COMPETITIVE EXAM MCQs SERIES of LIFE SCIENCES for CSIR-UGC NET/JRF, SLET, GATE, and other entrance tests: FUNDAMENTAL PROCESSES – Protein Synthesis and Processing.
Syllabus Outline
- Role of the ribosome in protein synthesis.
- Formation of initiation complex, initiation factors and their regulation.
- Elongation and elongation factors.
- Termination and genetic code.
- Aminoacylation of tRNA, tRNA-identity, aminoacyl tRNA synthetase, and translational proof-reading.
- Translational inhibitors
- Post-translational modification of proteins.
This quiz contains concept-based, most frequently asked 25 MCQs of “FUNDAMENTAL PROCESSES – Protein Synthesis and Processing”. Each question has a single correct/most appropriate answer.
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1. Human mitochondria encode only 22 tRNAs, but they can translate all 61 sense codons. Which mechanism allows this, and which codon reassignment in human mitochondria is correct?
A) Expanded wobble base pairing and UGA codes for Tryptophan
B) Import of many cytosolic tRNAs and AUA codes for Methionine
C) Ribosomal ambiguity and AGA codes for Serine
D) tRNA anticodon editing and UAG codes for Leucine
2. Which of the following functions is NOT attributed to Initiation Factor 3 (IF3)?
A) Preventing the premature association of the 50S and 30S ribosomal subunits.
B) Facilitating the dissociation of deacylated tRNA from the P-site during ribosome recycling.
C) Promoting the hydrolysis of GTP bound to IF2 upon 50S subunit joining.
D) Destabilising 30S initiation complexes containing non-canonical start codons or codon-anticodon mismatches.
3. A cell culture is treated with Harringtonine, a drug that blocks initiation by preventing the formation of the first peptide bond, but does not affect elongation of already initiated ribosomes. After 20 minutes of treatment, what change would be observed in the polysome profile compared to untreated cells?
A) Larger polysomes accumulate due to ribosome stalling
B) Polysomes collapse into monosomes
C) mRNAs shift into heavier polysome fractions
D) No change in the polysome profile
4. The antibiotic Puromycin is a structural analogue of:
A) The anticodon loop of tRNA.
B) The EF-Tu binding domain of the ribosome.
C) The pyrophosphate tail of GTP.
D) The 3′ end of aminoacyl-tRNA.
5. Diphtheria toxin is a lethal inhibitor of eukaryotic protein synthesis. Its molecular mechanism involves:
A) Proteolytic cleavage of eIF4G shuts down cap-dependent translation.
B) Phosphorylation of eIF2-alpha, inducing the integrated stress response.
C) Depurination of a specific adenine in the 28S rRNA sarcin-ricin loop.
D) ADP-ribosylation of a specific diphthamide residue on eEF2, inhibiting translocation.
6. Considering the energetics of protein synthesis, which statement correctly identifies the energy source for the formation of the peptide bond itself?
A) Hydrolysis of ATP by the 23S rRNA ribozyme during the peptidyl transfer reaction.
B) Hydrolysis of GTP by EF-Tu during the accommodation step.
C) The intrinsic high energy of the ester bond linking the amino acid to the tRNA.
D) The proton motive force across the membrane in coupled transcription-translation.
7. In bacteria, following peptide release, the ribosome remains bound to the mRNA with deacylated tRNA in the P-site. Disassembly of this complex requires:
A) The ATPase activity of ABCE1 (Rli1) to split the subunits.
B) The binding of Ribosome Recycling Factor (RRF) to the A-site.
C) The action of Initiation Factor 1 (IF1) to displace the tRNA from the P-site.
D) Spontaneous dissociation due to thermal fluctuations.
8. What is the molecular basis for the “Elongation Arrest” caused by Signal Recognition Particle (SRP) binding?
I – The SRP S-domain binds the signal sequence
II – The Alu-domain physically occupies the elongation factor binding site on the ribosome.
III – SRP phosphorylates the ribosome, inducing a conformational change that clamps the mRNA.
IV – SRP wraps around the exit tunnel and crosslinks the nascent chain to the tunnel wall.
V – SRP competes with aminoacyl-tRNA for binding to the A-site.
I only
I and II
II and III
IV and V
9. In Mucolipidosis, lysosomal enzymes are synthesised but are secreted outside the cell rather than being targeted to the lysosome. This phenotype is caused by a defect in:
A) The Mannose-6-Phosphate Receptor.
B) The Clathrin adaptor protein AP-1.
C) N-acetylglucosamine-1-phosphotransferase.
D) The proton pump responsible for lysosomal acidification.
10. What is the key mechanism of action involved in the inhibition of N-linked glycosylation by Tunicamycin?
A) Inhibiting the Oligosaccharyltransferase complex.
B) Blocking the formation of the first lipid-linked intermediate.
C) Preventing the “flipping” of the Dolichol-linked oligosaccharide from the cytosol to the lumen.
D) Inhibiting the glucosidases responsible for trimming the glycan in the Endoplasmic Reticulum.
11. Nonsense-Mediated Decay (NMD) degrades mRNAs containing premature termination codons (PTCs). How does the mammalian cell distinguish a PTC from a normal stop codon?
A) PTCs usually have a different sequence context.
B) PTCs cause the ribosome to stall longer than normal stops.
C) PTCs are recognised by a specialised release factor eRF4.
D) PTCs are located upstream of an Exon-Junction Complex.
12. What happens to an mRNA that lacks a stop codon in eukaryotes?
A) The ribosome translates into the Poly(A) tail.
B) The ribosome falls off at the end of the 3′ UTR and recycles normally.
C) The mRNA is circularised and translated indefinitely.
D) The ribosome initiates Nonsense-Mediated Decay (NMD).
13. Which of the following correctly pairs the ER retrieval signal with its receptor mechanism?
A) KDEL is found on membrane proteins and binds COPII.
B) KKXX is found on soluble proteins and binds the KDEL receptor.
C) KDEL is found on soluble ER residents and binds the KDEL receptor in the Golgi.
D) KKXX is found on membrane proteins and binds Clathrin.
14. Treatment of cells with Rapamycin results in a shift of mRNAs encoding Ribosomal Proteins from heavy polysomes to sub-polysomal fractions. This indicates:
A) Rapamycin promotes the degradation of these mRNAs.
B) Rapamycin induces premature termination.
C) Rapamycin inhibits elongation, causing runoff.
D) Rapamycin specifically inhibits the translation initiation of 5′ TOP mRNAs.
15. Protein splicing involves the excision of an internal segment called an:
A) Intron
B) Intein
C) Extein
D) Ubiquitin
16. Glycosylphosphatidylinositol anchors are attached to proteins in the:
A) Cytosol
B) ER Lumen
C) Golgi Lumen
D) Plasma Membrane
17. O-GlcNAcylation is a reversible modification occurring on Serine/Threonine residues in the cytosol/nucleus. It often competes directly with:
A) Methylation
B) Acetylation
C) Phosphorylation
D) Ubiquitination
18. Which lipid modification is reversible and used for dynamic regulation of membrane association?
A) Myristoylation
B) Farnesylation
C) Palmitoylation
D) Geranylgeranylation
19. In eukaryotes, the joining of the 60S subunit to the 40S subunit to form the 80S ribosome is the final step of initiation. This step is catalysed by eIF5B. Which statement regarding eIF5B is true?
A) eIF5B is a GTPase that is homologous to the bacterial factor IF2.
B) eIF5B hydrolysis of GTP is required for the release of eIF2-GDP from the P-site.
C) eIF5B binds to the A-site and checks the fidelity of the start codon recognition before subunit joining.
D) eIF5B functions as a scaffold protein to hold eIF1A and eIF1.
20. Protein Disulfide Isomerase (PDI) helps proteins form correct disulfide bonds in the endoplasmic reticulum (ER). If a protein forms wrong (incorrect) disulfide bonds, what form must PDI be in to fix them?
A) Oxidised form
B) Reduced form
C) It does not act directly
D) Phosphorylated form
21. In the ER-associated degradation (ERAD) pathway, misfolded proteins in the ER are transported back into the cytosol (retro-translocation) before being degraded. Which ATPase provides the mechanical force to extract these proteins from the ER membrane?
A) SecA
B) Hsp70
C) p97/VCP
D) 19S proteasome ATPases
22. The 26S proteasome is composed of a 20S Core Particle and a 19S Regulatory Particle. Which of the following functions is exclusively associated with the 19S Regulatory Particle?
A) Threonine-protease activity.
B) ATP-independent degradation of small, unfolded peptides.
C) Sequestration of the proteolytic active sites within a central chamber.
D) Recognition of poly-ubiquitin chains and ATP-dependent unfolding of the substrate.
23. In the trp operon of E. coli, attenuation depends on coupled transcription and translation of a leader peptide. When tryptophan levels are low, what happens?
A) The ribosome stalls at the two Trp codons in the leader peptide; the 2:3 anti-terminator stem-loop forms; transcription continues.
B) The ribosome rapidly translates the leader peptide; the 3:4 terminator stem-loop forms; transcription stops prematurely.
C) The Trp repressor binds the operator; transcription initiation is blocked.
D) The ribosome dissociates at the leader stop codon; Rho factor terminates transcription.
24. When expressing a eukaryotic protein in E. coli, “codon bias” can lead to truncated or misfolded products. What is the molecular basis for this problem?
A) Eukaryotic codons are recognised as stop codons by bacterial release factors.
B) The bacterial ribosome pauses at “rare” codons, leading to frameshifting.
C) The eukaryotic mRNA lacks a Shine-Dalgarno sequence.
D) The wobble rules in bacteria are stricter, preventing decoding.
25. According to the N-end rule, the half-life of a protein is determined by its N-terminal amino acid. In bacteria and mammals, which of the following N-terminal residues serves as a primary destabilising signal targeting the protein for ubiquitination?
A) Methionine
B) Valine
C) Arginine
D) Glycine
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References
- Nelson, David L. & Cox, Michael M. (2021). Lehninger Principles of Biochemistry, W. H. Freeman, 8th Edition.
- Alberts, B., Johnson, A., Lewis, J., Morgan, D., Raff, M., Roberts, K., & Walter, P. (2014). Molecular Biology of the Cell, Garland Science, 4th Edition.
- Geoffrey Cooper and Kenneth Adams (2022). The Cell: A Molecular Approach, Oxford University Press, 9th Edition
- Kuby, J., Kindt, T. J., Osborne, B. A., & Goldsby, R. A. (2019). Kuby Immunology, W. H. Freeman, 8th Edition.
- Gupta, P.K. (2022). Cell and Molecular Biology, Rastogi Publications, 5th Edition.
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