PCB3023C Cell Biology Exam 4 - Comprehensive Test 2026 |FGCU
1. Which molecule acts as a second messenger by being released from the
endoplasmic reticulum in response to phospholipase C activation?
A. cAMP
B. IP3 (Inositol triphosphate)
C. cGMP
D. Diacylglycerol (DAG)
Answer: B
Rationale: IP3 is released into the cytosol where it binds to and opens calcium channels in
the endoplasmic reticulum membrane.
2. In G-protein coupled receptor (GPCR) signaling, which subunit typically
possesses the GTPase activity?
A. Delta subunit
B. Beta subunit
C. Gamma subunit
D. Alpha subunit
Answer: D
Rationale: The alpha subunit of the G-protein binds GTP and has an intrinsic GTPase
activity that eventually hydrolyzes GTP to GDP to turn itself off.
,3. What is the primary function of Ras proteins in cell signaling?
A. To act as a secondary messenger
B. To relay signals from receptor tyrosine kinases to a MAP-kinase cascade
C. To inhibit the cell cycle during DNA damage
D. To transport proteins into the nucleus
Answer: B
Rationale: Ras is a monomeric GTPase that is activated by RTKs and initiates the MAP-
kinase phosphorylation pathway.
4. Which component of the cytoskeleton is responsible for forming the
contractile ring during cytokinesis?
A. Microtubules
B. Actin filaments
C. Intermediate filaments
D. Septins
Answer: B
Rationale: Actin filaments, interacting with myosin II, form the contractile ring that
pinches the cell into two daughter cells.
5. What characterizes ‘dynamic instability’ in microtubules?
A. The rapid switching between growth and shrinkage at the plus end
B. The movement of dynein along the tubulin wall
C. The constant length maintained by treadmilling
D. The formation of cross-links between protofilaments
Answer: A
Rationale: Dynamic instability refers to the sudden conversion from a growing state to a
shrinking state (catastrophe) or vice-versa (rescue) at the microtubule end.
, 6. Which motor protein moves toward the minus end of microtubules?
A. Dynein
B. Myosin V
C. Kinesin-1
D. Kinesin-5
Answer: A
Rationale: Dyneins are minus-end directed motor proteins, whereas most kinesins move
toward the plus end.
7. What is the structural arrangement of microtubules in a eukaryotic cilium or
flagellum?
A. 9+2 arrangement
B. 9+0 arrangement
C. Triple-helical arrangement
D. 7+2 arrangement
Answer: A
Rationale: The axoneme of cilia and flagella typically consists of nine outer doublet
microtubules surrounding two central single microtubules.
8. The phosphorylation of which protein by Cyclin-CDK complexes triggers the
entry into S-phase?
A. Retinoblastoma protein (Rb)
B. p53
C. Caspase-3
D. Histone H1
Answer: A
Rationale: Rb normally inhibits the E2F transcription factor; its phosphorylation by G1-
CDK releases E2F, allowing the transcription of S-phase genes.
1. Which molecule acts as a second messenger by being released from the
endoplasmic reticulum in response to phospholipase C activation?
A. cAMP
B. IP3 (Inositol triphosphate)
C. cGMP
D. Diacylglycerol (DAG)
Answer: B
Rationale: IP3 is released into the cytosol where it binds to and opens calcium channels in
the endoplasmic reticulum membrane.
2. In G-protein coupled receptor (GPCR) signaling, which subunit typically
possesses the GTPase activity?
A. Delta subunit
B. Beta subunit
C. Gamma subunit
D. Alpha subunit
Answer: D
Rationale: The alpha subunit of the G-protein binds GTP and has an intrinsic GTPase
activity that eventually hydrolyzes GTP to GDP to turn itself off.
,3. What is the primary function of Ras proteins in cell signaling?
A. To act as a secondary messenger
B. To relay signals from receptor tyrosine kinases to a MAP-kinase cascade
C. To inhibit the cell cycle during DNA damage
D. To transport proteins into the nucleus
Answer: B
Rationale: Ras is a monomeric GTPase that is activated by RTKs and initiates the MAP-
kinase phosphorylation pathway.
4. Which component of the cytoskeleton is responsible for forming the
contractile ring during cytokinesis?
A. Microtubules
B. Actin filaments
C. Intermediate filaments
D. Septins
Answer: B
Rationale: Actin filaments, interacting with myosin II, form the contractile ring that
pinches the cell into two daughter cells.
5. What characterizes ‘dynamic instability’ in microtubules?
A. The rapid switching between growth and shrinkage at the plus end
B. The movement of dynein along the tubulin wall
C. The constant length maintained by treadmilling
D. The formation of cross-links between protofilaments
Answer: A
Rationale: Dynamic instability refers to the sudden conversion from a growing state to a
shrinking state (catastrophe) or vice-versa (rescue) at the microtubule end.
, 6. Which motor protein moves toward the minus end of microtubules?
A. Dynein
B. Myosin V
C. Kinesin-1
D. Kinesin-5
Answer: A
Rationale: Dyneins are minus-end directed motor proteins, whereas most kinesins move
toward the plus end.
7. What is the structural arrangement of microtubules in a eukaryotic cilium or
flagellum?
A. 9+2 arrangement
B. 9+0 arrangement
C. Triple-helical arrangement
D. 7+2 arrangement
Answer: A
Rationale: The axoneme of cilia and flagella typically consists of nine outer doublet
microtubules surrounding two central single microtubules.
8. The phosphorylation of which protein by Cyclin-CDK complexes triggers the
entry into S-phase?
A. Retinoblastoma protein (Rb)
B. p53
C. Caspase-3
D. Histone H1
Answer: A
Rationale: Rb normally inhibits the E2F transcription factor; its phosphorylation by G1-
CDK releases E2F, allowing the transcription of S-phase genes.
