RNA 



The early work of Brachet (1941) and Caspersson (1941) clearly 

 demonstrated that RNA is a universal constituent of all living cells 

 and that the nucleus, particularly the nucleolus, contains appreciable 

 amounts. RNA is also a component of the chromosomes, and in the 

 case of rat liver constitutes as much as I 2 per cent of their total nucleic 

 acid (AWtTGy, et al, 1955). 



As previously indicated, sites of protein synthesis are rich in RNA, 

 notably nucleolus and cytoplasm. Since increase in protein frequently 



Figure 4-26. Autoradiograph Showing Active Incorporation of Adenine 

 — 8— C^^ by the Nucleolus of the Starfish Oocyte. (Courtesy of Dr. A. Ficq, 

 Universite Libre de Bruxelles.) 



accompanies cellular growth, a concomitant increase in nucleolar size 

 might be expected. This relationship appears to hold for nerve cells, 

 growing oocytes, proliferating normal and malignant cells, and for many 

 secretory cells. Further, the nucleolus is reduced in volume in cells of 

 animals fed on low or nonprotein diets. Also cells of the pancreas or 

 gastric mucosa, which are synthesizing large amounts of enzyme protein, 

 have nucleoli which possess a high content of RNA. 



Whether nucleolar RNA is synthesized in the nucleolus itself, or 

 elsewhere and then transported to this structure to be bound to protein, 

 is still something of an open question. There is no doubt from autoradio- 



108 / CHAPTER 4 



