PCB 3063 Exam 2 V3 | PCB 3063 Genetics
| Actual Q&A with Rationale (PCB3063
Exam 2) | University of Central Florida
1. Which enzyme is primarily responsible for synthesizing the leading strand during DNA
replication in E. coli?
A. DNA Polymerase III
B. DNA Polymerase I
C. Primase
D. Helicase
Answer: A
Rationale: DNA Polymerase III is the main replicative enzyme in prokaryotes. It adds
nucleotides in the 5’ to 3’ direction with high processivity. It also possesses 3’ to 5’
exonuclease activity for proofreading during synthesis.
2. In the context of the lac operon, what is the effect of high glucose levels on cAMP
concentration?
A. cAMP levels decrease
B. cAMP levels increase
C. cAMP levels remain constant
,D. cAMP is converted to lactose
Answer: A
Rationale: Glucose levels inversely regulate the activity of adenylyl cyclase in bacteria.
When glucose is high, the concentration of cAMP stays low, preventing the formation of the
CAP-cAMP complex. This represents a form of catabolite repression to ensure the cell uses
the most efficient energy source first.
3. What is the function of the sigma factor in bacterial transcription?
A. Termination of RNA synthesis
B. Recognition of the promoter sequence
C. Elongation of the RNA chain
D. Splicing of the primary transcript
Answer: B
Rationale: The sigma factor is a subunit of the RNA polymerase holoenzyme that provides
specificity for promoter binding. It recognizes the -10 and -35 consensus sequences in the
DNA. Without the sigma factor, the core enzyme would initiate transcription at random
sites.
4. Which of the following is a sequence-specific DNA-binding protein that inhibits
transcription in the lac operon?
A. Lac Repressor
, B. Beta-galactosidase
C. Permease
D. RNA Polymerase
Answer: A
Rationale: The Lac Repressor protein is encoded by the lacI gene and binds to the operator
site. This binding physically blocks RNA polymerase from proceeding with transcription.
When lactose is present, it is converted to allolactose, which binds the repressor and causes
it to release the DNA.
5. During eukaryotic RNA processing, what is added to the 5’ end of the pre-mRNA?
A. A 7-methylguanosine cap
B. An exon
C. A poly-A tail
D. A TATA box
Answer: A
Rationale: The 5’ cap consists of a 7-methylguanosine residue joined to the transcript via a
5’-5’ triphosphate bridge. This modification protects the mRNA from degradation by
exonucleases. It also serves as a recognition signal for the ribosome to initiate translation.
6. Which enzyme relieves torsional strain (supercoiling) ahead of the replication fork?
A. DNA Ligase
| Actual Q&A with Rationale (PCB3063
Exam 2) | University of Central Florida
1. Which enzyme is primarily responsible for synthesizing the leading strand during DNA
replication in E. coli?
A. DNA Polymerase III
B. DNA Polymerase I
C. Primase
D. Helicase
Answer: A
Rationale: DNA Polymerase III is the main replicative enzyme in prokaryotes. It adds
nucleotides in the 5’ to 3’ direction with high processivity. It also possesses 3’ to 5’
exonuclease activity for proofreading during synthesis.
2. In the context of the lac operon, what is the effect of high glucose levels on cAMP
concentration?
A. cAMP levels decrease
B. cAMP levels increase
C. cAMP levels remain constant
,D. cAMP is converted to lactose
Answer: A
Rationale: Glucose levels inversely regulate the activity of adenylyl cyclase in bacteria.
When glucose is high, the concentration of cAMP stays low, preventing the formation of the
CAP-cAMP complex. This represents a form of catabolite repression to ensure the cell uses
the most efficient energy source first.
3. What is the function of the sigma factor in bacterial transcription?
A. Termination of RNA synthesis
B. Recognition of the promoter sequence
C. Elongation of the RNA chain
D. Splicing of the primary transcript
Answer: B
Rationale: The sigma factor is a subunit of the RNA polymerase holoenzyme that provides
specificity for promoter binding. It recognizes the -10 and -35 consensus sequences in the
DNA. Without the sigma factor, the core enzyme would initiate transcription at random
sites.
4. Which of the following is a sequence-specific DNA-binding protein that inhibits
transcription in the lac operon?
A. Lac Repressor
, B. Beta-galactosidase
C. Permease
D. RNA Polymerase
Answer: A
Rationale: The Lac Repressor protein is encoded by the lacI gene and binds to the operator
site. This binding physically blocks RNA polymerase from proceeding with transcription.
When lactose is present, it is converted to allolactose, which binds the repressor and causes
it to release the DNA.
5. During eukaryotic RNA processing, what is added to the 5’ end of the pre-mRNA?
A. A 7-methylguanosine cap
B. An exon
C. A poly-A tail
D. A TATA box
Answer: A
Rationale: The 5’ cap consists of a 7-methylguanosine residue joined to the transcript via a
5’-5’ triphosphate bridge. This modification protects the mRNA from degradation by
exonucleases. It also serves as a recognition signal for the ribosome to initiate translation.
6. Which enzyme relieves torsional strain (supercoiling) ahead of the replication fork?
A. DNA Ligase
