Chapter 33 



POLYPEPTIDE 

 SYNTHESIS AND RNA 



T: 



|he metabolism of an organ- 

 ism is regulated primarily by 

 proteins whose purposes are 

 both structural (to make subcellular organ- 

 elles), and catalytic (to make enzymes). 

 Since the structure and function of an or- 

 ganism is so dependent upon protein, it is 

 not surprising that one of the primary ef- 

 fects of genes is to specify the amino acid 

 content of polypeptides (Chapter 32). 



Restricting our attention to DNA — the 

 genetic material in most kinds of organisms 

 — we ask, what can DNA do, or have done 

 to it, which will result in the formation of 

 particular polypeptide chains? Since we are 

 dealing with conserved DNA, that is, DNA 

 which remains part of a chromosome or 

 other structure, whatever DNA does must 

 happen in situ Since DNA is not protein, 

 it cannot be an enzyme and probably does 

 not act as a catalyst in producing its effect 

 on polypeptide formation. However. DNA 

 might possibly serve as a kind of template 

 for specifying a polypeptide. Because ribose 

 is more reactive than deoxy-D-ribose, RNA 

 is less stable than DNA. Consequently, be- 

 ing more inert, DNA is a more stable tem- 

 plate than RNA. 



We already know that after strand separa- 

 tion, each DNA strand serves as a template 

 for the formation of a complementary strand. 

 If DNA is also used as a template for gene 

 functioning, then the four different bases — 

 A, T, G, C — usually found in DNA must 

 play a role in determining the nature of the 

 423 



templates formed. In other words, the in- 

 formation for the genetic specification of a 

 polypeptide by a template mechanism would 

 have to be contained in the bases of DNA. 

 The nature of a polypeptide depends ulti- 

 mately upon its amino acid content. Many 

 polypeptide chains contain one or more of 

 each of the twenty amino acids commonly 

 found in organisms (Figure 32-4, p. 411), 

 and the number and sequence of these build- 

 ing blocks of polypeptides vary. Since both 

 polypeptides and DNA are linear structures, 

 the mechanism by which a sequence of DNA 

 nucleotides serve as a template for specify- 

 ing an amino acid sequence may be rela- 

 tively simple to visualize. 



Ribosomes 



The synthesis of hemoglobin occurs in the 

 cytoplasm of mammalian red blood corpus- 

 cles — cells which no longer have a nucleus. 

 Since the cytoplasm is the only site for pro- 

 tein synthesis in this case, it is desirable to 

 consider the structural components of the 

 cytoplasm in some detail. For all cells which 

 have been examined — plant, animal, and 

 microorganismal — electron micrographs re- 

 veal numerous bodies, called ribosomes (see 

 Figure 1-3 and Suppl. IX Fig. 2), in their 

 cytoplasm. These are particularly abundant 

 in cells actively synthesizing protein and are 

 also found in the nucleus and in chloroplasts. 

 Ribosomes isolated from ruptured cells are 

 characterized by their sedimentation rate — 

 measured in the ultracentrifuge and ex- 

 pressed in terms of sedimentation units, s. 

 The fewer the s units, the smaller the par- 

 ticle, although the relationship is not linear; 

 therefore, s values reflect ribosome size. E. 

 coli has four discrete ribosomal units: 30s, 

 50s, 70s, and 100s. The two basic sizes are 

 30s and 50s, the larger units being com- 

 posites of the basic ones, as indicated in 

 Figure 33-1. Both the 30s and 50s par- 

 ticles contain about 64% RNA and 36% 

 protein by weight. (Animal ribosomes are 



