Polypeptide Synthesis and RNA 435 



QUESTIONS FOR DISCUSSION 



33.1. What conclusions can you draw from the observation that under a variety of 

 conditions, the rate of protein synthesis is proportional to ribosome concentra- 

 tion? 



33.2. What can happen to an 80s ribosome that has just completed synthesis of a 

 polypeptide? 



33.3. Discuss the hypothesis that ribosomes are viruses. 



33.4. Since seventeen leucine molecules occur in reticulocyte hemoglobin, how can 

 you explain J. R. Warner's finding an average of 7.4 leucine molecules per 

 ribosome in a polysome synthesizing hemoglobin? 



33.5. To what do you attribute the difference between polysomes composed of five 

 or six ribosomes (in reticulocytes making hemoglobin) and fifty to seventy ribo- 

 somes (in a mammalian cell infected by poliomyelitis virus)? 



33.6. In what respects would you expect the properties of sRNA to be changed or 

 unchanged by methylation? 



33.7. The non-a dimer in Hb-Lepore seems to consist of an N-terminal portion of 

 the 8 chain joined to a C-terminal portion of the ft chain. This "hybrid" S-ft 

 chain is the same length as the 8 or ft chain. Recall that the genes for 8 and 

 ft chains are closely linked, and that the 8 gene probably originated as a du- 

 plication of the ft gene. Discuss the acceptability of the following genetic ex- 

 planations for the origin of Hb-Lepore: 



(a) Two-break deletion of parts of the ft and 8 genes and the region sepa- 

 rating them 



(b) Incorrect synapsis between the ft and 8 genes followed by a crossing over 

 which produces one crossover whose gene sequence is ft, ft-8, 8 (pheno- 

 typic effect, unknown) and the complementary crossover whose gene 

 content is 8-ft. 



How does your evaluation of these genetic explanations compare with that of 

 C. Baglioni, Proc. Nat. Acad. Sci., U.S., 48:1880-1886, 1962? 



33.8. Suggest a mechanism by which both DNA strands of </>X174 can be present in 

 the RF and only one in the mature phage. 



33.9. Compare the terminal nucleotides of TMV and sRNA. What can you infer 

 from this comparison? 



33.10. What conclusions can you draw from the observation that although the ribo- 

 somal RNAs from Pseudomonas aeruginosa and Bacillus megaterium are indis- 

 tinguishable, the DNA is 64% G + C in the former, and 44% in the latter? 



33.11. Although the average cell of the adult rat liver probably divides less often 

 than once a year, it synthesizes an amount of protein equivalent to its own con- 

 tent every six or so days. In bacteria, on the other hand, the time required 

 to double the protein content is roughly equal to the generation time. Com- 

 pare the turnover of messenger RNA in bacteria with that expected in adult rat 

 liver cells. 



33.12. When native RNase is treated with urea and sulfhydryl reagents, its disulfide 

 bonds are broken and the enzyme unfolds into an inactive linear form. When 

 O.. is bubbled slowly through a solution of this denatured enzyme, the disulfide 

 bonds reform and enzymatic activity resumes. What do these results tell you 

 about the genetic basis for the folding of polypeptides? 



