RNA AND CONTROL OF CELLULAR PROCESSES 105 



these granules appear normally associated with lamellae ( in so-called 

 "rough" reticulum ) , although lamellae can exist also without granules 

 ("smooth" reticulum). In some young animal cells and ovocytes, 

 most of the granules were observed free in the cytoplasm (Porter, 

 1954; Afzelius, 1957; Pasteels et ah, 1958), and in most plant cells 

 they are not associated with reticulum (Buvat and Carasso, 1957; 

 Sitte, 1958; Blondel and Turian, 1960) . Since the term ergastoplasm 

 means total basophilic cytoplasm, which encompasses granules and 

 lamellae, it mav still be appropriate to retain special names for the 

 lamellar system without granules and for the granules themselves. 



The proof that the granules and not the lamellae are the seat of 

 RNA was obtained when it was possible to separate granules by 

 centrifugation and analvze them chemicallv. When Claude ( 1946 ) 

 first devised a technique of differential centrifugation of homogen- 

 ized cells, he obtained a fraction which sedimented only at high 

 centrifugal forces. The fraction consisted of granules rich in RNA 

 called microsomes. Microsomes were not exact counterparts of 

 structures found in a living cell, but rather fractions of ergastoplasm, 

 appearing as vesicles of various sizes, carrying osmiophilic granules. 

 Further treatment of this material with desoxycholate (Littlefield 

 et al., 1955) dissolved the membranes of vesicles and allowed the 

 isolation of pure granules. The granules were also isolated from 

 bacteria and found to vary in size, having different sedimentation 

 constants (Tissieres and Watson, 1958). The bulk of the granules 

 have a sedimentation constant 70S; there are also lOOS particles and 

 smaller ones, 51 S and 32S. The molecular weight of 70S units is 

 2.8 X 10^ of 51S units - 1.85 X 10*^' and of 32S units - 0.95 X 10^ 

 The distribution into different sedimentation classes may vary in 

 different organisms (Ts'o et al., 1956; Peterman and Hamilton, 1957; 

 Chao, 1957). Chemical analysis showed that these granules are 

 practically pure nucleoproteins, consisting of from 40 to 60 per cent 

 RNA; the rest is protein, plus some occasional lipids. 



These granules are not an artefact of homogenization or fixation, 

 as some would believe (Sjostrand and Baker, 1958; Hanzon et al., 

 1959 ) , since it was possible to centrifuge them inside a living cell in 

 Neurospora (Zalokar, 1961) and to sediment them into a basophilic, 

 RNA rich layer near the centrifugal end of the cell. Electron micros- 

 copy revealed that this layer was composed of densely packed osmi- 

 ophilic granules of around 150 A diameter, and entirely devoid of 

 any cytoplasmic lamellae. 



