Regulation of enzymes – repression MCQs With Answer

Regulation of enzymes – repression MCQs With Answer

The regulation of enzymes by repression is a key concept for B. Pharm students studying biochemical control, gene regulation, and pharmacology. This introduction covers transcriptional repression, operon models (e.g., trp and lac), corepressors, negative control, attenuation, catabolite repression, allosteric control, and post‑translational mechanisms that reduce enzyme levels or activity. Understanding enzyme repression helps explain metabolic adaptation, drug interactions, CYP regulation, and targeted therapeutic strategies. Clear knowledge of molecular mechanisms, regulatory proteins, and clinical relevance is essential for exam success and rational drug design. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. Which mechanism most directly describes transcriptional repression in prokaryotic operons?

• Repressor protein binding to the operator to block RNA polymerase
• Activator binding to promoter to enhance RNA polymerase binding
• mRNA degradation by ribonucleases
• Translation inhibition by ribosomes

Correct Answer: Repressor protein binding to the operator to block RNA polymerase

Q2. In the trp operon, tryptophan functions as which of the following to repress transcription?

• Inducer
• Corepressor
• Activator
• Antisense RNA

Correct Answer: Corepressor

Q3. Catabolite repression in bacteria commonly involves which molecule acting as a global signal?

• ATP
• cAMP
• GMP
• cGMP

Correct Answer: cAMP

Q4. Which DNA-binding motif is frequently found in bacterial repressor proteins?

• Zinc finger
• Helix-turn-helix
• Leucine zipper
• Homeobox

Correct Answer: Helix-turn-helix

Q5. Attenuation as a regulatory mechanism primarily affects which step of gene expression?

• Translation initiation
• Transcription termination
• mRNA splicing
• Protein folding

Correct Answer: Transcription termination

Q6. Which of the following is an example of feedback repression at the genetic level?

• Enzyme competitive inhibition by substrate analogs
• End product binding to repressor protein to inhibit transcription
• Allosteric activation of enzyme by intermediate
• Proteolytic activation of zymogen

Correct Answer: End product binding to repressor protein to inhibit transcription

Q7. In the lac operon, the lac repressor is inactivated by which molecule to allow transcription?

• Glucose
• Lactose (or allolactose)
• Tryptophan
• Cyclic AMP

Correct Answer: Lactose (or allolactose)

Q8. Which process reduces enzyme activity by covalent modification rather than by reducing expression?

• Transcriptional repression
• Phosphorylation of the enzyme
• mRNA degradation
• Repressor binding to operator

Correct Answer: Phosphorylation of the enzyme

Q9. Which regulatory element is located immediately upstream of coding sequences and is the primary binding site for RNA polymerase?

• Operator
• Terminator
• Promoter
• Riboswitch

Correct Answer: Promoter

Q10. A corepressor is best described as:

• A molecule that binds RNA polymerase to start transcription
• A small molecule that binds the repressor, enabling it to bind DNA and block transcription
• A transcription factor that always activates genes
• An mRNA element that blocks translation

Correct Answer: A small molecule that binds the repressor, enabling it to bind DNA and block transcription

Q11. Which of the following mechanisms contributes to enzyme repression in eukaryotes but not typically in prokaryotes?

• DNA methylation and chromatin remodeling
• Repressor binding to operator sequences
• Attenuation through leader peptides
• Direct coupling of transcription and translation

Correct Answer: DNA methylation and chromatin remodeling

Q12. Riboswitches regulate gene expression by:

• Binding small metabolites and changing mRNA structure to control transcription or translation
• Recruiting histone acetyltransferases
• Cleaving repressor proteins
• Blocking RNA polymerase phosphorylation

Correct Answer: Binding small metabolites and changing mRNA structure to control transcription or translation

Q13. Which statement distinguishes repression from induction?

• Repression increases enzyme synthesis; induction decreases enzyme synthesis
• Repression decreases enzyme synthesis in response to a signal; induction increases enzyme synthesis in response to a signal
• Repression alters enzyme kinetic constants directly; induction changes mRNA half-life
• Repression only happens in eukaryotes; induction only in prokaryotes

Correct Answer: Repression decreases enzyme synthesis in response to a signal; induction increases enzyme synthesis in response to a signal

Q14. Catabolite repression of the lac operon in E. coli involves which protein complex binding under low glucose conditions?

• Lac repressor bound to operator
• CRP-cAMP complex binding to promoter region
• RNA polymerase blocked by histones
• Rho factor terminating transcription

Correct Answer: CRP-cAMP complex binding to promoter region

Q15. Which experimental technique is most appropriate for measuring changes in mRNA levels due to repression?

• Western blot
• Northern blot or qRT-PCR
• Mass spectrometry of metabolites
• Chromatin immunoprecipitation only

Correct Answer: Northern blot or qRT-PCR

Q16. Which is a post-transcriptional mechanism that can lead to decreased enzyme levels?

• Increased promoter activity
• mRNA degradation by RNases or microRNAs
• Formation of repressosome at operator
• Allosteric activation by substrate

Correct Answer: mRNA degradation by RNases or microRNAs

Q17. In bacterial negative control systems, what is the role of an inducer?

• It activates the repressor to bind DNA
• It inactivates the repressor, allowing transcription
• It degrades mRNA transcripts
• It phosphorylates RNA polymerase

Correct Answer: It inactivates the repressor, allowing transcription

Q18. Proteolytic degradation of enzymes as a regulatory mechanism primarily affects which aspect?

• Transcription initiation
• Enzyme half-life and cellular concentration
• mRNA splicing patterns
• Ribosome assembly

Correct Answer: Enzyme half-life and cellular concentration

Q19. Which of the following best exemplifies global repression of gene expression in bacteria?

• Specific repressor binding to a single operon operator
• Catabolite repression affecting many operons via cAMP-CRP
• Translational stalling of a single mRNA
• Proteasome degradation of histones

Correct Answer: Catabolite repression affecting many operons via cAMP-CRP

Q20. The lac operon is considered an example of which regulatory logic?

• Negative inducible operon
• Positive repressible operon
• Negative repressible operon
• Positive constitutive operon

Correct Answer: Negative inducible operon

Q21. Which type of molecule commonly serves as an inducer by binding and inactivating repressors?

• Small metabolites derived from substrate
• Large structural proteins
• Ribosomal RNA fragments
• DNA methyltransferases

Correct Answer: Small metabolites derived from substrate

Q22. Attenuation in the trp operon is sensitive to which cellular condition?

• Intracellular tryptophan levels affecting ribosome stalling
• Extracellular glucose concentration alone
• DNA methylation status
• Temperature-induced DNA melting

Correct Answer: Intracellular tryptophan levels affecting ribosome stalling

Q23. Which of the following describes an anti-repressor mechanism?

• A protein that enhances repressor DNA binding
• A molecule that sequesters the repressor, preventing DNA binding
• RNA polymerase binding to the operator
• Histone acetylation repressing transcription

Correct Answer: A molecule that sequesters the repressor, preventing DNA binding

Q24. In eukaryotes, transcriptional repression can be mediated by corepressors that recruit which enzymatic activity?

• Histone acetyltransferase
• Histone deacetylase
• DNA helicase
• RNA ligase

Correct Answer: Histone deacetylase

Q25. Which is a pharmacological implication of enzyme repression in drug metabolism?

• Repression of CYP enzymes can reduce drug clearance, increasing toxicity
• Repression always increases drug metabolism
• Enzyme repression has no effect on drug-drug interactions
• Repression only affects bacterial enzymes

Correct Answer: Repression of CYP enzymes can reduce drug clearance, increasing toxicity

Q26. A transcription factor that blocks RNA polymerase activity without binding DNA directly would most likely act through:

• Protein-protein interaction with the polymerase (repressor by blocking)
• Binding to operator to prevent access
• Cleaving mRNA to stop translation
• Altering tRNA charging

Correct Answer: Protein-protein interaction with the polymerase (repressor by blocking)

Q27. Which term describes a set of genes regulated by the same regulatory protein but located at different positions in the genome?

• Operon
• Regulon
• Plasmid
• Promoter cluster

Correct Answer: Regulon

Q28. An allosteric repressor differs from a constitutive repressor because it:

• Binds DNA without any ligand
• Changes conformation upon ligand binding to alter DNA affinity
• Is always degraded rapidly
• Acts only on eukaryotic genes

Correct Answer: Changes conformation upon ligand binding to alter DNA affinity

Q29. Which laboratory method can identify proteins bound to a specific DNA operator region in vivo?

• qRT-PCR of mRNA
• Chromatin immunoprecipitation (ChIP)
• Northern blot
• ELISA for small metabolites

Correct Answer: Chromatin immunoprecipitation (ChIP)

Q30. Which mechanism can silence enzyme expression at the translational level?

• Repressor binding to operator upstream of promoter
• MicroRNA binding to target mRNA to block translation
• Attenuation of transcription in bacteria
• Histone acetylation

Correct Answer: MicroRNA binding to target mRNA to block translation

Q31. Which feature distinguishes repressible operons from inducible operons?

• Repressible operons are normally active and can be turned off by a corepressor
• Inducible operons are normally active and are turned off by inducers
• Repressible operons only occur in eukaryotes
• Inducible operons never respond to metabolites

Correct Answer: Repressible operons are normally active and can be turned off by a corepressor

Q32. Which term best describes small RNAs that bind mRNA to prevent translation and can cause repression of enzyme synthesis?

• Ribozymes
• MicroRNAs or sRNAs
• tRNAs
• snRNAs only involved in splicing

Correct Answer: MicroRNAs or sRNAs

Q33. Which of the following is a consequence of promoter methylation in eukaryotic gene regulation?

• Increased transcription due to chromatin opening
• Transcriptional repression due to compacted chromatin
• Immediate mRNA translation in cytoplasm
• Enhanced ribosome binding to mRNA

Correct Answer: Transcriptional repression due to compacted chromatin

Q34. In a feedback repression network, a mutation that prevents end product binding to the repressor would most likely cause:

• Constitutive repression and no enzyme expression
• Loss of repression leading to constitutive expression of biosynthetic enzymes
• Increased degradation of the repressor protein only
• Immediate cell death

Correct Answer: Loss of repression leading to constitutive expression of biosynthetic enzymes

Q35. Which bacterial mechanism uses ribosome movement on a leader peptide to influence downstream transcription?

• Catabolite repression
• Attenuation
• mRNA capping
• DNA methylation

Correct Answer: Attenuation

Q36. A repressor protein that binds only as a dimer to palindromic operator sequences illustrates which principle?

• Sequence-specific RNA binding
• Symmetry recognition of DNA by dimeric proteins
• Random DNA association without specificity
• Histone-like wrapping of DNA

Correct Answer: Symmetry recognition of DNA by dimeric proteins

Q37. Which regulatory change would most rapidly decrease existing enzyme activity without waiting for reduced transcription?

• Reducing promoter strength
• Allosteric inhibition by a metabolic effector
• DNA methylation at promoter
• Decreasing ribosome biogenesis

Correct Answer: Allosteric inhibition by a metabolic effector

Q38. Which of the following best describes the effect of histone deacetylases (HDACs) on gene expression?

• They acetylate histones to activate transcription
• They remove acetyl groups from histones, leading to transcriptional repression
• They methylate DNA directly to activate genes
• They cleave promoter DNA to block transcription

Correct Answer: They remove acetyl groups from histones, leading to transcriptional repression

Q39. Which clinical scenario could result from repression of drug-metabolizing enzymes?

• Faster drug clearance requiring higher doses
• Reduced drug clearance leading to increased plasma levels and toxicity
• Complete immunity to drugs
• No change in drug pharmacokinetics

Correct Answer: Reduced drug clearance leading to increased plasma levels and toxicity

Q40. The lac repressor binds to operator sequences through recognition of:

• Specific DNA base pair sequences and structural conformation
• Random electrostatic interactions only
• Histone tails
• mRNA leader sequences

Correct Answer: Specific DNA base pair sequences and structural conformation

Q41. In bacteria, which factor terminates transcription in Rho-dependent termination and can influence operon regulation?

• Rho helicase
• DNA polymerase
• Ribosome subunit
• cAMP receptor protein

Correct Answer: Rho helicase

Q42. Which type of mutation in an operator sequence would likely cause constitutive expression of the operon?

• Mutation that increases repressor binding affinity
• Mutation that prevents repressor binding
• Mutation that creates an additional promoter
• Mutation that enhances attenuation sensitivity

Correct Answer: Mutation that prevents repressor binding

Q43. A synthetic small molecule designed to stabilize a repressor-DNA complex could be used therapeutically to:

• Increase expression of target enzyme
• Repress expression of a disease-associated protein
• Enhance ribosome assembly
• Promote mRNA splicing

Correct Answer: Repress expression of a disease-associated protein

Q44. Which of the following accurately links a regulatory mechanism with its target in enzyme repression?

• MicroRNA acts on chromatin to prevent transcription
• Corepressor binds repressor protein to promote DNA binding and repress transcription
• Allosteric activator increases repressor binding to DNA
• Proteasome directly methylates promoter DNA

Correct Answer: Corepressor binds repressor protein to promote DNA binding and repress transcription

Q45. In bacteria, what role does the operator sequence play?

• Acts as the ribosome binding site for translation
• Serves as the binding site for regulatory repressors or activators affecting transcription
• Signals termination of translation
• Modifies tRNA charging efficiency

Correct Answer: Serves as the binding site for regulatory repressors or activators affecting transcription

Q46. Which measurement would directly show decreased enzyme synthesis due to repression at the protein level?

• Reduced mRNA levels by qRT-PCR only
• Lower protein abundance measured by Western blot or mass spectrometry
• Increased promoter occupancy by activator
• Increased DNA replication rate

Correct Answer: Lower protein abundance measured by Western blot or mass spectrometry

Q47. Which regulatory mechanism allows fast reversible repression of enzyme activity without altering protein abundance?

• Proteolytic degradation
• Allosteric inhibition by effector molecules
• Promoter methylation
• Transcriptional attenuation

Correct Answer: Allosteric inhibition by effector molecules

Q48. Which is true about inducible and repressible systems in metabolic regulation?

• Inducible systems are typically used for catabolic pathways; repressible systems for anabolic pathways
• Inducible systems control anabolic pathways; repressible systems regulate catabolic pathways only
• Both systems only exist in eukaryotes
• Neither system responds to metabolite levels

Correct Answer: Inducible systems are typically used for catabolic pathways; repressible systems for anabolic pathways

Q49. Which term refers to a protein complex of transcription factors and cofactors that repress transcription by steric hindrance or chromatin change?

• Enhanceosome
• Repressosome
• Spliceosome
• Ribosome

Correct Answer: Repressosome

Q50. Why is understanding enzyme repression important for B. Pharm students in relation to drug therapy?

• It only matters for basic research and has no clinical relevance
• It helps predict drug-drug interactions, dosage adjustments, and effects on metabolic pathways when enzymes are downregulated
• It ensures that all drugs will always be activated by enzymes
• It solely determines the taste of oral medications

Correct Answer: It helps predict drug-drug interactions, dosage adjustments, and effects on metabolic pathways when enzymes are downregulated

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com