Answers to Weaver end of chapter questions
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
-globin polypeptides, showing that translation starts at the amino
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
and obtained the polypeptide fMet-Phe
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
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.
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.
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
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
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.
it was released by puromycin, unlike the labeled methionine in Met-tRNA
, which goes to the
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.