428 



CHAPTER 47 



2 (30s) + 2 (50s); 



MOLECULAR 

 WEIGHTS 



XIO* 



r>^ .8 



1.8 



2 (70s) 

 2.7 



1 (100s) 

 5.4 



FIGURE 47-1. Relation among ribosomes having different sedimentation rates. 



consider the nature of certain particular 

 cytoplasmic components, because of the 

 possibility that these may be concerned with 

 protein synthesis. 



Electron micrographs of thin sections of 

 cells reveal numerous ribosomes in the cyto- 

 plasm of all cells which have been examined 

 for them -^ plant, animal, or microorgan- 

 ismal. They vary in size from 100-200 A 

 in diameter and are particularly abundant 

 in cells actively synthesizing protein. (Some 

 are also found in the nucleus.) It is possible 

 to rupture cells and characterize these ribo- 

 somes by size according to their sedimenta- 

 tion rate in the ultracentrifuge. Thus, in 

 terms of sedimentation units, s (the smaller 

 the number of units the smaller the particle, 

 although the relationship is not linear), 

 there are four discrete sizes in E. coli: 30s, 

 50s, 70s, and 100s. There are two basic 

 sizes, 30s and 50s, the larger ones being 

 composites of these units, as indicated in 

 Figure 47-1. Both the 30s and 50s particles 

 contain 63% RNA and 37% protein. Sev- 

 eral enzymes appear to be attached to the 

 ribosomes, including much, if not all, of the 

 cell's RNAase and part of the cell's DN Aase.^ 

 The smaller particles aggregate into the 

 larger ones when Mg++ or other divalent 

 cations are added. Most (about 80%) of the 

 RNA in a cell is contained in ribosomes. 

 (Small amounts of RNA are also reported 

 in mitochondria and plastids.) Analysis 

 shows that the RNA in ribosomes has the 

 relatively high molecular weight of .56-1.1 

 X 10*^ (having about 1000-2000 bases). 



2 See M. Tal and D. Elson (1961). 



A series of experiments can be performed ^ 

 in which radioactive amino acids are injected 

 into the body, and tissues rapidly synthe- 

 sizing proteins examined at intervals. The 

 first experiment involves injection of a large 

 dose of labeled amino acid, and shows that 

 the ribosomes become labeled almost im- 

 mediately. A second experiment uses a very 

 small dose of labeled amino acid, which is 

 expected to be used up rapidly in protein 

 synthesis. In this case, the label in the ribo- 

 some increases quickly at first, and then de- 

 creases. Finally, a third experiment can be 

 performed which demonstrates that the 

 labeled amino acid which moves out of the 

 ribosomes is actually incorporated into pro- 

 tein, for example, hemoglobin. Here, then, 

 is clear evidence that ribosomes are asso- 

 ciated with protein synthesis. Moreover, it 

 would seem that the manufacture of hemo- 

 globin takes place in the cytoplasm. But the 

 amino acid sequence in hemoglobin is sup- 

 posed to be determined as the primary func- 

 tion of DNA cistrons (Chapter 34)! How 

 can the DNA template, which apparently 

 remains in the nucleus, function to order 

 the amino acid sequence of hemoglobin, 

 which is apparently manufactured in the 

 cytoplasm? Clearly, in this respect, the DNA 

 cannot be functioning as a template directly, 

 but would have to be doing so indirectly. 



Suppose the DNA made another template 

 which was neither DNA nor protein, which 

 could leave the nucleus, enter the cytoplasm, 

 and there serve as template for protein syn- 

 thesis. A reasonable candidate for this func- 



3 Following the work of P. C. Zamecnik and co- 

 workers, and of M. Rabinovits and M. E. Olson. 



