BIOL 304: MOLECULAR CELL BIOLOGY -
EXAM 4 – VERSION B QUESTIONS AND
ANSWERS WITH RATIONALES/ GRADED
A+/2026 UPDATE /100% PASS
Section I: Multiple Choice (1.5 points each, 60 points total)
1. Which of the following cytoskeletal filaments is correctly paired with its primary associated motor
protein and directionality?
a) Microtubules – Kinesin (moves toward the minus end)
b) Actin filaments – Dynein (moves toward the plus end)
c) Intermediate filaments – Myosin (no directional movement)
d) Microtubules – Dynein (moves toward the minus end)
e) Actin filaments – Kinesin (moves toward the barbed end)
Answer: d) Microtubules – Dynein (moves toward the minus end)
Rationale: Dynein is a microtubule-associated motor protein that moves toward the minus end (typically
the centrosome). Kinesin moves toward the plus end. Myosin moves on actin filaments. Intermediate
filaments have no known motor proteins.
2. A patient presents with a mutation that prevents the addition of the prenyl group to the small
GTPase Rho. What cellular process would be most directly impaired?
a) Association of Rho with the plasma membrane
b) Hydrolysis of GTP to GDP by Rho
c) Binding of Rho to its downstream effector, ROCK
d) The intrinsic guanine nucleotide exchange factor (GEF) activity of Rho
Answer: a) Association of Rho with the plasma membrane
Rationale: Prenylation is a lipid modification (e.g., farnesylation or geranylgeranylation) that targets
, small GTPases like Rho to the inner leaflet of the plasma membrane, where they interact with upstream
regulators and downstream effectors. Without this, Rho would remain cytosolic and inactive.
3. The "sliding filament model" of muscle contraction relies on the cyclical interaction between actin
and myosin. What is the direct role of ATP hydrolysis in this cycle?
a) To cause the power stroke that moves the thin filament
b) To allow the myosin head to detach from the actin filament
c) To expose the myosin-binding site on actin by moving tropomyosin
d) To phosphorylate the myosin light chain, initiating contraction
Answer: b) To allow the myosin head to detach from the actin filament
Rationale: ATP binding to the myosin head causes a conformational change that reduces its affinity for
actin, allowing detachment (rigor mortis occurs when ATP is absent and detachment cannot happen).
The subsequent hydrolysis of ATP primes the myosin head for the next power stroke.
4. Which of the following statements best describes the mechanism of action of colchicine, a drug
used to treat gout?
a) It stabilizes microtubules, preventing mitotic spindle disassembly.
b) It binds to the plus ends of actin filaments, preventing polymerization.
c) It binds to tubulin dimers, preventing their polymerization into microtubules.
d) It inhibits myosin light-chain kinase, leading to smooth muscle relaxation.
Answer: c) It binds to tubulin dimers, preventing their polymerization into microtubules.
Rationale: Colchicine binds to soluble tubulin heterodimers, preventing their addition to growing
microtubules. This disrupts microtubule dynamics, which is particularly effective against rapidly dividing
cells (like inflammatory cells in gout) and is the basis for its use as a chemotherapeutic agent as well.
5. Integrins are critical for cell adhesion and signaling. Which of the following correctly describes their
function?
a) They form tight, impermeable seals between adjacent epithelial cells.
b) They mediate homophilic (same-type) adhesion between cells.
c) They are transmembrane receptors that bind to extracellular matrix proteins like fibronectin.
d) They link cadherins to the intermediate filament cytoskeleton.
Answer: c) They are transmembrane receptors that bind to extracellular matrix proteins like
fibronectin.
Rationale: Integrins are heterodimeric transmembrane receptors that bind to ECM components (e.g.,
collagen, fibronectin, laminin) on the outside and link to the actin cytoskeleton on the inside via adaptor
proteins like talin and vinculin.
6. A researcher adds a drug that inhibits the activity of the Arp2/3 complex to a culture of migrating
fibroblasts. What would be the most immediate effect on cell motility?
a) The cell would be unable to extend lamellipodia.
EXAM 4 – VERSION B QUESTIONS AND
ANSWERS WITH RATIONALES/ GRADED
A+/2026 UPDATE /100% PASS
Section I: Multiple Choice (1.5 points each, 60 points total)
1. Which of the following cytoskeletal filaments is correctly paired with its primary associated motor
protein and directionality?
a) Microtubules – Kinesin (moves toward the minus end)
b) Actin filaments – Dynein (moves toward the plus end)
c) Intermediate filaments – Myosin (no directional movement)
d) Microtubules – Dynein (moves toward the minus end)
e) Actin filaments – Kinesin (moves toward the barbed end)
Answer: d) Microtubules – Dynein (moves toward the minus end)
Rationale: Dynein is a microtubule-associated motor protein that moves toward the minus end (typically
the centrosome). Kinesin moves toward the plus end. Myosin moves on actin filaments. Intermediate
filaments have no known motor proteins.
2. A patient presents with a mutation that prevents the addition of the prenyl group to the small
GTPase Rho. What cellular process would be most directly impaired?
a) Association of Rho with the plasma membrane
b) Hydrolysis of GTP to GDP by Rho
c) Binding of Rho to its downstream effector, ROCK
d) The intrinsic guanine nucleotide exchange factor (GEF) activity of Rho
Answer: a) Association of Rho with the plasma membrane
Rationale: Prenylation is a lipid modification (e.g., farnesylation or geranylgeranylation) that targets
, small GTPases like Rho to the inner leaflet of the plasma membrane, where they interact with upstream
regulators and downstream effectors. Without this, Rho would remain cytosolic and inactive.
3. The "sliding filament model" of muscle contraction relies on the cyclical interaction between actin
and myosin. What is the direct role of ATP hydrolysis in this cycle?
a) To cause the power stroke that moves the thin filament
b) To allow the myosin head to detach from the actin filament
c) To expose the myosin-binding site on actin by moving tropomyosin
d) To phosphorylate the myosin light chain, initiating contraction
Answer: b) To allow the myosin head to detach from the actin filament
Rationale: ATP binding to the myosin head causes a conformational change that reduces its affinity for
actin, allowing detachment (rigor mortis occurs when ATP is absent and detachment cannot happen).
The subsequent hydrolysis of ATP primes the myosin head for the next power stroke.
4. Which of the following statements best describes the mechanism of action of colchicine, a drug
used to treat gout?
a) It stabilizes microtubules, preventing mitotic spindle disassembly.
b) It binds to the plus ends of actin filaments, preventing polymerization.
c) It binds to tubulin dimers, preventing their polymerization into microtubules.
d) It inhibits myosin light-chain kinase, leading to smooth muscle relaxation.
Answer: c) It binds to tubulin dimers, preventing their polymerization into microtubules.
Rationale: Colchicine binds to soluble tubulin heterodimers, preventing their addition to growing
microtubules. This disrupts microtubule dynamics, which is particularly effective against rapidly dividing
cells (like inflammatory cells in gout) and is the basis for its use as a chemotherapeutic agent as well.
5. Integrins are critical for cell adhesion and signaling. Which of the following correctly describes their
function?
a) They form tight, impermeable seals between adjacent epithelial cells.
b) They mediate homophilic (same-type) adhesion between cells.
c) They are transmembrane receptors that bind to extracellular matrix proteins like fibronectin.
d) They link cadherins to the intermediate filament cytoskeleton.
Answer: c) They are transmembrane receptors that bind to extracellular matrix proteins like
fibronectin.
Rationale: Integrins are heterodimeric transmembrane receptors that bind to ECM components (e.g.,
collagen, fibronectin, laminin) on the outside and link to the actin cytoskeleton on the inside via adaptor
proteins like talin and vinculin.
6. A researcher adds a drug that inhibits the activity of the Arp2/3 complex to a culture of migrating
fibroblasts. What would be the most immediate effect on cell motility?
a) The cell would be unable to extend lamellipodia.
