Chapter 27 Protein Metabolism
Multiple Choice Questions
1. A certain bacterial mRNA is known to represent only one gene and to contain about 800 nucleotides.
If you assume that the average amino acid residue contributes 110 to the peptide molecular weight,
the largest polypeptide that this mRNA could code for would have a molecular weight of about:
A) 800.
B) 5,000.
C) 30,000.
D) 80,000.
E) An upper limit cannot be determined from the data given.
2. Assuming that the average amino acid residue contributes 110 to the peptide molecular weight, what
will be the minimum length of the mRNA encoding a protein of molecular weight 50,000?
A) 133 nucleotides
B) 460 nucleotides
C) 1,400 nucleotides
D) 5,000 nucleotides
E) A minimum length cannot be determined from the data given.
3. Which of the following are features of the wobble hypothesis?
A) A naturally occurring tRNA exists in yeast that can read both arginine and lysine codons.
B) A tRNA can recognize only one codon.
C) Some tRNAs can recognize codons that specify two different amino acids if both are nonpolar.
D) The “wobble” occurs only in the first base of the anticodon.
E) The “wobble” occurs only in the third base of the anticodon.
4. Which one of the following is true about the genetic code?
A) All codons recognized by a given tRNA encode different amino acids.
B) It is absolutely identical in all living things.
C) Several different codons may encode the same amino acid.
D) The base in the middle position of the tRNA anticodon sometimes permits “wobble” base pairing
with two or three different codons.
E) The first position of the tRNA anticodon is always adenosine.
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5. Which one of the following is not true about RNA editing?
A) RNA editing occurs mostly in mitochondria and chloroplasts.
B) RNA editing requires guide RNAs that have sequences complementary to the edited mRNA.
C) Deamination of G is a common reaction in RNA editing.
D) Deamination of A is common reaction in RNA editing.
E) RNA editing allows for the synthesis of two different proteins from one gene.
6. Which of the following is not one of the five steps of protein synthesis?
A) Termination and ribosome degradation
B) Elongation of the peptide chain
C) Initiation of peptide synthesis
D) Activation of the amino acids
E) Protein folding and posttranslational processing
7. Which one of the following statements about ribosomes is true?
A) The large subunit contains rRNA molecules; the small subunit does not.
B) The RNA in ribosomes plays a structural, not catalytic, role.
C) There are about 25 ribosomes in an E. coli cell.
D) There are two major ribosomal subunits, each with multiple proteins.
E) Ribosomes are relatively small, with molecular weights less than 10,000.
8. Which of the following statements about tRNA molecules is false?
A) A, C, G, and U are the only bases present in the molecule.
B) Although composed of a single strand of RNA, each molecule contains several short, double–
helical regions.
C) Any given tRNA will accept only one specific amino acid.
D) The amino acid attachment is always to an A nucleotide at the 3‘ end of the molecule.
E) There is at least one tRNA for each of the 20 amino acids.
9. Which of the following statements about the tRNA that normally accepts phenylalanine is false?
(mRNA codons for phenylalanine are UUU and UUC.)
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A) It interacts specifically with the Phe synthetase.
B) It will accept only the amino acid phenylalanine.
C) Its molecular weight is about 25,000.
D) Phenylalanine can be specifically attached to an —OH group at the 3‘ end.
E) The tRNA must contain the sequence UUU.
10. Which of the following is not true of tRNA molecules?
A) The 3‘-terminal sequence is —CCA.
B) Their anticodons are complementary to the triplet codon in the mRNA.
C) They contain more than four different bases.
D) They contain several short regions of double helix.
E) With the right enzyme, any given tRNA molecule will accept any of the 20 amino acids.
11. Aminoacyl-tRNA synthetases (amino acid activating enzymes):
A) “recognize” specific tRNA molecules and specific amino acids.
B) in conjunction with another enzyme attach the amino acid to the tRNA.
C) interact directly with free ribosomes.
D) occur in multiple forms for each amino acid.
E) require GTP to activate the amino acid.
12. In E. coli, aminoacyl-tRNA synthetases:
A) activate amino acids in 12 steps.
B) are amino acid–specific; there is at least one enzyme specific for each amino acid.
C) fall into two classes, each of which attaches amino acids to different ends of the tRNA.
D) have no proofreading activities.
E) require a tRNA, an amino acid, and GTP as substrates.
13. Which of the following statements about aminoacyl-tRNA synthetases is false?
A) Some of the enzymes have an editing/proofreading capability.
B) The enzyme attaches an amino acid to the 3‘ end of a tRNA.
C) The enzyme splits ATP to AMP + PPi.
D) The enzyme will use any tRNA species but is highly specific for a given amino acid.
E) There is a different synthetase for every amino acid.
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14. The enzyme that attaches an amino acid to a tRNA (aminoacyl-tRNA synthetase):
A) always recognizes only one specific tRNA.
B) attaches a specific amino acid to any available tRNA species.
C) attaches the amino acid at the 5‘ end of the tRNA.
D) catalyzes formation of an ester bond.
E) splits ATP to ADP + Pi.
15. In the “activation” of an amino acid for protein synthesis:
A) leucine can be attached to tRNAPhe, by the aminoacyl-tRNA synthetase specific for leucine.
B) methionine is first formylated, then attached to a specific tRNA.
C) the amino acid is attached to the 5‘ end of the tRNA through a phosphodiester bond.
D) there is at least one specific activating enzyme and one specific tRNA for each amino acid.
E) two separate enzymes are required, one to form the aminoacyl adenylate, the other to attach the
amino acid to the tRNA.
16. Which of the following is/are true for protein synthesis in eukaryotes?
A) All proteins are initially synthesized with methionine at their C-terminus.
B) All proteins are initially synthesized with methionine at their N-terminus.
C) All proteins are initially synthesized with tryptophan at their C-terminus.
D) All proteins are initially synthesized with a multiple of 3 amino acids in their sequence.
E) None of the above
17. Formation of the ribosomal initiation complex for bacterial protein synthesis does not require:
A) EF-Tu.
B) formylmethionyl tRNAfMet.
C) GTP.
D) initiation factor 2 (IF-2).
E) mRNA.
18. In bacteria the elongation stage of protein synthesis does not involve:
A) aminoacyl-tRNAs.
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B) EF-Tu.
C) GTP.
D) IF-2.
E) peptidyl transferase.
19. Which one of the following statements about the elongation phase of protein synthesis is true?
A) At least five high-energy phosphoryl groups are expended for each peptide bond formed.
B) During elongation, incoming aminoacylated tRNAs are first bound in the P site.
C) Elongation factor EF-Tu facilitates translocation.
D) Peptidyl transferase catalyzes the attack of the carboxyl group of the incoming amino acid on an
ester linkage in the nascent polypeptide.
E) Peptidyl transferase is a ribozyme.
20. Which of the following statements about bacterial mRNA is true?
A) A ribosome usually initiates translation near the end of the mRNA that is synthesized last.
B) An mRNA is never degraded but is passed on to the daughter cells at cell division.
C) During polypeptide synthesis, ribosomes move along the mRNA in the direction 5‘ → 3‘.
D) Ribosomes cannot initiate internally in a polycistronic transcript.
E) The codon signaling peptide termination is located in the mRNA near its 5‘ end.
21. Bacterial ribosomes:
A) bind tightly to specific regions of DNA, forming polysomes.
B) contain at least one catalytic RNA molecule (ribozyme).
C) contain three species of RNA and five different proteins.
D) have specific, different binding sites for each of the 20 tRNAs.
E) require puromycin for normal function.
22. The large structure consisting of a mRNA molecule being translated by multiple copies of the
macromolecular complexes that carry out protein synthesis is called a:
A) lysosome.
B) polysome.
C) proteosome.
D) ribosome.
E) synthosome.
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23. It is possible to convert the Cys that is a part of Cys-tRNACys to Ala by a catalytic reduction. If the
resulting Ala-tRNACys were added to a mixture of (1) ribosomes, (2) all the other tRNAs and amino
acids, (3) all of the cofactors and enzymes needed to make protein in vitro, and (4) mRNA for
hemoglobin, where in the newly synthesized hemoglobin would the Ala from Ala-tRNACys be
incorporated?
A) Nowhere; this is the equivalent of a nonsense mutation.
B) Wherever Ala normally occurs
C) Wherever Cys normally occurs
D) Wherever either Ala or Cys normally occurs
E) Wherever the dipeptide Ala-Cys normally occurs
24. Approximately how many NTPs must be converted to NDPs to incorporate one amino acid into a
protein?
A) 0
B) 1
C) 2
D) 4
E) 8
25. Which one of the following antibiotics does not function by interfering with the translational process?
A) Chloramphenicol
B) Cycloheximide
C) Penicillin
D) Puromycin
E) Streptomycin
26. Which of the following is true about the sorting pathway for proteins destined for incorporation into
lysosomes or the plasma membrane of eukaryotic cells?
A) Binding of SRP to the signal peptide and the ribosome temporarily accelerates protein synthesis.
B) The newly synthesized polypeptides include a signal peptide at their carboxyl termini.
C) The signal peptide is cleaved off inside the mitochondria by signal peptidase.
D) The SRP binds to the signal peptide soon after it appears outside the ribosome.
E) The signal sequence is added to the polypeptide in a posttranslational modification reaction.
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27. Glycosylation of proteins inside the endoplasmic reticulum does not involve:
A) a His residue on the protein.
B) an Asn residue on the protein.
C) dolichol phosphate.
D) glucose.
E) N-acetylglucosamine.
28. Posttranslational glycosylation of proteins is inhibited specifically by:
A) chloramphenicol.
B) cycloheximide.
C) puromycin.
D) streptomycin.
E) tunicamycin.
29. The signal sequences that direct proteins to the nucleus are:
A) always at the amino terminus of the targeted protein.
B) cleaved after the protein arrives in the nucleus.
C) glycosyl moieties containing mannose 6-phosphate residues.
D) not located at the ends of the peptide, but in its interior.
E) the same as those that direct certain proteins to lysosomes.
30. The pathway for polypeptides exported from E. coli includes the following steps, which occur in what
order for correct export?
1. A chaperone, SecA, binds to the polypeptide.
2. A chaperone, SecB, binds to the polypeptide.
3. ATP is hydrolyzed by Sec A.
4. SecA pushes 20 amino acids of the polypeptide into the translocation complex.
A) 1, 2, 3, 4
B) 1, 2, 4, 3
C) 2, 1, 4, 3
D) 2, 3, 1, 4
E) 3, 1, 4, 2
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31. Which of the following is not utilized for protein degradation in eukaryotes?
A) ATP
B) Ubiquitin ligases
C) Deubiquitinases
D) Lon protease
E) Proteasome
32. Ubiquitin-mediated protein degradation is a complex process, and many of the signals remain
unknown. One known signal involves recognition of amino acids in a processed protein that are
either stabilizing (Ala, Gly, Met, Ser, etc.) or destabilizing (Arg, Asp, Leu, Lys, Phe, etc.) and are
located at:
A) a helix-turn-helix motif in the protein.
B) a lysine-containing target sequence in the protein.
C) a zinc finger structure in the protein.
D) the amino-terminus of the protein.
E) the carboxy-terminus of the protein
Short Answer Questions
33. Explain how insertions and deletions were used to provide evidence that the genetic code was a triplet
code.
34. Describe two ways in which synthetic polynucleotides were used in solving the genetic code (you
need not describe how the synthetic polynucleotides were made).
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35. You have isolated a fragment of viral DNA that totally encodes at least two proteins, 120 and 80
amino acids long. The DNA fragment is 400 base pairs long. (a) Why might you consider this
unusual? (b) You sequence the two proteins and find no sequence homology. Propose a model to
account for these findings.
36. Consider the following hypothetical short mRNA; what would be the sequence of the protein
produced if this were translated in an E. coli cell?
5’–AUAGGAGGUUUGACCUAUGCCUCGUUUAUAGCC-3’
37. The template strand of a segment of double-stranded DNA contains the sequence:
(5‘)-CTT TGA TAA GGA TAG CCC TTC–(3’)
(a) What is the base sequence of the mRNA that can be transcribed from this strand? (b) What amino
acid sequence could be coded by the mRNA base sequence in (a), using only the first reading frame
starting at the 5‘ end? (Refer to Fig. 27-7, p. 1107.) (c) Suppose the other (complementary) strand is
used as a template for transcription. What is the amino acid sequence of the resulting peptide, again
starting from the 5‘ end and using only the first reading frame?
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38. Describe the possible outcomes that could occur because of a single base change in an mRNA.
39. The following sequence of four amino acids occurred in the structure of a polypeptide found in a
wild-type organism: Leu-Ser-Ile-Arg. Several mutants were isolated, each of which carried a single
base pair change in the region of DNA that coded for this amino acid sequence. Their corresponding
amino acid sequences are:
Mutant
1 MET-Ser-Ile-Arg
2 Leu-TRP-Ile-Arg
3 Leu-Ser-ARG-Arg
4 Leu-Ser-Ile-PRO
5 Leu-Ser-Ile-TRP
What was the nucleotide sequence of the region of mRNA that coded for the amino acid sequence in
the wild-type organism? (Refer to Fig. 27-7, p. 1107.)
40. In protein synthesis, 61 codons specify the 20 amino acids. Base pairing between the codon and the
tRNA anticodon assures that the correct amino acid will be inserted into the nascent polypeptide
chain. Why then does the cell require only 32 different tRNAs to recognize 61 different codons?
41. Indicate whether the following statements are true (T) or false (F).
___A ribosome is the complex within which protein synthesis occurs.
___Ribosomes contain many separate proteins.
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___Ribosomes contain many different RNAs.
___The three ribosomal RNAs in a bacterial ribosome are distributed in three separate, large
ribosomal subunits.
___The catalytic machinery of the ribosome is protein-based.
___There are four binding sites for aminoacyl-tRNAs on a ribosome.
42. The process of charging tRNAs with their cognate amino acids involves multiple proofreading steps
to increase the overall fidelity. Briefly describe these steps.
Ans: There are two main stages of selection: (1) The synthetase strongly favors activation of the
43. The recognition of an amino acid by its cognate aminoacyl-tRNA synthetase is said to involve a
“second genetic code.” What is meant by this?
44. In 1961, Howard Dintzis carried out an experiment that defined the direction of polypeptide chain
growth during protein synthesis in cells. The experiment involved the analysis of hemoglobin
molecules that were being synthesized in reticulocytes in the presence of radioactive amino acids.
Describe the analysis and how it demonstrated the direction of chain growth.
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45. A given mRNA sequence might be translated in any of three reading frames. Describe how
prokaryotes and eukaryotes determine the correct reading frame.
46. Match the factor or enzyme at the right with the stage(s) of protein synthesis at which it acts. If a
factor or enzyme participates in two stages of protein synthesis, indicate both of them.
___ Amino acid activation (a) RF1
___ Initiation (b) EF-Tu
___ Elongation (c) Aminoacyl-tRNA
___ Termination (d) Shine-Dalgarno sequence
47. Indicate whether each of the following statements is true (T) or false (F).
___Assembly of a complete ribosome onto an mRNA requires ATP hydrolysis.
___Aminoacylation or “charging” of tRNA requires the formation of an aminoacyl-AMP
intermediate.
___Aminoacyl-tRNA binding to the A site of the ribosome requires the accessory factor EF–G
and GTP hydrolysis.
___Translocation of a growing polypeptide from the A to the P site on the ribosome requires EF–
G and GTP hydrolysis.
___Termination of translation requires release factors, but no NTP hydrolysis.
48. Briefly describe the role of the following components in bacterial protein synthesis.
(a) Initiation factor 2 (IF-2)
(b) 16S RNA
(c) Peptidyl transferase
(d) Release factors
(e) Elongation factor G (EF-G)
(f) N10-formyltetrahydrofolate
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(g) ATP
(h) tRNAfMet
49. Number the following steps in the proper order with regard to protein synthesis.
___ Aminoacyl-tRNA binds to the A site.
___ Deacylated tRNA is released from ribosome.
___ Peptide bond formation shifts the growing peptide from the P to the A site.
___ The 50S subunit binds to the initiation complex of the 30S subunit and mRNA.
50. Describe what would happen to bacterial protein synthesis if EF-Tu could recognize tRNAfMet.
51. Indicate whether each of the following statements is true (T) or false (F).
___ Bacterial mRNA is broken down within a few minutes of its formation in E. coli.
___ Bacterial mRNA consists only of the bases that code for amino acids.
___ Polysomes do not necessarily contain mRNA.
___ Bacterial mRNA normally occurs as a double-stranded structure, with one strand containing
codons, the other containing anticodons.
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___ Bacterial mRNA can be translated while it is still being synthesized.
Protein synthesis
Pages: 1124–1136 Difficulty: 2
52. Regarding translation in eukaryotes versus that in prokaryotes (bacteria), indicate whether each of the
following statements is true (T) or false (F).
___ In eukaryotes, the 3‘ end of the mRNA is associated with the 5‘ end during initiation, whereas
in prokaryotes, it is not.
___ In prokaryotes, it is initiated at an AUG near a Shine-Dalgarno sequence in the mRNA,
whereas in eukaryotes, it is initiated at an AUG near the 3‘ end of the mRNA.
___ In prokaryotes, it is initiated with Met, whereas in eukaryotes, it is initiated with fMet.
___ In prokaryotes, translation and transcription are coupled, whereas in eukaryotes, they are not.
Protein synthesis
Pages: 1124–1136 Difficulty: 2 Ans: B
53. Polypeptide chain elongation in E. coli occurs by the cyclical repetition of three steps. What are these
steps and what cellular components are necessary for each of them to occur?
54. The ribosome does not have a comparable protease proofreading function like the exonuclease
activity found in DNA polymerase. What is the “technical difficulty” that perhaps prevented the easy
evolution of such an activity?
55. A new antibiotic was recently discovered that inhibits prokaryotic protein synthesis. In the presence
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of the antibiotic, protein synthesis can be initiated, but only dipeptides that remain bound to the
ribosome are formed. What specific step of protein synthesis is likely to be blocked by this
antibiotic?
56. Following the synthesis of their polypeptide chain, many proteins require further posttranslational
modifications before they attain their full biological activity or function. List and describe briefly at
least four possible types of modification that can occur.
57. In no more than three sentences, describe a nonsense suppressor tRNA and how it differs from a
normal tRNA.
58. When first synthesized, proinsulin has an additional leader or signal peptide at its amino terminus.
This complete molecule is called preproinsulin and the signal peptide is cleaved off to give
proinsulin. Briefly, what is the likely function of the signal peptide?
59. Describe the sequence of events between the transcription of an mRNA for a secreted protein and the
arrival of that protein in the lumen of the endoplasmic reticulum.
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60. What are the stages in targeting of nuclear proteins, and why are the targeting sequences not removed
upon arrival of the protein in the nucleus?
61. Describe the role of ubiquitin in mediating intracellular protein breakdown.