COMPETITIVE EXAM MCQs SERIES of LIFE SCIENCES for CSIR-UGC NET/JRF, SLET, GATE, and other entrance tests: FUNDAMENTAL PROCESSES – Control of Gene Expression.
Syllabus Outline
- Control of gene expression at the transcription and translation levels.
- Regulating the expression of phages, viruses, prokaryotic and eukaryotic genes.
- Role of chromatin in gene expression and gene silencing.
This quiz contains concept-based, most frequently asked 25 MCQs of “FUNDAMENTAL PROCESSES – Control of Gene Expression” Each question has a single correct/most appropriate answer.
*****
1. In an E. coli lac operon mutant lacking a functional catabolite activator protein (CAP) binding site, which of the following is expected when glucose is absent, and lactose is present?
A) Lac operon is constitutively expressed at high levels.
B) Lac operon expression remains low.
C) Lac operon is repressed by LacI repressor even in the presence of lactose.
D) Lac operon expression is unaffected by the CAP site mutation.
2. The lac operon is induced by allolactose. Which of the following best describes the molecular mechanism of induction?
A) Allolactose binds to RNA polymerase and enhances its affinity for the lac promoter
B) Allolactose methylates the operator sequence, preventing repressor binding
C) Allolactose binds to CAP and stimulates its interaction with the promoter
D) Allolactose binds to the repressor and reduces its affinity for the operator
3. In the trp operon, tryptophan acts as a co-repressor. If a mutation abolishes the ability of tryptophan to bind to the Trp repressor, which of the following outcomes is expected?
A) Constitutive transcription of the trp operon regardless of tryptophan levels
B) Constitutive repression of the trp operon regardless of tryptophan levels
C) Enhanced attenuation of the trp operon when tryptophan is limiting
D) Loss of attenuation but retention of operator-mediated repression
4. DNA methylation in eukaryotic promoters generally leads to:
A) Inhibition of gene transcription.
B) Increased transcriptional activation.
C) RNA polymerase stalling in transcribed regions.
D) Histone acetylation.
5. In eukaryotes, a gene is found to be expressed in liver but not in muscle cells, despite identical DNA sequences in both cell types. Which of the following mechanisms is least likely to explain this cell-type-specific expression?
A) Differential methylation of CpG islands in the gene’s promoter
B) Presence of a liver-specific enhancer bound by tissue-specific transcription factors
C) Different splicing of the primary transcript in liver versus muscle
D) A somatic mutation in the coding sequence, specifically in muscle cells
6. In eukaryotes, which RNA polymerase transcribes most protein-coding (mRNA) genes?
A) RNA polymerase I
B) RNA polymerase II
C) RNA polymerase III
D) RNA polymerase IV
7. Which of the following features distinguishes an enhancer from a promoter in eukaryotic gene regulation?
A) Enhancers function only in a 5′ to 3′ orientation relative to the transcription start site
B) Enhancers must be immediately adjacent to the TATA box to function
C) Enhancers can activate transcription in an orientation-independent
D) Enhancers recruit RNA polymerase II directly without transcription factors
8. Histone acetylation by histone acetyltransferase usually results in:
A) Chromatin condensation and gene silencing.
B) Loosened chromatin structure and increased gene expression.
C) DNA demethylation and gene silencing.
D) Formation of heterochromatin and induction of gene expression.
9. In RNA interference (RNAi), Dicer cleaves dsRNA into siRNA duplexes of approximately 21–23 bp. The RNA-induced silencing complex then uses one strand as a guide. Which strand is preferentially loaded into the RNA-induced silencing complex?
A) The strand with greater thermodynamic stability at its 5′ end
B) The strand with lesser thermodynamic stability at its 5′ end
C) The strand with a 5′-cap structure
D) The strand complementary to the Argonaute PAZ domain recognition sequence
10. During X-chromosome inactivation in mammals, which of the following represents the correct sequence of molecular events?
A) XIST RNA coating → DNA methylation → H3K27me3 deposition → H4 hypoacetylation
B) TSIX expression → XIST coating → H3K4me3 deposition → DNA methylation
C) H3K9me3 → XIST RNA coating → DNMT3a recruitment → H3K27me3 deposition
D) DNA methylation → XIST RNA coating → H3K27me3 deposition → chromatin compaction
11. Riboswitches are RNA elements found in the 5′ UTR of bacterial mRNAs. The thiamine pyrophosphate riboswitch regulates gene expression by which mechanism?
A) Thiamine pyrophosphate binding promotes translation by recruiting the 30S ribosomal subunit
B) Thiamine pyrophosphate binding stabilises the antiterminator hairpin, promoting read-through
C) Thiamine pyrophosphate binding activates RNase E cleavage of the mRNA
D) Thiamine pyrophosphate binding causes a conformational change that inhibits translation
12. Which of the following statements about the TATA-binding protein (TBP) is INCORRECT?
A) TBP binds in the minor groove of DNA and causes approximately 80° bending
B) TBP is a component of TFIID and is required for transcription from TATA-containing promoters
C) TBP is required for transcription by RNA polymerase I, II, and III
D) TBP directly catalyses the phosphodiester bond formation during transcription
13. Retroviruses like HIV produce their reverse transcriptase through a special translation mechanism. How is the viral Pol polyprotein produced?
A) Separate monocistronic mRNAs encode Pol.
B) A –1 ribosomal frameshift during Gag polyprotein translation.
C) Protease cleavage of a long Env precursor.
D) Alternative splicing of viral transcripts.
14. In eukaryotes, the Mediator complex functions as:
A) A direct component of the pre-initiation complex that melts promoter DNA
B) A kinase that phosphorylates histone H3 at serine 10 during transcription
C) A histone chaperone that deposits H3.3 at actively transcribed genes
D) A bridge between enhancer-bound activators and the RNA pol II holoenzyme
15. CRISPR-dCas9 fused to the KRAB domain silences gene expression by which mechanism?
A) Introducing double-strand breaks at the promoter, disrupting transcription factor binding
B) Recruiting H3K9 methyltransferases and HP1 via KRAB, creating a repressive chromatin state
C) Directly methylating CpG islands in the target gene promoter
D) Blocking mRNA export by tethering the transcript to the nuclear matrix
16. Trimethylation of histone H3 on lysine 4 (H3K4me3) is most often associated with:
A) Transcriptional activation of genes.
B) Constitutive heterochromatin formation.
C) DNA replication initiation.
D) X-chromosome inactivation.
17. A eukaryotic gene normally expressed ubiquitously is found to be silenced specifically in cancer cells. Bisulfite sequencing reveals hypermethylation of its CpG island promoter. Which of the following additional epigenetic marks would you expect to find associated with this silenced gene?
A) H3K4me3, H3K9ac, H4K16ac
B) H3K27me3, H3K9me3, H4K20me3
C) H3K36me3, H3K79me2, H2BK120ub
D) H3K4me1, H3K27ac, H3K9me1
18. Which of the following enzymes in eukaryotes is primarily responsible for the maintenance of DNA methylation after replication?
A) de novo methylation
B) maintenance methyltransferase
C) maintenance methylation
D) tRNA methylation
19. The phenomenon of co-suppression in plants was first observed when:
A) Antisense RNA was introduced to silence the chalcone synthase gene in petunia
B) Overexpression of a transcription factor silenced downstream target genes via promoter competition
C) Viroid infection triggered methylation of host gene promoters
D) Extra copies of the chalcone synthase gene in petunia caused silencing of both the transgene and the endogenous gene.
20. Regarding the regulation of eukaryotic mRNA stability, which of the following sequences or elements promotes mRNA degradation?
A) Poly(A) tail in the 3′ UTR
B) AU-rich elements in the 3′ UTR that recruit deadenylases
C) m⁷G cap at the 5′ end
D) The Kozak consensus sequence surrounding the AUG start codon
21. Which of the following statements about the CAP–cAMP complex in the lac operon is correct:
I – CAP–cAMP binds upstream of the lac promoter and directly contacts the α subunit of RNA polymerase
II – CAP–cAMP increases the melting temperature of the promoter DNA
III – cAMP levels rise when glucose is abundant
IV – CAP–cAMP binding results in DNA bending of approximately 90°
A) I and IV
B) I, II, and IV
C) II and III
D) I, III, and IV
22. Which of the following statements is correct regarding eukaryotic gene regulation by nuclear receptors:
I – Unliganded nuclear receptors often associate with HDAC-containing corepressor complexes
II – Ligand binding triggers the exchange of corepressors for coactivators with HAT activity
III – Nuclear receptors exclusively bind as monomers to hormone response elements
IV – Heat shock protein 90 keeps cytoplasmic nuclear receptors in an inactive conformation
A) I, II, and IV only
B) I and II only
C) II, III, and IV only
D) I, II, III, and IV
23. Which of the following regulatory RNA elements and their mechanism is correct:
I – Riboswitch: Conformational change upon binding small molecules (ligands)
II – microRNA: Base-pairing with mRNAs to inhibit translation or induce cleavage
III – siRNA: RISC-mediated cleavage of complementary mRNA.
A) I only
B) I and II only
C) II and III only
D) I, II and III
24. In the trp operon attenuation, which of the following statements are correct?
I – High tryptophan → formation of the 3–4 terminator hairpin.
II – Low tryptophan → 2–3 antiterminator hairpin forms.
III – The leader peptide contains two Trp codons.
IV – If the ribosome stalls at the leader, transcription terminates.
I, and III only.
I, III and IV only.
I, II and III only.
I and IV only.
25. Assertion (A): Histone acetylation opens chromatin and activates transcription.
Reason (R): Acetylation neutralises positive lysine charges on histones, weakening DNA binding.
A and R are true, and R explains A.
A and R are true, but R does not explain A.
A is true, R is false.
A is false, R is true.
*****
Previous: Protein Synthesis and Processing
Next:
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.
🔗 Explore More MCQs:
