BIOL 331 ADVANCED CELL BIOLOGY -
EXAM 3 PREP QUESTIONS & VERIFIED
ANSWERS 2026/2027 UPDATE
1. Which of the following events occurs immediately after a G-protein coupled receptor
(GPCR) binds to its ligand?
A. The G-alpha subunit exchanges GDP for GTP.
B. The G-alpha subunit hydrolyzes GTP to GDP.
C. The receptor undergoes a conformational change that acts as a Guanine Nucleotide
Exchange Factor (GEF).
D. The beta-gamma complex dissociates from the receptor to activate adenylyl cyclase.
Answer: C
Conceptual Explanation: Upon ligand binding, the GPCR undergoes a conformational
change that allows it to act as a GEF for the G-alpha subunit, triggering the exchange of GDP
for GTP.
2. In the MAP kinase signaling pathway, which protein acts as the direct upstream activator of
MAP Kinase Kinase (MEK)?
A. Ras
B. Grb2
,C. ERK
D. Raf
Answer: D
Conceptual Explanation: Raf is the MAP Kinase Kinase Kinase (MAPKKK) that
phosphorylates and activates MEK (MAPKK).
3. Cholera toxin causes massive diarrhea by targeting which signaling component?
A. Preventing the activation of Gi-alpha.
B. Inhibiting the GTPase activity of Gs-alpha.
C. Degrading Phospholipase C.
D. Blocking the binding of cAMP to PKA.
Answer: B
Conceptual Explanation: Cholera toxin ADP-ribosylates the Gs-alpha subunit, inhibiting
its GTPase activity. This keeps the subunit permanently ‘on,’ leading to constitutive cAMP
production.
4. Which of the following molecules is responsible for the release of Calcium from the
Endoplasmic Reticulum?
A. Diacylglycerol (DAG)
B. Inositol 1,4,5-trisphosphate (IP3)
C. Phosphatidylinositol 4,5-bisphosphate (PIP2)
, D. Cyclic GMP
Answer: B
Conceptual Explanation: IP3 is a soluble second messenger that binds to IP3-gated
calcium channels on the ER membrane, causing the release of stored calcium into the
cytosol.
5. What is the function of the SH2 domain in signaling proteins?
A. Binding to proline-rich sequences.
B. Anchoring proteins to the plasma membrane.
C. Catalyzing the hydrolysis of ATP.
D. Binding to phosphorylated tyrosine residues.
Answer: D
Conceptual Explanation: SH2 (Src Homology 2) domains specifically recognize and bind
to phosphorylated tyrosine residues, allowing proteins to dock onto activated Receptor
Tyrosine Kinases (RTKs).
6. During which phase of the cell cycle is the DNA replicated?
A. G1 phase
B. G2 phase
C. S phase
D. M phase
EXAM 3 PREP QUESTIONS & VERIFIED
ANSWERS 2026/2027 UPDATE
1. Which of the following events occurs immediately after a G-protein coupled receptor
(GPCR) binds to its ligand?
A. The G-alpha subunit exchanges GDP for GTP.
B. The G-alpha subunit hydrolyzes GTP to GDP.
C. The receptor undergoes a conformational change that acts as a Guanine Nucleotide
Exchange Factor (GEF).
D. The beta-gamma complex dissociates from the receptor to activate adenylyl cyclase.
Answer: C
Conceptual Explanation: Upon ligand binding, the GPCR undergoes a conformational
change that allows it to act as a GEF for the G-alpha subunit, triggering the exchange of GDP
for GTP.
2. In the MAP kinase signaling pathway, which protein acts as the direct upstream activator of
MAP Kinase Kinase (MEK)?
A. Ras
B. Grb2
,C. ERK
D. Raf
Answer: D
Conceptual Explanation: Raf is the MAP Kinase Kinase Kinase (MAPKKK) that
phosphorylates and activates MEK (MAPKK).
3. Cholera toxin causes massive diarrhea by targeting which signaling component?
A. Preventing the activation of Gi-alpha.
B. Inhibiting the GTPase activity of Gs-alpha.
C. Degrading Phospholipase C.
D. Blocking the binding of cAMP to PKA.
Answer: B
Conceptual Explanation: Cholera toxin ADP-ribosylates the Gs-alpha subunit, inhibiting
its GTPase activity. This keeps the subunit permanently ‘on,’ leading to constitutive cAMP
production.
4. Which of the following molecules is responsible for the release of Calcium from the
Endoplasmic Reticulum?
A. Diacylglycerol (DAG)
B. Inositol 1,4,5-trisphosphate (IP3)
C. Phosphatidylinositol 4,5-bisphosphate (PIP2)
, D. Cyclic GMP
Answer: B
Conceptual Explanation: IP3 is a soluble second messenger that binds to IP3-gated
calcium channels on the ER membrane, causing the release of stored calcium into the
cytosol.
5. What is the function of the SH2 domain in signaling proteins?
A. Binding to proline-rich sequences.
B. Anchoring proteins to the plasma membrane.
C. Catalyzing the hydrolysis of ATP.
D. Binding to phosphorylated tyrosine residues.
Answer: D
Conceptual Explanation: SH2 (Src Homology 2) domains specifically recognize and bind
to phosphorylated tyrosine residues, allowing proteins to dock onto activated Receptor
Tyrosine Kinases (RTKs).
6. During which phase of the cell cycle is the DNA replicated?
A. G1 phase
B. G2 phase
C. S phase
D. M phase
