SITES WITHIN THE CELL 47 



magnesium ions. At low magnesium concentrations, the particles dissociate 

 into nucleoprotein fragments (Chao, 1957), 



Yeast and moulds contain nuclei, mitochondria and ribosomes which 

 resemble those of higher organisms (Yotsuyanagi, 1955, 1956; Shatkine 

 and Tatum, 1959; Blondel and Turian, 1960). In bacteria, no mitochon- 

 dria have been recognized, the nuclear apparatus is not very sharply 

 separated from the surrounding material, no nuclear membrane and 

 no structure resembling the endoplasmic reticulum has been found. 

 However, electron microscopy on thin sections of bacteria shows a very 

 dense population of particles which resemble the ribosomes of animal cells 

 (Bradfield, 1956). In bacterial extracts, up to 85 per cent of the RNA is 

 associated with sedimentable particles (Schachman et ah, 1952). Much 

 attention is now being paid to these long neglected bacterial particles 

 (see Roberts, 1958). 



The ribosomes isolated from Azotohacter vinelandii (Gillchriest and 

 Bock, 1958) or from E. colt (Bolton et al, 1958; Wagman and Trawick, 

 1958; Tissieres et al., 1959; Spahr and Tissieres, 1959) are very similar to 

 the yeast particles. Ribosomes from liver and from micro-organisms are 

 quite comparable in size range, composition, response to magnesium ion 

 concentration and behaviour in the ultracentrifuge (Hall and Doty, 1958; 

 Peterman et ah, 1958). Cheng (1957) remarked that the RNA content of a 

 ribosome of rat liver, pea seedlings and yeast as well, can be estimated as 

 equal to l-7-lO^ as expressed in molecular weight units. The same figure 

 was later found for certain ribosomes of E. coli (Tissieres and Watson, 

 1958), tobacco plant (Gierer, 1958) and mouse brain (Cheng, 1960). It 

 would seem therefore that a class of ribosomes might all contain about the 

 same absolute amount of RNA. Another striking coincidence is that the 

 ribonucleoprotein viruses so far studied in this respect also contain one 

 RNA molecule v/ith a molecular weight of about 2-10^ per virus particle. 

 One should probably not give too much weight to a coincidence of numbers 

 which may still be fortuitous. The more so, in that ribosomes can dissociate 

 into smaller particles (Elson and Tal, 1959; Hall and Slayter, 1959; 

 McCarthy, 1960; Huxley and Zubay, 1960) and sometimes constitute a 

 physiologically heterogeneous population (Siekevitz and Palade, 1959). 

 Nevertheless, Chen's observations raise the question of the existence of a 

 RNA 'quantum' and make one suspect similarities of structure and func- 

 tion between ribosome RNA and virus RNA. 



In animal cells, the ribosomes of the cytoplasmic ground substance are 

 certainly an important part of the protein-making system, they are most 

 probably the sites where polypeptides arise. Their function in bacteria was 

 more difficult to ascertain. The total amount of ribosomes is greatest when 

 the bacteria are in the logarithmic phase of growth (Mendelsohn and 

 Tissieres, 1959). But no correlation was found between the concentration 



