Gene expression and regulation - operon concept, mutations, DNA repair - Question Bank

1. Which repair mechanism is crucial for maintaining the fidelity of DNA replication by correcting mispaired bases after replication has occurred?
A) Nucleotide Excision Repair
B) Base Excision Repair
C) Mismatch Repair
D) Direct Repair
2. What is the consequence of a deletion of the operator region in the lac operon?
A) The operon will always be repressed.
B) The operon will always be induced, regardless of lactose presence.
C) Transcription will be significantly reduced.
D) The repressor protein will bind more strongly.
3. In the context of operons, what does 'polycistronic mRNA' refer to?
A) An mRNA molecule that codes for only one protein.
B) An mRNA molecule that codes for multiple proteins from a single transcript.
C) An mRNA molecule that is transcribed from a single gene.
D) An mRNA molecule that is highly regulated.
4. Which of the following is NOT a structural gene in the lac operon?
A) lacZ
B) lacY
C) lacA
D) lacI
5. What is the primary role of cyclic AMP (cAMP) in the regulation of the lac operon?
A) It binds to the repressor protein, inactivating it.
B) It binds to CAP, forming a complex that binds to the promoter.
C) It directly induces the transcription of lacZ.
D) It inhibits the synthesis of beta-galactosidase.
6. The enzyme that fixes UV-induced pyrimidine dimers in E. coli is part of:
A) Base Excision Repair
B) Mismatch Repair
C) Nucleotide Excision Repair
D) Direct Repair (photolyase)
7. Which of the following is a type of spontaneous mutation?
A) Mutation induced by UV radiation
B) Mutation induced by chemical mutagens
C) Tautomeric shift of bases
D) Mutation induced by X-rays
8. A mutation that changes a codon so that it still codes for the same amino acid is known as a:
A) Missense mutation
B) Nonsense mutation
C) Silent mutation
D) Frameshift mutation
9. What is the function of the leader sequence (trpL) in the trp operon?
A) It encodes the repressor protein.
B) It contains the operator sequence.
C) It plays a role in attenuation, regulating transcription termination.
D) It is the promoter region.
10. The trp operon is an example of:
A) An inducible operon
B) A constitutive operon
C) A repressible operon
D) A simple gene system
11. What is the effect of high glucose concentration on CAP binding to the promoter of the lac operon?
A) CAP binds strongly, activating transcription.
B) CAP binds weakly or not at all, reducing activation.
C) CAP is degraded.
D) CAP binds to the operator instead of the promoter.
12. Which type of DNA repair directly reverses the damage without removing any nucleotides?
A) Base Excision Repair
B) Nucleotide Excision Repair
C) Mismatch Repair
D) Direct Reversal Repair
13. A mutation that converts a codon for an amino acid into a stop codon is called a:
A) Missense mutation
B) Silent mutation
C) Nonsense mutation
D) Neutral mutation
14. What is the function of transacetylase (encoded by lacA) in the lac operon?
A) To transport lactose.
B) To hydrolyze lactose.
C) To acetylate thiogalactosides, a detoxification function.
D) To regulate the operon.
15. Which of the following is an example of a constitutive gene in the context of the lac operon?
A) lacZ
B) lacY
C) lacI
D) None of the above
16. The structural genes of the lac operon are transcribed as a single polycistronic mRNA because they are under the control of:
A) Multiple promoters
B) A single promoter
C) The operator region
D) The repressor protein
17. In the absence of lactose, the lac repressor protein is:
A) Inactive and cannot bind to the operator.
B) Active and binds to the operator, blocking transcription.
C) Degraded by the cell.
D) Bound to cAMP.
18. What is the primary consequence of a frameshift mutation?
A) A single amino acid change.
B) No change in the amino acid sequence.
C) A complete alteration of the amino acid sequence downstream of the mutation.
D) Formation of a stop codon.
19. Which DNA repair mechanism is responsible for fixing depurination or deamination errors?
A) Nucleotide Excision Repair
B) Mismatch Repair
C) Base Excision Repair
D) Photoreactivation
20. Direct repair of DNA damage involves:
A) Removal of a damaged nucleotide.
B) Enzymatic reversal of DNA damage, like photoreactivation of pyrimidine dimers.
C) Resynthesis of a DNA segment.
D) Mismatch correction.
21. What is the role of permease (encoded by lacY) in the lac operon?
A) To break down lactose.
B) To transport lactose into the cell.
C) To modify lactose.
D) To block transcription.
22. Which enzyme is encoded by the lacZ gene in the lac operon?
A) Permease
B) Transacetylase
C) Beta-galactosidase
D) Repressor protein
23. In the trp operon, when tryptophan levels are low, the repressor protein is:
A) Active and bound to the operator.
B) Inactive and not bound to the operator, allowing transcription.
C) Degraded by the cell.
D) Bound to tryptophan, making it active.
24. What is the function of the operator in an operon?
A) It is a binding site for RNA polymerase.
B) It is a binding site for the repressor protein or activator protein.
C) It codes for structural genes.
D) It is a regulatory gene.
25. Which type of mutation is caused by the insertion or deletion of nucleotides in a number not divisible by three?
A) Point mutation
B) Substitution
C) Frameshift mutation
D) Silent mutation
26. A mutation that results in a codon specifying a different amino acid is a:
A) Nonsense mutation
B) Silent mutation
C) Missense mutation
D) Frameshift mutation
27. What happens to the lac operon when lactose is present and glucose is absent?
A) The operon is repressed.
B) The operon is induced and CAP is bound, leading to high transcription.
C) Transcription is blocked by the repressor.
D) CAP cannot bind, so transcription is low.
28. In the lac operon, the promoter is the site where:
A) The repressor protein binds.
B) RNA polymerase binds to initiate transcription.
C) The inducer molecule binds.
D) The operator sequence is located.
29. What is the role of the repressor protein in a repressible operon, like the trp operon?
A) It binds to the promoter to activate transcription.
B) It binds to the operator to block transcription when the end product is abundant.
C) It acts as an inducer to start transcription.
D) It is always bound to the operator, preventing transcription.
30. Nucleotide Excision Repair (NER) is particularly effective at repairing:
A) Single base mismatches.
B) Depurination.
C) Bulky lesions like thymine dimers caused by UV radiation.
D) Deamination of cytosine.
31. The mismatch repair system primarily corrects:
A) UV-induced pyrimidine dimers.
B) Errors in base pairing that escape proofreading during replication.
C) Bulky DNA adducts.
D) Oxidized bases.
32. Which DNA repair pathway involves the removal of a damaged nucleotide and its replacement with a correct one?
A) Mismatch repair
B) Nucleotide excision repair (NER)
C) Base excision repair (BER)
D) Direct repair
33. What is the primary function of DNA repair mechanisms?
A) To increase the rate of mutations.
B) To correct errors introduced during DNA replication or damage to DNA.
C) To synthesize new DNA strands.
D) To regulate gene expression.
34. Which of the following is a common mutagen?
A) Oxygen
B) Ultraviolet (UV) radiation
C) Water
D) Carbon dioxide
35. What is a silent mutation?
A) A mutation that results in cell death.
B) A mutation that changes an amino acid to a stop codon.
C) A mutation that alters a codon but does not change the encoded amino acid.
D) A mutation that causes a frameshift.
36. Which type of mutation results in a shift in the reading frame of the mRNA sequence?
A) Point mutation
B) Substitution
C) Insertion or deletion of nucleotides (not in multiples of three)
D) Silent mutation
37. A mutation where a single nucleotide is replaced by another, resulting in a different amino acid, is known as a:
A) Nonsense mutation
B) Silent mutation
C) Missense mutation
D) Frameshift mutation
38. A mutation that changes a codon to a stop codon is called a:
A) Silent mutation
B) Missense mutation
C) Nonsense mutation
D) Neutral mutation
39. Which type of mutation involves the substitution of a single nucleotide base?
A) Insertion
B) Deletion
C) Point mutation
D) Frameshift mutation
40. What is a mutation?
A) A process of DNA replication.
B) A permanent alteration in the DNA sequence.
C) The synthesis of proteins from mRNA.
D) The regulation of gene expression.
41. Which component of the trp operon binds to the operator to block transcription when tryptophan is present?
A) Trp repressor complex
B) RNA polymerase
C) Promoter
D) Leader sequence
42. When tryptophan levels are high in E. coli, what is the state of the trp operon?
A) It is fully transcribed.
B) It is repressed.
C) It is induced.
D) It is partially transcribed.
43. In the trp operon of E. coli, what molecule acts as the corepressor?
A) Tryptophan
B) Repressor protein
C) ATP
D) RNA polymerase
44. What is the role of CAP (Catabolite Activator Protein) in the lac operon?
A) It binds to the operator and blocks transcription.
B) It binds to the promoter and enhances transcription in the absence of glucose.
C) It is synthesized in the presence of lactose to induce the operon.
D) It degrades lactose into simpler sugars.
45. What is catabolite repression in the context of the lac operon?
A) The repression of the lac operon by the presence of lactose.
B) The activation of the lac operon by the absence of glucose.
C) The repression of the lac operon by high levels of glucose.
D) The induction of the lac operon by the repressor protein.
46. Under conditions of high glucose levels in E. coli, what is the effect on the lac operon's transcription?
A) Transcription is maximally activated.
B) Transcription is repressed.
C) Transcription is induced by lactose.
D) Transcription is slightly reduced due to catabolite repression.
47. What is the function of the lacI gene in the lac operon?
A) Codes for beta-galactosidase.
B) Codes for permease.
C) Codes for the repressor protein.
D) Codes for transacetylase.
48. Which gene in the lac operon codes for the enzyme that hydrolyzes lactose into glucose and galactose?
A) lacZ
B) lacY
C) lacA
D) lacI
49. In the lac operon model of E. coli, what molecule acts as the inducer?
A) Lactose
B) Allolactose
C) Repressor protein
D) Beta-galactosidase
50. What is an operon?
A) A single gene that codes for a protein.
B) A cluster of genes transcribed together under the control of a single promoter.
C) A regulatory protein that binds to DNA.
D) A type of RNA molecule involved in protein synthesis.
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