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Weaver eoc answers 17

Weaver eoc answers 17 - Answers to Weaver end of chapter...

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Answers to Weaver end of chapter questions 1Chapter 17 The Mechanism of Translation I: Initiation 1. See Figures 17.3 and 17.4. Matthew Meselson and colleagues labeled ribosomes in E. coli with heavy isotopes, then shifted the bacteria to medium containing the corresponding light isotopes for 3.5 generations. Then they measured the densities of the ribosomes and found that they were hybrids of heavy and light ribosomal subunits. Thus, ribosomes must have dissociated and reassociated. 2. IF1 stimulates ribosome dissociation, and IF3 binds to dissociated 30S particles and prevents their reassociation with 50S particles. Severo Ochoa and colleagues provided evidence for the role of IF3 by using sucrose gradient ultracentrifugation to measure the dissociation of ribosomes. They found that the more IF3 they added, the more the equilibrium shifted toward dissociation. See Figure 17.5. 3. See Figure 17.6. Sabol and Ochoa mixed E. coli ribosomes with [ 35 S]IF3, then performed sucrose gradient ultracentrifugation. Again, the more IF3, the greater the degree of subunit dissociation. Furthermore, the peak of labeled IF3 coincided with the 30S subunit peak, but no label could be found in the 70S (actually 63S) ribosome peak, even when 50S subunits were added to stimulate ribosomal particle association. 4. The two methionyl tRN As are called tRNA Met f and tRNA Met m . The former participates in initiation by inserting fMet at the amino terminus of a nascent polypeptide. The latter participates in elongation by inserting Met into the interior of a nascent polypeptide. 5. See Figure 17.9. The start codon for the replicase gene is buried in secondary structure that involves a sequence within the coat gene. As the coat gene is being translated, the ribosome melts this secondary structure, freeing up the replicase start codon so the replicase gene can be translated. 6. The most convincing evidence for the importance of base-pairing between the Shine-Dalgarno (SD) sequence in the mRNA and the 16S rRNA comes from an intergenic suppression experiment by Anna Hui and Herman De Boer (p. 529). They cloned a human growth hormone (hGH) gene into an E. coli expression vector, and obtained large quantities of hGH. Then they changed the SD sequence of the expression vector so it could no longer base-pair with the 16S rRNA. This blocked production of hGH. Finally, they provided a copy of the 16S rRNA gene with its sequence altered to be complementary to the mutated SD sequence. This restored expression of the mutant gene. 7. See Table 17.1. IF3 by itself can stimulate binding of mRNA to either 30S ribosomal particles or whole ribosomes. IF1 and IF2 can help IF3 in this task, but they cannot provide any stimulation on their own, or even together. 8. See Figure 17.11. GTP and GDPCP are equally efficient in stimulating formation of the 30S initiation complex. Because GDPCP cannot be hydrolyzed, this means that GTP hydrolysis is not necessary for initiation complex formation.
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9. See Figure 17.14. GTP, but not GDPCP, can support dissociation of labeled IF2 from the ribosome. Thus, GTP hydrolysis is required for this dissociation.
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