Chapter 40 The Mechanism of Protein Synthesis
Matching Questions
Use the following to answer questions 1–10:
Choose the correct answer from the list below. Not all of the answers will be used.
a) initiation factors
b) ATP
c) ricin
d) one
e) Shine-Dalgarno
f) elongation factor Tu
g) polysome
h) two
i) kanamycin
j) release factor
k) N-formylmethionine
l) signal sequence
m) GTP
1.
The sequence that interacts with a complementary sequence on the 3 end of the 16S rRNA is
_______________.
2.
_______________ include(s) IF1, IF2, and IF3 in prokaryotes.
Ans: a
Section: 40.1
3.
_______________ is the first amino acid in bacteria and typically is chemically modified.
Ans: k
Section: 40.1
4.
EF-G translocase proteins use the energy of _______________ hydrolysis.
Ans: m
Section: 40.2
5.
_______________ inhibits the interaction between tRNA and 16S rRNA.
Section: 40.4
6.
The biomolecule _______________ delivers the aminoacyl-tRNA to the “A site” in E. coli.
Ans: f
Section: 40.1
7.
The _______________ proteins recognize UAA, UAG, and UGA codons.
Ans: j
Section: 40.2
Ans: e
Section: 40.1
Chapter 40 The Mechanism of Protein Synthesis
2
8.
_______________ is a sequence of 9–12 hydrophobic amino acid residues, sometimes
containing positively charged amino acids.
9.
How many release factors do eukaryotes have?
Ans: d
Section: 40.3
10.
The protein _______________ is toxic because it hydrolyzes a critical adenine base from the
28S RNA molecule of eukaryotes.
Ans: c
Section: 40.4
Fill–in-the-Blank Questions
11.
In a ribosome, A stands for _____ and P stands for _____.
Ans: aminoacyl; peptidyl Section: 40.1
12.
mRNA that codes for more than one protein is considered _____.
Ans: polycistronic Section: 40.1
13.
The first amino acid in bacterial proteins is _____.
Ans: N-formylmethionine Section: 40.1
14.
After the peptide bond forms, the mRNA and tRNA are _____, or moved, a distance of one
codon.
Ans: translocated Section: 40.2
15.
GTP binds to the _____ protein in the translocation mechanism.
Ans: EF–G Section: 40.2
16.
Release factors function by allowing _____ to attack the ester linkage between tRNA and amino
acid.
Ans: water Section: 40.2
17.
Proteins are synthesized in the _____ direction.
Ans: N → C, or amino to carboxyl Section: Introduction
18.
In E. coli, the signal sequence that properly aligns the initiation codon in the P site is called the
_____ sequence.
Ans: Shine–Dalgarno Section: 40.1
19.
Initiation, elongation, and termination all require factors that are members of the _____ family.
Ans: G-protein Section: 40.2
20.
The signal sequence that marks a protein for translocation across the endoplasmic reticulum
membrane is located near the _____ end of the protein.
Ans: N-terminal or amino-terminal Section: 40.5
Ans: l
Section: 40.5
Chapter 40 The Mechanism of Protein Synthesis
3
Multiple-Choice Questions
21.
Eukaryotic mRNA is
A)
linear.
B)
circular.
C)
modified only at the ribose.
D)
discontinuously translated.
E)
None of the above.
22.
In vanishing white matter disease, nerve cells disappear due to
A)
phagocytosis of living cells.
B)
an autoimmune result of mutations in mRNA.
C)
mutation in release factors.
D)
mutations in initiation factor 2.
E)
mutations in the tRNA charging mechanism.
Ans: D Section: 40.3
23.
Chloramphenicol acts by
A)
binding to the peptidase cell-wall enzyme.
B)
inhibiting peptidyl transferase activity.
C)
inhibiting protein synthesis.
D)
binding to fMET tRNA.
E)
changing the shape of the mRNA.
Ans: B Section: 40.4
24.
Ricin is made from castor beans and is a strong toxin, which
A)
removes an adenine from 28S rRNA.
B)
glycosylates tRNA.
C)
inhibits protein synthesis initiation.
D)
blocks the A binding site.
E)
coats the mRNA.
Ans: A Section: 40.4
25.
Which of the following is required for the initiation of protein synthesis in prokaryotes?
A)
mRNA
B)
the 30S subunit
C)
fMet-tRNAf
D)
GTP
E)
All of the above.
Ans: E Section: 40.1
26.
What is the order of the tRNA binding sites on the 70S ribosome with respect to the 5′ → 3′
direction of the mRNA?
A)
E P A
B)
P A E
C)
A E P
D)
P E A
E)
E A P
Ans: A Section: 40.1
Ans: B Section: 40.3
Chapter 40 The Mechanism of Protein Synthesis
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27.
What is the direction of translation of mRNA?
A)
bidirectional
B)
5 → 3
C)
3 → 5
D)
All of the above.
E)
None of the above.
28.
Signals that define the beginning and end of protein synthesis are contained in
A)
rRNA.
B)
tRNA.
C)
mRNA.
D)
the ribosome.
E)
protein.
Ans: C Sections: 40.1, 40.2
29.
Which of the following E. coli proteins is responsible for translocation of the mRNA as a result
of a conformation change due to the hydrolysis of GTP?
A)
RRF
B)
EF-Tu
C)
EF–G
D)
Shine–Delgarno
E)
None of the above.
Ans: C Section: 40.2
30.
What factor(s) is (are) necessary for the correct placement of the mRNA and initiating tRNA on
the ribosome?
A)
IF1
B)
IF2
C)
IF3
D)
A and B.
E)
A, B, and C.
Ans: E Section: 40.1
31.
Eukaryotic and prokaryotic translation differs primarily in which step?
A)
initiation
B)
elongation
C)
termination
D)
All of the above.
E)
None of the above.
Ans: A Section: 40.3
32.
What is the mechanism of action of the A fragment of diphtheria toxin?
A)
binding the 60S subunit
B)
cleaving tRNA
C)
the chemical modification of EF2
D)
All of the above.
E)
None of the above.
Ans: B Section: 40.1
Chapter 40 The Mechanism of Protein Synthesis
5
33.
An error in the synthesis of an RNA molecule or a protein molecule will not be passed on to
subsequent generations. Suggest a reason why there is a proofreading mechanism in protein
synthesis but not RNA synthesis?
A)
Protein proofreading occurs during protein folding events; thus, the rate of protein
synthesis is not reduced.
B)
Protein proofreading occurs during post-translational splicing of proteins (such as
insulin); thus, the rate of protein synthesis is not reduced.
C)
Protein synthesis takes place at a much slower rate than RNA synthesis; thus, giving time
for improper codon-anticodon binding destabilization.
D)
The mischarging of some tRNAs is corrected by the hydrolytic action of aminoacyl-
tRNA synthetase; thus, mistakes are prevented.
E)
Some tRNAs are proofread in the E site triggering hydrolytic action of peptidyl
transferase; thus, mistakes are corrected as they are made.
34.
What would happen if you introduced a substitution of two of the Gs for two Cs in the Shine-
Dalgarno sequence?
A)
Weak base-pairing between the mRNA and the 3 end of the 16S rRNA could terminate
initiation.
B)
Weak base-pairing between the 5 end of the tRNA and the 18S rRNA could cause the
insertion of the wrong amino acid.
C)
Weak base-pairing between the codon and the anticodon sequences could cause the
insertion of the wrong amino acid.
D)
The Shine-Dalgarno sequence allows for the proper pairing of each amino acid with its
tRNA, which would lead to an incorrect protein sequence.
E)
The 16S and 18S ribosomal subunits would not be bound tightly enough and early chain
termination would occur.
Ans: A Section: 40.3
35.
Suppose you added 20 amino acids, each carrying a 14C label to as protein synthesis system with
only one mRNA present. After one minute you purify the newly synthesized protein and then
cleave it through several rounds first with an aminopeptidase and then a carboxypeptidase. The
majority of the radioactive label is in the carboxypeptidase digest. What does this tell you?
A)
tRNA molecules are not bound to amino acids until the mRNA binds to the 16S
ribosomal site.
B)
The newly synthesized protein is not labeled because it is screened by the 18S rRNA.
C)
Protein synthesis occurs relative to the 3 to 5 direction of the mRNA.
D)
Protein synthesis occurs in the amino-terminal to carboxy-terminal direction.
E)
Carboxypeptidase has a much higher kcat than aminopeptidase.
Ans: D Section: 40.2
Ans: D Section: 40.1
Chapter 40 The Mechanism of Protein Synthesis
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36.
Even if the protein synthesis machinery in the ribosomes is flawless, could there be other pitfalls
in the formation of a biologically active protein? Why?
A)
Yes, a defect in the signal-recognition particle fails to recognize the termination signal.
B)
Yes, a defect in the signal-recognition particle fails to recognize the signal sequence.
C)
Yes, The splicing mechanism of the 18S rRNA is defective so proteins do not get spliced
properly.
D)
No, proper protein folding is driven by hydrophobic interactions.
E)
No, protein tertiary structure is determined by the primary structure.
37.
What explains the remarkable toxicity of diphtheria toxin?
A)
Initiation is inhibited causing misreading of the mRNA.
B)
Binding to the 30S subunit inhibits the binding of aminoacyl-tRNAs.
C)
EF2’s capacity to carry out the translocation of the growing polypeptide chain is blocked.
D)
Causes premature chain termination by acting as an analog of aminoacyl-tRNA.
E)
Binding to the 50S ribosomal subunit inhibits peptidyl transferase.
Ans: C Section: 40.4
Short-Answer Questions
38.
What is the final destination of proteins directed to the secretory pathway?
become integral membrane proteins.
Section: 40.5
39.
Can a protein fully translated and released from a ribosome enter the mitochondria or ER
lumen?
Ans:
No, entry into the mitochondria or ER lumen must happen while the newly synthesized
protein is in transit through the ribosome.
Section: 40.5
40.
What is the function of RF1?
Ans:
RF1 is a release factor that recognizes UAA or UAG. When it binds to the ribosome, it
unfolds to bridge the gap between the Stop codon and the peptidyl transferase.
Section: 40.2
41.
What is an iron response element?
An IRE is a 5 untranslated stem-loop structure on mRNA that binds an IRE-binding
protein that blocks initiation.
Section: 40.6
42.
What proteins are involved in the circularization of eukaryotic mRNA?
Ans:
The eIF-4E protein that binds to the mRNA cap is also bound to the poly(A) tail through
the eIF-4G protein and the poly(A)-binding protein.
Section: 40.3
Ans: B Section: 40.5
Chapter 40 The Mechanism of Protein Synthesis
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43.
Describe RNAi.
44.
Describe the subunits of E. coli ribosomes.
sedimentation coefficient of 70S. The 30S subunit contains 21 proteins and a 16S RNA
molecule. The large subunit contains 34 proteins and 5S and 23S RNA molecules.
Section: 40.1
45.
What are the names for the tRNA binding sites on the ribosome?
Ans:
They are called the A, P, and E sites (aminoacyl, peptidyl, and exit).
Section: 40.1
46.
What is the role of the translocon in protein synthesis?
Ans:
The translocon is a multi-subunit protein-conducting channel in the ER membrane. The
translocon allows the growing peptide chain to pass into the ER.
Section: 40.5
47.
What role does the amino acid itself play in the accurate placement of the charged tRNA at the
codon?
Ans:
None. Experiments prove that only the anticodon–codon interactions participate, and the
amino acid is not involved in the recognition.
Section: 40.2
48.
What is the first step in the elongation cycle?
Ans:
The first step is the placement of the correct aminoacyl-tRNA into the A site by the EF-
Tu factor.
Section: 40.2
49.
Give the chemical mechanism that is involved in the peptide bond formation as catalyzed by
peptidyl transferase on the 23S sRNA.
Section: 40.2; Figure 40.7
Ans:
RNA interference is a means for a cell to regulate protein synthesis. Small microRNA
molecules bind to mRNA, targeting that RNA for degradation.
Section: 40.6
Chapter 40 The Mechanism of Protein Synthesis
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50.
How does puromycin inhibit protein synthesis?