Subtopic: Fundamentals of Cell structure and signaling
Question Statement:
Which of the following technique(s) can be used to study conformational changes in
myoglobin?
P. Mass spectrometry
Q. Fluorescence spectroscopy
R. Circular dichroism spectroscopy
S. Light microscopy
(A) P only (B) P and S only (C) Q and R only (D) S only
Options:
(A) proliferate faster
(B) proliferate normally
(C) undergo cell cycle arrest
(D) undergo immediate apoptosis
Official Answer Key:
C
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2018 Q.12)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (1 Mark)
- **Topic**: Cell Biology / Animal Cell Culture (Cell Cycle Regulation & Growth Factors)
**2. Detailed Step-by-Step Analysis**:
- In mammalian cell culture (such as HEK cells), serum (e.g., Fetal Bovine Serum, FBS) provides vital mitogens and polypeptide growth factors (such as EGF, PDGF, and IGF) required for cell survival, proliferation, and cell cycle progression.
- During the early to mid-$G_1$ phase, cells require continuous mitogenic signaling through the RTK/Ras/MAPK pathway to induce Cyclin D expression, which activates Cdk4/6 to phosphorylate the retinoblastoma protein (pRb).
- If serum is withdrawn from the culture medium (serum starvation), cells lack the mitogens necessary to pass the **Restriction Point (R point)** located in late $G_1$.
- Consequently, the cells exit the active cell cycle and arrest reversibly in the quiescent **$G_0 / G_1$ phase**. They do not arrest in $S$, $G_2$, or $M$ because cells already past the restriction point complete the ongoing cycle before arresting in the next $G_1$.
- **Option-by-Option Evaluation**:
- **Option (A) [INCORRECT]**: Cells do not arrest in $M$ phase; mitotic arrest typically requires spindle-disrupting agents like nocodazole or taxol.
- **Option (B) [INCORRECT]**: $G_2$ arrest is induced by DNA damage checkpoints (via ATM/ATR and Chk1/Chk2), not serum deprivation.
- **Option (C) [CORRECT]**: Serum starvation specifically arrests cells at the $G_0 / G_1$ phase boundary due to the absence of mitogenic growth factors.
- **Option (D) [INCORRECT]**: Arrest in $S$ phase is caused by replication stress or inhibitors like aphidicolin and hydroxyurea.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **C**
- **Scoring Criteria**: +1.0 Mark (Negative marking: -0.33 marks).
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