37. nucleic acid and protein synthesis 353 



1. The Ribosomes 



The ubiquitous ribonucleoprotein particles, recently rechristened ribo- 

 somes, 16 have come to be regarded as the major cellular site of protein 

 synthesis. The evidence that they were the initial site of appearance of new 

 protein came early in the development of our knowledge. However, we 

 shall reverse the chronology and first consider what is known about ribo- 

 somes in a physical and chemical context and then discuss the evidence 

 supporting the conclusion that they are in fact the major site of cellular 

 protein synthetic activity. 



a. Physicochemical Properties of Ribosomes 



(1) Morphology. In the electron microscope the ribosomes from a variety 

 of mammalian, bacterial, fungal, and higher plant sources appear as uni- 

 form, round electron dense particles having an average diameter of 100 to 

 200 A (when unflattened) (cf. the review by Palade 16a ). The ribosomes con- 

 tain 80-90 % of the total cellular RNA. They exist apparently free in the 

 cytoplasm in some tissues (notably bacteria and rapidly growing mam- 

 malian cells) , perhaps sometimes attached to the limiting membrane of the 

 cell (in bacteria) and frequently, in exocrine tissues, associated with lipo- 

 protein-rich membranous material of the cytoplasm. The ribosomes in 

 natural association with this membranelike material comprise what has 

 been variously named the "cytoplasmic ground substance," "ergastro- 

 plasm," or "endoplasmic reticulum" by the electron microscopists. Further- 

 more, it appears that ribonucleoprotein particles very similar to those 

 found in the cytoplasm may also be found in the nucleus of mammalian 

 cells. 1226 " 28 There is evidence that RNA exists in mitochondria 12,29 and 

 chloroplasts, 12 suggesting the possibility, at least, that particles may reside 

 in these subcellular structures as well. 



Much careful work in correlating electron microscopic observations and 

 biochemical studies 30, 31 (and cf. review by Palade 16a ) has now established 

 the fact that microsomes, the material sediment ed by centrifuging the 

 mitochondria-free supernatant fraction of a mammalian tissue homogenate 

 at 100,000 g for an hour or more, are in fact ribosomes still attached to 



26 E. de Robertis, J. Biophys. Biochem. Cytol. 2, 785 (1956). 



27 S. Osawa, K. Takata, and Y. Hotta, Biochim. et Biophys. Acta 25, 656 (1957). 



28 P. O. P. Ts'o and C. S. Sato, J. Biophys. Biochem. Cytol. 5, 59 (1959). 



29 H. M. Bates, V. M. Craddock, and M. V. Simpson, J. Am. Chem. Soc. 80, 1000 

 (1958). 



30 J. W. Littlefield, E. B. Keller, J. Gross, and P. C. Zamecnik, J. Biol. Chem. 217, 

 111 (1955). 



31 G. E. Palade and P. Siekevitz, J . Biophys. Biochem. Cytol. 2, 171 (1956). 



