VII. SYNTHESIS OF RNA AND RIBOSOMES 349 



Whether or not ribosomes appear in electron micrographs of cellular 

 sections depend strongly on the methods of preparation. The pattern 

 observed in the analytical centrifuge is quite different when the cells 

 were broken by lysozyme-freeze-thaw and then put through the pressure 

 cell from the pattern obtained when the processes were interchanged. 

 The quantity of /?-galactosidase associated with the ribosomes can vary 

 10- fold, depending on the method of breaking. The ribosomes obtained 

 by breaking the cells in the presence of 10'- M Mg and dialyzing to 

 10"* M are not identical to ribosomes obtained by breaking in lO'Mf 

 Mg. The protein content of 308 ribosomes from cells broken in low 

 magnesium concentrations is unexpectedly high. Some find ribonuclease 

 equally distributed between 30S and 50S particles, but others find it 

 only in the 30S particles. 



Perhaps all of these anomalies indicate that the ribosome as it exists 

 in the cell has quite a different form and the ribosome as observed in 

 the broken cell juice depends both on the metabolic state of the living 

 cell and on the particular conditions at the time of breakage. 



Inforaiation can be derived from studies of the particles even though 

 they may be formed at the instant of breakage as they cany reproducible 

 samples of cellular components. In other respects there may be important 

 differences. The low rates of protein synthesis and the difficulties of 

 removing nascent protein from the ribosomes in vitro may be due to a 

 less favorable configuration of the particles in vitro than in vivo. Models 

 of cellular function and structure should not be limited to those con- 

 ceived in terms of the little round balls observed in the electron micro- 

 scope. Extended intertwining strands might equally well, or better, 

 represent the organization of ribosomes in the living cell. If this were 

 the case, then at the moment of breakage the strands might gather into 

 their coils quite different proportions of cellular material depending on 

 the conditions at that particular instant. 



The freedom given by this point of view is particularly necessaiy in 

 trying to picture how template RNA might be associated with a pre- 

 existing ribosome. The template must contain 600 nucleotides at least 

 if it is to carry information for a polypeptide strand of 200 amino acids. 

 Thus, it is long enough to wrap three times around the 70S particle seen 

 in the electron microscope. It is, however, quite compatible with an 

 extended ribosome. 



The extended picture is also more helpful in visualizing ribosome 

 synthesis. The experiments of Caro indicate that newly formed RNA 

 shows the same localization in the cell as does DNA. Thus, eosomes 

 may initially be extended along DNA strands. The hybrid RNA-DNA 

 complexes found by Hall and Spiegelman indicate a considerable binding 



