PCB 3023C | PCB3023C Exam 3: Cell Biology
Updated and Latest Questions and Correct
Answers with Rationale - Florida Gulf Coast
University
1. Which of the following describes the primary role of the G1 checkpoint in the eukaryotic
cell cycle?
A. Ensuring all chromosomes are properly attached to the spindle fibers.
B. Verifying that DNA replication has been completed accurately during S phase.
C. Checking for DNA damage and ensuring the cell has adequate resources for division.
D. Triggering the separation of sister chromatids during anaphase.
Correct Answer: C
Expert Explanation: The G1 checkpoint is the primary point where the cell decides
whether or not to divide based on external and internal signals. It specifically monitors for
DNA integrity and the availability of necessary growth factors and nutrients. If DNA is
damaged, the cell cycle is arrested to allow for repair or to initiate apoptosis. This
checkpoint prevents the replication of faulty DNA which could lead to mutations or cancer.
Progression past this point usually commits the cell to the rest of the cell cycle stages.
2. During DNA replication, which enzyme is responsible for relieving the torsional strain
caused by the unwinding of the double helix?
A. Topoisomerase
B. DNA Ligase
C. DNA Helicase
D. DNA Polymerase III
Correct Answer: A
Expert Explanation: As DNA helicase unwinds the double helix, it creates supercoiling and
tension ahead of the replication fork. Topoisomerase acts by cutting the phosphate
backbone, allowing the DNA to swivel and relieve this stress. Once the tension is released,
the enzyme reseals the DNA strands to maintain structural integrity. Without this action,
the replication fork would eventually stall due to extreme overwinding. This process is
essential for the continuous and efficient movement of the replication machinery.
3. In eukaryotic transcription, what is the specific role of the TATA box within the promoter
region?
A. It acts as a termination signal for RNA polymerase II.
,B. It is the site where introns are spliced out of the pre-mRNA.
C. It encodes the first few amino acids of the polypeptide chain.
D. It serves as the binding site for transcription factors that recruit RNA polymerase.
Correct Answer: D
Expert Explanation: The TATA box is a conserved DNA sequence found in the promoter
region of many eukaryotic genes. It is recognized by the TATA-binding protein which is a
subunit of the TFIID complex. This recognition is the first step in assembling the
transcription initiation complex required for RNA polymerase II binding. By positioning the
polymerase correctly, it ensures that transcription starts at the precise nucleotide location.
Therefore, the TATA box is fundamental for the spatial regulation of gene expression
initiation.
4. During translation, in which site of the ribosome does a peptide bond form between the
incoming amino acid and the growing polypeptide chain?
A. The A (Aminoacyl) site
B. The P (Peptidyl) site
C. The E (Exit) site
D. The mRNA binding site
Correct Answer: A
Expert Explanation: The ribosome contains three distinct sites known as A, P, and E for
processing tRNA molecules. The A site is where the incoming aminoacyl-tRNA carrying a
new amino acid binds to the mRNA codon. Peptidyl transferase activity then catalyzes the
formation of a peptide bond between the amino acid in the A site and the chain in the P site.
This reaction transfers the growing chain from the P-site tRNA to the A-site tRNA.
Consequently, the A site is the location where the polypeptide chain physically extends.
5. A mutation that prevents the degradation of Cyclin B would most likely have what effect
on the cell cycle?
A. The cell would be unable to enter the S phase.
B. The cell would exit mitosis prematurely.
C. The cell would remain stuck in mitosis and fail to enter cytokinesis.
D. The cell would immediately undergo apoptosis before G2.
Correct Answer: C
Expert Explanation: Cyclin B levels must rise to trigger entry into mitosis by activating
Cdk1. For the cell to exit mitosis and complete division, Cyclin B must be rapidly degraded
by the Anaphase-Promoting Complex. If Cyclin B remains stable, the Cdk1 activity stays
, high, preventing the transition back to an interphase state. This result in the cell being
arrested in the M phase, specifically in a late mitotic stage. Thus, regulated proteolysis of
cyclins is as critical as their synthesis for proper cell cycle progression.
6. Which of the following best describes the function of the Lac repressor in the absence of
lactose?
A. It binds to the promoter to enhance RNA polymerase affinity.
B. It degrades the mRNA produced by the lac operon.
C. It binds to the operator and physically blocks transcription.
D. It converts lactose into glucose and galactose.
Correct Answer: C
Expert Explanation: The lac operon is a classic example of negative inducible regulation in
bacteria. When lactose is absent, the Lac repressor protein is active and binds tightly to the
operator DNA sequence. This binding creates a physical roadblock that prevents RNA
polymerase from moving past the promoter to the structural genes. This mechanism
ensures that the cell does not waste energy producing enzymes for lactose metabolism
when the substrate is unavailable. Once lactose is present, it is converted to allolactose,
which inactivates the repressor.
7. What is the role of the 5’ cap added to eukaryotic mRNA during processing?
A. It identifies the start codon for the ribosome.
B. It protects mRNA from degradation and assists in ribosome binding.
C. It provides a signal for the termination of transcription.
D. It aids in the splicing of introns from the primary transcript.
Correct Answer: B
Expert Explanation: Shortly after transcription begins, a modified guanine nucleotide is
added to the 5’ end of the pre-mRNA. This 5’ cap is essential for protecting the transcript
from exonucleases that would otherwise digest the RNA from that end. Additionally, it
serves as a recognition signal for the eukaryotic translation initiation machinery to bind the
mRNA. The cap also plays a role in the nuclear export of the processed mRNA into the
cytoplasm. Without the cap, translation efficiency would be severely reduced and the
mRNA would be unstable.
8. In DNA replication, why is a primer required for DNA polymerase to begin synthesis?
A. DNA polymerase can only add nucleotides to an existing 3’ hydroxyl group.
B. The primer provides the energy required for phosphodiester bond formation.
C. DNA polymerase needs a signal to identify the origin of replication.
Updated and Latest Questions and Correct
Answers with Rationale - Florida Gulf Coast
University
1. Which of the following describes the primary role of the G1 checkpoint in the eukaryotic
cell cycle?
A. Ensuring all chromosomes are properly attached to the spindle fibers.
B. Verifying that DNA replication has been completed accurately during S phase.
C. Checking for DNA damage and ensuring the cell has adequate resources for division.
D. Triggering the separation of sister chromatids during anaphase.
Correct Answer: C
Expert Explanation: The G1 checkpoint is the primary point where the cell decides
whether or not to divide based on external and internal signals. It specifically monitors for
DNA integrity and the availability of necessary growth factors and nutrients. If DNA is
damaged, the cell cycle is arrested to allow for repair or to initiate apoptosis. This
checkpoint prevents the replication of faulty DNA which could lead to mutations or cancer.
Progression past this point usually commits the cell to the rest of the cell cycle stages.
2. During DNA replication, which enzyme is responsible for relieving the torsional strain
caused by the unwinding of the double helix?
A. Topoisomerase
B. DNA Ligase
C. DNA Helicase
D. DNA Polymerase III
Correct Answer: A
Expert Explanation: As DNA helicase unwinds the double helix, it creates supercoiling and
tension ahead of the replication fork. Topoisomerase acts by cutting the phosphate
backbone, allowing the DNA to swivel and relieve this stress. Once the tension is released,
the enzyme reseals the DNA strands to maintain structural integrity. Without this action,
the replication fork would eventually stall due to extreme overwinding. This process is
essential for the continuous and efficient movement of the replication machinery.
3. In eukaryotic transcription, what is the specific role of the TATA box within the promoter
region?
A. It acts as a termination signal for RNA polymerase II.
,B. It is the site where introns are spliced out of the pre-mRNA.
C. It encodes the first few amino acids of the polypeptide chain.
D. It serves as the binding site for transcription factors that recruit RNA polymerase.
Correct Answer: D
Expert Explanation: The TATA box is a conserved DNA sequence found in the promoter
region of many eukaryotic genes. It is recognized by the TATA-binding protein which is a
subunit of the TFIID complex. This recognition is the first step in assembling the
transcription initiation complex required for RNA polymerase II binding. By positioning the
polymerase correctly, it ensures that transcription starts at the precise nucleotide location.
Therefore, the TATA box is fundamental for the spatial regulation of gene expression
initiation.
4. During translation, in which site of the ribosome does a peptide bond form between the
incoming amino acid and the growing polypeptide chain?
A. The A (Aminoacyl) site
B. The P (Peptidyl) site
C. The E (Exit) site
D. The mRNA binding site
Correct Answer: A
Expert Explanation: The ribosome contains three distinct sites known as A, P, and E for
processing tRNA molecules. The A site is where the incoming aminoacyl-tRNA carrying a
new amino acid binds to the mRNA codon. Peptidyl transferase activity then catalyzes the
formation of a peptide bond between the amino acid in the A site and the chain in the P site.
This reaction transfers the growing chain from the P-site tRNA to the A-site tRNA.
Consequently, the A site is the location where the polypeptide chain physically extends.
5. A mutation that prevents the degradation of Cyclin B would most likely have what effect
on the cell cycle?
A. The cell would be unable to enter the S phase.
B. The cell would exit mitosis prematurely.
C. The cell would remain stuck in mitosis and fail to enter cytokinesis.
D. The cell would immediately undergo apoptosis before G2.
Correct Answer: C
Expert Explanation: Cyclin B levels must rise to trigger entry into mitosis by activating
Cdk1. For the cell to exit mitosis and complete division, Cyclin B must be rapidly degraded
by the Anaphase-Promoting Complex. If Cyclin B remains stable, the Cdk1 activity stays
, high, preventing the transition back to an interphase state. This result in the cell being
arrested in the M phase, specifically in a late mitotic stage. Thus, regulated proteolysis of
cyclins is as critical as their synthesis for proper cell cycle progression.
6. Which of the following best describes the function of the Lac repressor in the absence of
lactose?
A. It binds to the promoter to enhance RNA polymerase affinity.
B. It degrades the mRNA produced by the lac operon.
C. It binds to the operator and physically blocks transcription.
D. It converts lactose into glucose and galactose.
Correct Answer: C
Expert Explanation: The lac operon is a classic example of negative inducible regulation in
bacteria. When lactose is absent, the Lac repressor protein is active and binds tightly to the
operator DNA sequence. This binding creates a physical roadblock that prevents RNA
polymerase from moving past the promoter to the structural genes. This mechanism
ensures that the cell does not waste energy producing enzymes for lactose metabolism
when the substrate is unavailable. Once lactose is present, it is converted to allolactose,
which inactivates the repressor.
7. What is the role of the 5’ cap added to eukaryotic mRNA during processing?
A. It identifies the start codon for the ribosome.
B. It protects mRNA from degradation and assists in ribosome binding.
C. It provides a signal for the termination of transcription.
D. It aids in the splicing of introns from the primary transcript.
Correct Answer: B
Expert Explanation: Shortly after transcription begins, a modified guanine nucleotide is
added to the 5’ end of the pre-mRNA. This 5’ cap is essential for protecting the transcript
from exonucleases that would otherwise digest the RNA from that end. Additionally, it
serves as a recognition signal for the eukaryotic translation initiation machinery to bind the
mRNA. The cap also plays a role in the nuclear export of the processed mRNA into the
cytoplasm. Without the cap, translation efficiency would be severely reduced and the
mRNA would be unstable.
8. In DNA replication, why is a primer required for DNA polymerase to begin synthesis?
A. DNA polymerase can only add nucleotides to an existing 3’ hydroxyl group.
B. The primer provides the energy required for phosphodiester bond formation.
C. DNA polymerase needs a signal to identify the origin of replication.
