PCB 3023 Exam 3 | Questions and Correct Answers with
Rationale | Already Graded A+ | Newly Updated 2026-2027
1. Restriction nucleases are bacterial enzymes fundamental to molecular biology
techniques. What is the primary biological role of these enzymes in the bacteria
that naturally express them?
A) To repair damaged bacterial DNA
B) To degrade exogenous or foreign DNA, thereby protecting the bacterium from
threats like viruses
C) To replicate bacterial chromosomes
D) To transcribe bacterial genes
Correct Answer: B) To degrade exogenous or foreign DNA, thereby protecting the
bacterium from threats like viruses
Rationale: Restriction nucleases are part of the bacterial defense system
(restriction-modification system). They recognize specific DNA sequences and
cleave foreign DNA (e.g., from bacteriophages), protecting the bacterium from
viral infection. The bacterial host's own DNA is protected by methylation.
2. Gel electrophoresis is a technique used to separate DNA fragments after
restriction enzyme digestion. If a large, linear DNA molecule is digested by a
restriction enzyme and run on an agarose gel, how are the fragments physically
separated?
A) Separation occurs based on the size of the DNA fragments, with smaller
fragments traveling fastest toward the positive pole
B) Separation occurs based on the charge of the DNA fragments, with larger
fragments traveling fastest toward the negative pole
C) Separation occurs based on the sequence of the DNA fragments, with GC-rich
fragments traveling faster
D) Separation occurs based on the shape of the DNA fragments, with circular
fragments traveling faster
,Correct Answer: A) Separation occurs based on the size of the DNA fragments,
with smaller fragments traveling fastest toward the positive pole
Rationale: DNA fragments are negatively charged due to their phosphate
backbone. In an agarose gel, smaller fragments migrate faster through the matrix
toward the positive electrode, while larger fragments are slowed by the gel's
sieving effect. Size is the primary determinant of migration.
3. In the process of visualizing DNA fragments in an agarose gel, Ethidium
Bromide is added. Which statement best describes how Ethidium Bromide
allows researchers to see the DNA under UV light?
A) It binds to the phosphate backbone of DNA, causing it to absorb UV light
B) It intercalates into the DNA double helix, acting as a dye that fluoresces upon
UV exposure
C) It covalently attaches to the 5' end of DNA, creating a fluorescent product
D) It denatures the DNA, allowing it to be visualized
Correct Answer: B) It intercalates into the DNA double helix, acting as a dye that
fluoresces upon UV exposure
Rationale: Ethidium Bromide intercalates (inserts) between the stacked base pairs
of double-stranded DNA. When exposed to UV light, it fluoresces brightly,
allowing DNA bands to be visualized. This intercalation does not alter DNA
sequence but allows detection.
4. A researcher is attempting to clone a gene into a circular plasmid vector. The
plasmid map indicates that it contains one EcoR1 site, one HindIII site, and one
Pst1 site. If the researcher performs a complete restriction digest using only
HindIII and Pst1 enzymes simultaneously on this circular DNA, how many linear
DNA fragments would be generated?
A) 1 fragment
B) 2 fragments
C) 3 fragments
D) 4 fragments
,Correct Answer: B) 2 fragments
Rationale: Restriction enzymes cut at specific recognition sites. In a circular
plasmid, each cut by a restriction enzyme linearizes the circle. With two different
enzymes each cutting once (HindIII and PstI), there are two cuts total, producing
two linear fragments of different sizes.
5. The fundamental process of creating a Recombinant DNA molecule, which
initiates DNA cloning, requires two distinct enzymatic activities working
together. These activities are:
A) Cutting the DNA at specific sequences and then sealing the DNA backbone
B) Cutting the DNA randomly and then sealing the DNA backbone
C) Copying the DNA and then cutting it
D) Transcribing the DNA and then translating it
Correct Answer: A) Cutting the DNA at specific sequences and then sealing the
DNA backbone
Rationale: Recombinant DNA formation requires: (1) restriction endonucleases to
cut DNA at specific sequences, generating fragments with compatible ends, and
(2) DNA ligase to seal the sugar-phosphate backbone between the vector and
insert fragments, creating a stable recombinant molecule.
6. A crucial component of successful cloning is the use of a vector (plasmid). If
the goal is to amplify the target DNA sequence to make many identical copies
within a host bacterium, which element is absolutely necessary for replication in
the bacterial cell?
A) An antibiotic resistance gene
B) An origin of replication (ORI)
C) A multiple cloning site
D) A promoter sequence
Correct Answer: B) An origin of replication (ORI)
Rationale: The origin of replication (ORI) is a DNA sequence that allows the
plasmid to be replicated independently by the host's replication machinery.
, Without an ORI, the plasmid cannot be copied and will be lost as bacteria divide,
preventing amplification of the target DNA.
7. G-Protein-Coupled Receptors (GPCRs) represent the largest family of cell-
surface receptors in humans and are crucial targets for therapeutic drugs.
Structurally, all GPCRs share a defining characteristic that facilitates signal
transmission across the plasma membrane. What is this shared structural
feature?
A) They possess two polypeptide chains that span the membrane four times each
B) They possess a single polypeptide chain that spans the membrane exactly
seven times
C) They possess a single polypeptide chain that spans the membrane once
D) They possess three polypeptide chains that form a channel
Correct Answer: B) They possess a single polypeptide chain that spans the
membrane exactly seven times
Rationale: GPCRs are characterized by seven transmembrane α-helices (a single
polypeptide chain threading through the membrane seven times). This
"serpentine" structure positions the extracellular N-terminus for ligand binding
and the intracellular C-terminus for G-protein interaction, enabling signal
transmission across the membrane.
8. G proteins function as molecular switches in cell signaling pathways. What
mechanism causes the G protein α subunit to transition from its active (ON)
state back to its inactive (OFF) state, effectively terminating the signal?
A) The α subunit is ubiquitinated and degraded
B) The α subunit possesses intrinsic GTPase activity, hydrolyzing bound GTP to
GDP
C) The α subunit is phosphorylated by a kinase
D) The α subunit dissociates from the βγ complex permanently
Correct Answer: B) The α subunit possesses intrinsic GTPase activity, hydrolyzing
bound GTP to GDP
Rationale: G proteins are GTPases. The α subunit hydrolyzes GTP to GDP (intrinsic
Rationale | Already Graded A+ | Newly Updated 2026-2027
1. Restriction nucleases are bacterial enzymes fundamental to molecular biology
techniques. What is the primary biological role of these enzymes in the bacteria
that naturally express them?
A) To repair damaged bacterial DNA
B) To degrade exogenous or foreign DNA, thereby protecting the bacterium from
threats like viruses
C) To replicate bacterial chromosomes
D) To transcribe bacterial genes
Correct Answer: B) To degrade exogenous or foreign DNA, thereby protecting the
bacterium from threats like viruses
Rationale: Restriction nucleases are part of the bacterial defense system
(restriction-modification system). They recognize specific DNA sequences and
cleave foreign DNA (e.g., from bacteriophages), protecting the bacterium from
viral infection. The bacterial host's own DNA is protected by methylation.
2. Gel electrophoresis is a technique used to separate DNA fragments after
restriction enzyme digestion. If a large, linear DNA molecule is digested by a
restriction enzyme and run on an agarose gel, how are the fragments physically
separated?
A) Separation occurs based on the size of the DNA fragments, with smaller
fragments traveling fastest toward the positive pole
B) Separation occurs based on the charge of the DNA fragments, with larger
fragments traveling fastest toward the negative pole
C) Separation occurs based on the sequence of the DNA fragments, with GC-rich
fragments traveling faster
D) Separation occurs based on the shape of the DNA fragments, with circular
fragments traveling faster
,Correct Answer: A) Separation occurs based on the size of the DNA fragments,
with smaller fragments traveling fastest toward the positive pole
Rationale: DNA fragments are negatively charged due to their phosphate
backbone. In an agarose gel, smaller fragments migrate faster through the matrix
toward the positive electrode, while larger fragments are slowed by the gel's
sieving effect. Size is the primary determinant of migration.
3. In the process of visualizing DNA fragments in an agarose gel, Ethidium
Bromide is added. Which statement best describes how Ethidium Bromide
allows researchers to see the DNA under UV light?
A) It binds to the phosphate backbone of DNA, causing it to absorb UV light
B) It intercalates into the DNA double helix, acting as a dye that fluoresces upon
UV exposure
C) It covalently attaches to the 5' end of DNA, creating a fluorescent product
D) It denatures the DNA, allowing it to be visualized
Correct Answer: B) It intercalates into the DNA double helix, acting as a dye that
fluoresces upon UV exposure
Rationale: Ethidium Bromide intercalates (inserts) between the stacked base pairs
of double-stranded DNA. When exposed to UV light, it fluoresces brightly,
allowing DNA bands to be visualized. This intercalation does not alter DNA
sequence but allows detection.
4. A researcher is attempting to clone a gene into a circular plasmid vector. The
plasmid map indicates that it contains one EcoR1 site, one HindIII site, and one
Pst1 site. If the researcher performs a complete restriction digest using only
HindIII and Pst1 enzymes simultaneously on this circular DNA, how many linear
DNA fragments would be generated?
A) 1 fragment
B) 2 fragments
C) 3 fragments
D) 4 fragments
,Correct Answer: B) 2 fragments
Rationale: Restriction enzymes cut at specific recognition sites. In a circular
plasmid, each cut by a restriction enzyme linearizes the circle. With two different
enzymes each cutting once (HindIII and PstI), there are two cuts total, producing
two linear fragments of different sizes.
5. The fundamental process of creating a Recombinant DNA molecule, which
initiates DNA cloning, requires two distinct enzymatic activities working
together. These activities are:
A) Cutting the DNA at specific sequences and then sealing the DNA backbone
B) Cutting the DNA randomly and then sealing the DNA backbone
C) Copying the DNA and then cutting it
D) Transcribing the DNA and then translating it
Correct Answer: A) Cutting the DNA at specific sequences and then sealing the
DNA backbone
Rationale: Recombinant DNA formation requires: (1) restriction endonucleases to
cut DNA at specific sequences, generating fragments with compatible ends, and
(2) DNA ligase to seal the sugar-phosphate backbone between the vector and
insert fragments, creating a stable recombinant molecule.
6. A crucial component of successful cloning is the use of a vector (plasmid). If
the goal is to amplify the target DNA sequence to make many identical copies
within a host bacterium, which element is absolutely necessary for replication in
the bacterial cell?
A) An antibiotic resistance gene
B) An origin of replication (ORI)
C) A multiple cloning site
D) A promoter sequence
Correct Answer: B) An origin of replication (ORI)
Rationale: The origin of replication (ORI) is a DNA sequence that allows the
plasmid to be replicated independently by the host's replication machinery.
, Without an ORI, the plasmid cannot be copied and will be lost as bacteria divide,
preventing amplification of the target DNA.
7. G-Protein-Coupled Receptors (GPCRs) represent the largest family of cell-
surface receptors in humans and are crucial targets for therapeutic drugs.
Structurally, all GPCRs share a defining characteristic that facilitates signal
transmission across the plasma membrane. What is this shared structural
feature?
A) They possess two polypeptide chains that span the membrane four times each
B) They possess a single polypeptide chain that spans the membrane exactly
seven times
C) They possess a single polypeptide chain that spans the membrane once
D) They possess three polypeptide chains that form a channel
Correct Answer: B) They possess a single polypeptide chain that spans the
membrane exactly seven times
Rationale: GPCRs are characterized by seven transmembrane α-helices (a single
polypeptide chain threading through the membrane seven times). This
"serpentine" structure positions the extracellular N-terminus for ligand binding
and the intracellular C-terminus for G-protein interaction, enabling signal
transmission across the membrane.
8. G proteins function as molecular switches in cell signaling pathways. What
mechanism causes the G protein α subunit to transition from its active (ON)
state back to its inactive (OFF) state, effectively terminating the signal?
A) The α subunit is ubiquitinated and degraded
B) The α subunit possesses intrinsic GTPase activity, hydrolyzing bound GTP to
GDP
C) The α subunit is phosphorylated by a kinase
D) The α subunit dissociates from the βγ complex permanently
Correct Answer: B) The α subunit possesses intrinsic GTPase activity, hydrolyzing
bound GTP to GDP
Rationale: G proteins are GTPases. The α subunit hydrolyzes GTP to GDP (intrinsic
