Weaver eoc answers 18

Weaver eoc answers 18 - Answers to Weaver end of chapter...

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Answers to Weaver end of chapter questions 1Chapter 18 The Mechanism of Translation II: Elongation and Termination 1. See Figures 18.1 and 18.2. Labeled leucine was incorporated more into the carboxyl terminal peptides of growing 2200 - and 5 -globin polypeptides, showing that translation starts at the amino terminus. Thus, polypeptides that were partially finished before the labeled leucine was added would have unlabeled amino termini, but labeled carboxyl termini. 2. Ochoa and colleagues translated the synthetic mRNA with the sequence AUGUUU n in vitro and obtained the polypeptide fMet-Phe n . Because we know that the polypeptide was made in the N–>C direction, and because fMet is at the N-terminus of the polypeptide, it must have been inserted first. We know that AUG encodes fMet, and it is at the 5'-end of the synthetic mRNA. Therefore, the mRNA was read in the 5'–>3' direction. 3. a. Single base changes in mRNAs lead to at most one altered amino acid in the resulting proteins. If the code were overlapping, some base changes would alter two or three adjacent amino acids. b. Single nucleotide deletions or insertions have very severe consequences. They change the reading frame such that all codons downstream of the deletion or insertion are altered. If the code had commas setting off the codons, the ribosome could get back in the right reading frame after the deletion or insertion, and the consequences would be much less severe. c. Deleting or inserting three nucleotides in a row has very mild effects, strongly suggesting that the codon contains three nucleotides, and these deletions or insertions merely subtract or add a codon without changing the reading frame. Moreover, experiments with synthetic mRNAs containing repeating dinucleotides, trinucleotides, and tetranucleotides are compatible with a code of three or nine nucleotides – and three makes much more sense. d. Many amino acids are specified by more than one codon, as revealed by the experiments that led to the genetic code shown in Figure 18.6. 4. See Figure 18.5. Both AAA and AAG caused binding of labeled lysyl-tRNA to ribosomes in vitro. Therefore, both AAA and AAG code for lysine. 5. See Figure 18.7. In your answer, you could just outline the bases without showing atoms. 6. See Figure 18.10. 7. See Figure 18.11b. 8. See Figure 18.12. The idea is to add labeled fMet-tRNA Met f and see whether the labeled fMet is released by puromycin. If so, the fMet-tRNA is in the P site. If not, it is in the A site. In fact, it was released by puromycin, unlike the labeled methionine in Met-tRNA Met m , which goes to the A site. 9. See Figure 18.19. Miller and Weissbach made a complex with EF-Tu and labeled GDP, then added EF-Ts and used gel filtration chromatography to measure GDP release from the complex.
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The more EF-Ts they added, the more GDP was released. 10. Chloramphenicol blocks the peptidyl transferase step in bacterial translation.
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Weaver eoc answers 18 - Answers to Weaver end of chapter...

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