Subtopic: Fundamentals of Cell structure and signaling
Question Statement:
In eukaryotes, cytokinesis is inhibited by
Options:
(A) cytochalasin D
(B) vinblastine
(C) nocodazole
(D) colchicine
Official Answer Key:
A or C
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2017 Q.18)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (1 Mark)
- **Topic**: Cell Biology (Cytokinesis, Contractile Ring & Cytoskeletal Inhibitors)
**2. Detailed Step-by-Step Analysis**:
- In animal and fungal cells, **cytokinesis** (cell division) is executed by the assembly and constriction of an equatorial **contractile actomyosin ring** composed of actin filaments and non-muscle myosin II motors:
- **Cytochalasin D** is a fungal toxin that specifically binds to the barbed (+), fast-growing ends of actin microfilaments, inhibiting monomer addition and depolymerizing the actin cytoskeleton. By disrupting actin filament dynamics, cytochalasin D prevents contractile ring assembly and completely blocks cytokinesis, resulting in binucleated cells.
- **Nocodazole** is a synthetic compound that rapidly binds to $\beta$-tubulin, preventing microtubule assembly and depolymerizing the mitotic spindle. While its primary classical block is during metaphase, dynamic astral microtubules are also essential for determining the equatorial cleavage plane and signaling cytokinesis onset. Consequently, nocodazole treatment also halts the progression into and completion of cytokinesis.
- Recognizing that both actin and microtubule dynamics directly influence the eukaryotic division machinery, the official GATE key accepted both **A (cytochalasin D)** and **C (nocodazole)** as valid correct answers.
- **Option-by-Option Evaluation**:
- **Option (A) [CORRECT]**: Cytochalasin D disrupts actin microfilaments, which directly constitute the contractile ring of cytokinesis.
- **Option (B) [INCORRECT]**: Vinblastine destabilizes microtubules primarily utilized in antineoplastic mitotic spindle poisoning.
- **Option (C) [CORRECT]**: Nocodazole disrupts microtubules, preventing mitotic exit and cytokinesis signaling (officially accepted).
- **Option (D) [INCORRECT]**: Colchicine depolymerizes microtubules, predominantly studied for metaphase karyotype arrest.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **A or C**
- **Scoring Criteria**: +1.0 Mark (Negative marking: -0.33 marks).
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