BIOLOGICAL ROLE OF PENTOSE NUCLEIC ACIDS 497 



activity for microsomes than for mitochondria. On the other hand, Barnum 

 and Huseby'-^ found practically no difference between the two fractions 

 in this respect, and this observation has been confirmed by recent work 

 from Davidson's laboratory (Smellie et al}"^^). 



The same group (Smellie et al}'^'^) has studied the incorporation of two 

 other labeled precursors, glycine-N^^ and formate-C^'*, into the PXA of 

 various cytoplasmic fractions: in confirmation of their earlier results with 

 P^2, they found no great difference between mitochondria and microsomes. 

 The supernatant (cell sap) fraction showed a higher activity than the 

 particulate fractions (mitochondria and microsomes); but, as Smellie et alP^ 

 have been careful to point out, the possibility cannot be excluded that this 

 fraction is contaminated with PNA of nuclear origin, with a high specific 

 activity, which might leach out of nuclei during the experimental procedure. 



It should finally be mentioned that work with P^- by Jeener^^s q^ ^^g 

 flagellate Polytomella coeca, has led him to conclusions which are not com- 

 patible with Chantrenne's"^ hypothesis: the experiments indicate that 

 there are probably, as Chantrenne believed, more than two types of par- 

 ticle, the mitochondria and the microsomes. But the view that the smallest 

 particles serve as nuclei for the synthesis of the larger is not confirmed by 

 the experimental results, which show that the quantity of PNA synthe- 

 sized at any instant of time is proportional to the quantity of PNA present. 

 No significant difference in specific activity is to be found between micro- 

 somes and mitochondria in these experiments. 



The fact that, in Polytomella, the quantity of PNA synthesized at any 

 moment is proportional to the quantity of PNA present leads Jeener'-* to 

 the interesting conclusion that this relationship can be best explained by 

 assuming that all types of ribonucleoprotein particles, whatever their size, 

 are capable of independent autocatalytic multiplication. 



Such a hypothesis is certainly in agreement with most, if not all, the 

 experimental work done with radio-isotopes on cell particles. It would now 

 appear that microsomes and mitochondria are independent units; their 

 syntheses inside the cell go on side by side and might well be the result of 

 some autocatalytic multiplication process. The question which now arises 

 is whether this synthesis of the cytoplasmic particles is limited by the 

 production of PNA in the nucleus and its diffusion in the cytoplasm. 



(2) Interrelations between Nuclear PNA and Cytoplasmic PNA. Caspers- 

 son's theory was largely based on the observations made in his laboratory 

 that an accumulation of ultraviolet-absorbing material, rich in PNA, took 



"« C. P. Barnum and R. A. Huseby, Arch. Biochem. 29, 7 (1950). 



1" R. M. S. Smellie, W. M. Mclndoe, and J. N. Davidson, Biochim. and Biophys. 



Acta 11, 559 (1953). 

 1" R. Jeener, Biochim. et Biophys. Acta 8, 270 (1952). 



