PCB 3063 Exam 4 V1 | PCB 3063 Genetics
| Actual Q&A with Rationale (PCB3063
Exam 4) | University of Central Florida
1. In the lac operon of E. coli, what is the specific function of the lacI gene product?
A. It encodes the permease protein responsible for transporting lactose into the cell.
B. It functions as a structural gene that encodes the enzyme beta-galactosidase.
C. It serves as the promoter region where RNA polymerase initiates transcription.
D. It encodes a repressor protein that binds to the operator to inhibit transcription.
Answer: D
Rationale: The lacI gene is a regulatory gene located upstream of the lac operon that
produces a repressor protein. This protein binds to the operator site (lacO) to prevent RNA
polymerase from transcribing the structural genes when lactose is absent. This mechanism
is a classic example of negative inducible regulation in prokaryotic systems.
2. Which of the following mutations results in a change from a purine to a pyrimidine or vice
versa?
A. Transition mutation
B. Transversion mutation
C. Silent mutation
D. Synonymous mutation
,Answer: B
Rationale: A transversion mutation occurs when a purine (A or G) is replaced by a
pyrimidine (C or T), or a pyrimidine is replaced by a purine. This is distinct from a
transition mutation, where a base is replaced by another of the same chemical class.
Understanding these distinctions is critical for predicting the impact of point mutations on
DNA structure and coding capacity.
3. Under which condition is the lac operon expressed at the highest levels in E. coli?
A. Presence of glucose and absence of lactose
B. Presence of both glucose and lactose
C. Absence of both glucose and lactose
D. Absence of glucose and presence of lactose
Answer: D
Rationale: High expression of the lac operon requires the presence of lactose to remove
the repressor and the absence of glucose to allow for high cAMP levels. High cAMP allows
the CAP-cAMP complex to bind the CAP site, which significantly recruits RNA polymerase
to the promoter. This dual control ensures that the cell prioritizes the more efficient energy
source, glucose, over lactose.
4. A mutation that changes a codon for an amino acid into a stop codon is known as a:
A. Missense mutation
B. Neutral mutation
, C. Nonsense mutation
D. Frameshift mutation
Answer: C
Rationale: A nonsense mutation is a point mutation that results in a premature stop codon
in the transcribed mRNA. This leads to the production of a truncated, and usually non-
functional, protein product. Such mutations are often deleterious depending on where in
the sequence the truncation occurs.
5. In the tryptophan (trp) operon, what role does tryptophan play in regulation?
A. It acts as an inducer by binding to the repressor.
B. It acts as a corepressor by binding to the aporepressor.
C. It binds to the operator to stimulate transcription.
D. It acts as an enhancer for the RNA polymerase binding.
Answer: B
Rationale: The trp operon is a repressible system where the end product, tryptophan,
regulates its own synthesis. Tryptophan binds to the inactive aporepressor protein,
changing its conformation so it can bind to the operator. This binding blocks RNA
polymerase, effectively shutting down the operon when tryptophan levels are sufficient.
| Actual Q&A with Rationale (PCB3063
Exam 4) | University of Central Florida
1. In the lac operon of E. coli, what is the specific function of the lacI gene product?
A. It encodes the permease protein responsible for transporting lactose into the cell.
B. It functions as a structural gene that encodes the enzyme beta-galactosidase.
C. It serves as the promoter region where RNA polymerase initiates transcription.
D. It encodes a repressor protein that binds to the operator to inhibit transcription.
Answer: D
Rationale: The lacI gene is a regulatory gene located upstream of the lac operon that
produces a repressor protein. This protein binds to the operator site (lacO) to prevent RNA
polymerase from transcribing the structural genes when lactose is absent. This mechanism
is a classic example of negative inducible regulation in prokaryotic systems.
2. Which of the following mutations results in a change from a purine to a pyrimidine or vice
versa?
A. Transition mutation
B. Transversion mutation
C. Silent mutation
D. Synonymous mutation
,Answer: B
Rationale: A transversion mutation occurs when a purine (A or G) is replaced by a
pyrimidine (C or T), or a pyrimidine is replaced by a purine. This is distinct from a
transition mutation, where a base is replaced by another of the same chemical class.
Understanding these distinctions is critical for predicting the impact of point mutations on
DNA structure and coding capacity.
3. Under which condition is the lac operon expressed at the highest levels in E. coli?
A. Presence of glucose and absence of lactose
B. Presence of both glucose and lactose
C. Absence of both glucose and lactose
D. Absence of glucose and presence of lactose
Answer: D
Rationale: High expression of the lac operon requires the presence of lactose to remove
the repressor and the absence of glucose to allow for high cAMP levels. High cAMP allows
the CAP-cAMP complex to bind the CAP site, which significantly recruits RNA polymerase
to the promoter. This dual control ensures that the cell prioritizes the more efficient energy
source, glucose, over lactose.
4. A mutation that changes a codon for an amino acid into a stop codon is known as a:
A. Missense mutation
B. Neutral mutation
, C. Nonsense mutation
D. Frameshift mutation
Answer: C
Rationale: A nonsense mutation is a point mutation that results in a premature stop codon
in the transcribed mRNA. This leads to the production of a truncated, and usually non-
functional, protein product. Such mutations are often deleterious depending on where in
the sequence the truncation occurs.
5. In the tryptophan (trp) operon, what role does tryptophan play in regulation?
A. It acts as an inducer by binding to the repressor.
B. It acts as a corepressor by binding to the aporepressor.
C. It binds to the operator to stimulate transcription.
D. It acts as an enhancer for the RNA polymerase binding.
Answer: B
Rationale: The trp operon is a repressible system where the end product, tryptophan,
regulates its own synthesis. Tryptophan binds to the inactive aporepressor protein,
changing its conformation so it can bind to the operator. This binding blocks RNA
polymerase, effectively shutting down the operon when tryptophan levels are sufficient.
