44 



THE BIOSYNTHESIS OF PROTEINS 



antibody from ribonucleoprotein particles of lymph nodes or spleen of 

 immunized animals, by destroying the RNA with ribonuclease. 



All the experiments reviewed above point to the microsomes and more 

 especially to the ribonucleoprotein they contain as important centres of 

 synthesis of soluble cell proteins within the living cell. It must be em- 

 phasized that 'microsome' is not the name of any intact cell structure. This 

 term simply refers to a certain type of small particles which can be separated 

 from cell homogenates by high speed centrifugation in an appropriate 

 medium. Microsome pellets are jelly-like, transparent and slightly coloured. 

 When redispersed in aqueous medium they form an opalescent suspension 

 in which regular light microscopy or even phase contrast does not reveal 



20 



5 10 15 20 25 5 10 15 



Time, min Time, min 



Fig. 19. In vivo incorporation of I'^C leucine into microsomes of rat liver 

 fractionated with sodium deoxycholate. 



(a) Saturating dose of ^'^C leucine. 



(b) Tracer dose of ^'^C leucine. 

 (Littlefield et al., 1955). 



any structure. Dark field examination shows a great number of particles 

 animated with intense brownian motion. 



No such particles can be observed in living cells and it has long been 

 suspected that microsomes form after disruption of the cell, during the 

 dispersion of the cell contents in the extracting medium (Brachet and 

 Jeener, 1944; Claude, 1947; Porter, 1953). The fact that they contain most 

 of the cellular RNA indicated that microsomes originate from the ergasto- 

 plasm, also called cytoplasmic ground substance. 



Thanks to the development of electronmicroscopy of ultrathin tissue 

 sections (Porter and Blum, 1953; Sjostrand, 1953), knowledge of the 

 structure of cytoplasm made great progress during the last few years. A 

 network of lamellar formations was observed in the ground substance of 



