in rapidly growing cells (Brachet and Jeener^'^) such 

 as yeast cells and amphibian or chick embryos. It is probable that this 

 soluble fraction has something to do with cell division, as indicated by its 

 definite increase in hepatomas (Price et al}^^). 



The possible relationships between microsomes and mitochondria (see 

 Chapter 21) are still very much open to discussion: in the opinion of 

 Claude, ^^^"^ the original discoverer of the microsomes, both fractions are 

 entirely independent of each other; evidence in favor of this view can be 

 found in Claude's"^ demonstration that, in ultracentrifuged liver cells, the 

 mitochondrial and basophilic (presumably microsomal) layers are well 

 separated. On the other hand, D. Green" ^ believes that microsomes are 

 little more than breakdown products of mitochondria, an opinion which is 

 hard to reconcile with the fact that microsomes and mitochondria are very 



no V. Allfrey, H. Stern., A. E. Mirsky, and H. Saetren, /. Gen. Physiol. 35, 529 



(1952). 

 "1 H. Stern and A. E. Mirsky, J. Gen. Physiol. 36, 181 (1952). 



112 J. Brachet and R. Jeener, Enzymologia 11, 196 (1944). 



113 J. M. Price, J. A. Miller, E. C. Miller, and G. M. Weber, Cancer Research 9, 103 

 (1949). 



"^ A. Claude, Science 87, 467 (1938). 



116 A. Claude, Biol. Symposia 10, 111 (1943). 



11* D. E. Green, 2nd Intern. Congr. Biochem., Paris p. 1 (1952). 



