310 CELL HEREDITY 



generating particles like the mitochondria of higher forms; the cyto- 

 chromes of bacteria are confined to the plasma membrane. Bacteria do 

 have ribosomes, the RNA-containing particles of the cytoplasm, which 

 seem to be comparable with those of mammalian cells. In rapidly grow- 

 ing cells of E. coli, the ribosomes contain over 80 per cent of the total 

 RNA. 



hi mammalian cells, as in bacteria, the bulk of protein synthesis ap- 

 pears to occur on the ribosomes, but the quantitative evidence is less 

 complete than in the E. coli system. So far as the analysis of protein 

 synthesis has progressed, no significant differences have been found 

 among plant, animal, and microbial systems. 



Although the knowledge about this pathway of protein synthesis is 

 still fragmentary, several points of importance have already emerged. 

 From the viewpoint of the energetics of the problem, the formation of 

 the amino acid-adenylate complex (Figure 11.2) provides the energy for 

 peptide-bond formation, so that, subsequently, when the amino acids are 

 lined up on the protein-forming template, polymerization may be ac- 

 complished with little additional energy requirement. 



With respect to arrangement of amino acids in the unique array re- 

 quired for protein specificity, it now appears likely that this occurs in 

 or on the ribosomes. Reasoning by analogy with the viruses in which 

 the protein serves as a genetically inert wrapper for the genetically 

 potent nucleic acid, many investigators now view the ribosomal RNA and 

 not the protein as the carrier of the genetic code which determines amino 

 acid sequence and, thereby, protein specificity. One obstacle in this line 

 of thinking had been the difficulty of providing a sterically acceptable 

 model in which a linear sequence of nucleotides could determine a cor- 

 responding sequence of amino acids. This particular dilemma has been 

 overcome by the unexpected finding of s-RNA as an intermediate carrier 

 of amino acids. On one proposed model, the sequential arrangement of 

 amino acids is determined by base pairing between the ribosomal RNA, 

 which carries the code, and s-RNA which carries the amino acids. 

 Among other questions, this leaves unanswered why the s-RNA molecules 

 are so large, since on formal grounds it has been shown that no more 

 than three or four nucleotides would be enough to distinguish among 20 

 different possibilities, i.e., 20 different s-RNA's. 



One way of studving this problem would be to determine whether the 

 s-RNA carries any further specificity beyond that required to dis- 

 criminate between individual amino acids. If a system for the in vitro 

 synthesis of a single well-characterized protein could be established, 

 then one might test the effects of s-RNA fractions as well as ribosomal 

 fractions from different sources upon the specificity of the product pro- 



