Mock Test 1

Cell Biology Mock Test 01 (CSIR-NET/GATE Level)

45:00

1. The KDEL receptor retrieves ER-resident proteins from the Golgi complex. Its affinity for the KDEL sequence is: A) High in the ER and high in the Golgi B) Low in the ER and high in the Golgi C) High in the ER and low in the Golgi D) Low in the ER and low in the Golgi

Ans: B. The KDEL receptor has high affinity in the slightly acidic environment of the Golgi (facilitating binding) and low affinity in the neutral ER (facilitating release).

2. Cholera toxin causes massive secretory diarrhea by modifying the Gs alpha subunit. What is the molecular mechanism? A) It phosphorylates Gs alpha, blocking GTP binding B) It ADP-ribosylates Gs alpha, locking it in the active GTP-bound state C) It proteolytically cleaves Gs alpha D) It ADP-ribosylates Gi alpha, locking it in the inactive GDP-bound state

Ans: B. Cholera toxin prevents Gs alpha from hydrolyzing GTP, leading to continuous activation of Adenylyl Cyclase and high cAMP levels.

3. In the sorting of lysosomal enzymes, the addition of Mannose-6-phosphate (M6P) tags occurs primarily in the: A) Rough Endoplasmic Reticulum B) Cis-Golgi network C) Trans-Golgi network D) Early endosomes

Ans: B. GlcNAc phosphotransferase catalyzes the first step of M6P addition in the cis-Golgi network.

4. The drug colchicine affects the cytoskeleton by: A) Binding to actin monomers to prevent polymerization B) Binding to tubulin dimers to prevent microtubule polymerization C) Stabilizing microtubules against depolymerization (like Taxol) D) Cleaving intermediate filaments

Ans: B. Colchicine binds to free tubulin dimers, preventing their addition to the growing plus end, causing microtubule depolymerization.

5. The directionality of nuclear transport is primarily maintained by the Ran-GTP gradient. Where is Ran-GEF (RCC1) exclusively localized? A) In the cytoplasm B) On the nuclear envelope outer membrane C) Bound to chromatin inside the nucleus D) Inside the nucleolus only

Ans: C. Ran-GEF is tethered to chromatin, ensuring that Ran is in the GTP-bound state inside the nucleus. Ran-GAP is cytoplasmic.

6. COPII-coated vesicles are responsible for mediating transport: A) From the Golgi to the ER (Retrograde) B) From the Trans-Golgi Network to endosomes C) From the plasma membrane to endosomes D) From the ER to the Golgi (Anterograde)

Ans: D. COPII mediates ER to Golgi transport. COPI mediates retrograde transport (Golgi to ER).

7. SH2 (Src Homology 2) domains in intracellular signaling proteins specifically recognize and bind to: A) Proline-rich sequences B) Phosphorylated Tyrosine residues C) Phosphorylated Serine/Threonine residues D) Inositol phospholipids (PIP3)

Ans: B. SH2 domains are critical for Receptor Tyrosine Kinase (RTK) signaling cascades as they dock onto phospho-tyrosines.

8. In the regulation of the mammalian cell cycle, the phosphorylation of the Retinoblastoma (Rb) protein results in: A) The inhibition of the E2F transcription factor B) The release and activation of the E2F transcription factor C) The activation of p53 D) Arresting the cell at the G2/M checkpoint

Ans: B. Hyperphosphorylated Rb releases E2F, allowing it to transcribe genes necessary for S-phase entry.

9. "Treadmilling" in actin filaments occurs when: A) The rate of addition at the plus end exactly equals the rate of loss at the minus end B) Both ends are growing rapidly C) The filament breaks in the middle D) ATP hydrolysis stops completely

Ans: A. At steady state, the filament maintains a constant length while monomers continuously flux through it from the plus end to the minus end.

10. The Anaphase-Promoting Complex/Cyclosome (APC/C) triggers sister chromatid separation by ubiquitinating: A) Separase B) Cohesin C) Securin D) Cyclin E

Ans: C. APC/C tags Securin for degradation. Its destruction frees the protease Separase, which then cleaves Cohesin.

11. A single gap junction channel (connexon) in vertebrate cells is composed of how many connexin subunits? A) 4 B) 6 C) 8 D) 12

Ans: B. Six connexins assemble to form a hemichannel (connexon), and two connexons join to form the full gap junction.

12. Integrins mediate cell adhesion to the extracellular matrix by recognizing which specific amino acid sequence in proteins like fibronectin? A) KDEL B) NLS (PKKKRKV) C) RGD (Arg-Gly-Asp) D) PTS1 (SKL)

Ans: C. The RGD motif is the primary integrin-binding sequence found in many ECM proteins.

13. During intrinsic apoptosis, Apaf-1 binds to which two molecules to form the active apoptosome? A) Cytochrome c and ATP/dATP B) Bcl-2 and Cytochrome c C) Fas Ligand and FADD D) Caspase 8 and Smac/DIABLO

Ans: A. The apoptosome requires Cytochrome c (released from mitochondria) and dATP/ATP to oligomerize and activate Caspase-9.

14. The nuclear lamina provides structural support to the nuclear envelope. It is composed of lamins, which belong to which cytoskeletal family? A) Microtubules B) Microfilaments C) Intermediate filaments D) Septins

Ans: C. Nuclear lamins are Type V intermediate filaments.

15. The Signal Recognition Particle (SRP) is a ribonucleoprotein. Which RNA molecule is a structural component of eukaryotic SRP? A) 5S rRNA B) 7S RNA (7SL RNA) C) U1 snRNA D) snoRNA

Ans: B. The 7SL RNA provides the scaffold for the six proteins of the SRP.

16. Zellweger syndrome is a lethal metabolic disorder characterized by the absence of functional: A) Lysosomes B) Mitochondria C) Peroxisomes D) Golgi bodies

Ans: C. It is caused by mutations in PEX genes, severely impairing peroxisome biogenesis and protein import.

17. The primary biochemical function of the p53 tumor suppressor protein is: A) A tyrosine kinase B) A DNA-specific endonuclease C) A sequence-specific transcription factor D) A GTPase-activating protein (GAP)

Ans: C. p53 acts as a transcription factor, upregulating genes like p21 (a CDK inhibitor) in response to DNA damage.

18. Glycosylphosphatidylinositol (GPI) anchors are attached to proteins targeted for the plasma membrane. This attachment occurs in the: A) Cytosol B) Endoplasmic reticulum lumen C) Golgi lumen D) Extracellular space

Ans: B. The GPI anchor is preassembled in the ER and attached to the C-terminus of proteins in the ER lumen.

19. In the Spindle Assembly Checkpoint (SAC), unattached kinetochores generate a "wait" signal primarily through the activation of which protein? A) Cyclin B B) Mad2 C) Cdk1 D) Separase

Ans: B. Mad2 undergoes a conformational change at unattached kinetochores to inhibit Cdc20, thus keeping the APC/C inactive.

20. Phospholipase C-beta (PLC-β) is classically activated by: A) Receptor Tyrosine Kinases (RTKs) B) The Gq alpha subunit of GPCRs C) cAMP-dependent Protein Kinase (PKA) D) Calmodulin

Ans: B. Gq activates PLC-β. (Note: PLC-gamma is activated by RTKs).

21. Lipid rafts are specialized microdomains in the plasma membrane highly enriched in: A) Phosphatidylcholine and unsaturated fatty acids B) Cholesterol and sphingolipids C) Cardiolipin and phosphatidylethanolamine D) Triglycerides and sterols

Ans: B. The saturated tails of sphingolipids pack tightly with cholesterol to form these thicker, less fluid domains.

22. The TOM and TIM complexes are essential for protein translocation into which organelle? A) Chloroplasts B) Nucleus C) Mitochondria D) Endoplasmic Reticulum

Ans: C. Translocase of the Outer/Inner Mitochondrial Membrane (TOM/TIM).

23. The structural unit of a clathrin coat is the "triskelion," which consists of: A) 3 heavy chains and 3 light chains B) 6 identical polypeptide chains C) 2 heavy chains and 4 light chains D) A hexamer of dynamin proteins

Ans: A. Clathrin triskelions self-assemble into a polyhedral lattice to shape vesicles.

24. Progression through the G1 restriction point in mammalian cells is primarily driven by: A) Cyclin B / CDK1 B) Cyclin A / CDK2 C) Cyclin D / CDK4/6 D) Cyclin E / CDK1

Ans: C. Cyclin D pairs with CDK4 and CDK6 to initiate Rb phosphorylation in early G1.

25. Which of the following is classified as an "initiator caspase" in the extrinsic apoptotic pathway? A) Caspase-3 B) Caspase-8 C) Caspase-9 D) Caspase-7

Ans: B. Caspase-8 is recruited to the Death-Inducing Signaling Complex (DISC) upon death receptor activation (extrinsic). Caspase-9 is intrinsic.