Mock Test 2

Cell Biology Mock Test 02 (Advanced Level)

45:00

1. Which of the following motions of membrane lipids is the slowest and least frequent in a biological membrane? A) Lateral diffusion B) Flexion of hydrocarbon tails C) Transverse diffusion (flip-flop) D) Rotation around the long axis

Ans: C. Transverse diffusion (flip-flop) requires the polar head group to pass through the hydrophobic core, making it thermodynamically unfavorable without enzymes like flippases.

2. The Na+/K+ pump is an electrogenic pump because it: A) Moves 3 Na+ out of the cell for every 2 K+ moved in B) Moves 2 Na+ out of the cell for every 3 K+ moved in C) Hydrolyzes ATP to move ions down their concentration gradients D) Is a voltage-gated ion channel

Ans: A. Moving 3 positive charges out and only 2 in results in a net movement of one positive charge outwards per cycle, contributing to the resting membrane potential.

3. In N-linked glycosylation, the 14-sugar core oligosaccharide is initially transferred to an asparagine residue from a lipid carrier called: A) Phosphatidylinositol B) Dolichol phosphate C) Sphingomyelin D) Farnesyl pyrophosphate

Ans: B. Dolichol phosphate is a specialized lipid anchored in the ER membrane that serves as a carrier for assembling the core oligosaccharide.

4. During vesicle budding, which GTPase is required to "pinch off" clathrin-coated vesicles from the membrane? A) Rab B) Sar1 C) ARF D) Dynamin

Ans: D. Dynamin forms a ring around the neck of the budding vesicle and hydrolyzes GTP to catalyze membrane fusion and scission.

5. The highly acidic interior of lysosomes (pH ~5.0) is maintained primarily by the action of: A) Na+/H+ exchangers B) V-type H+ ATPases C) F-type ATP synthases D) Passive proton channels

Ans: B. V-type (Vacuolar) ATPases pump protons into the lysosomal lumen against their concentration gradient using ATP hydrolysis.

6. The TIM23 complex is found in the: A) Outer mitochondrial membrane B) Inner mitochondrial membrane C) Outer chloroplast membrane D) Nuclear envelope

Ans: B. The Translocase of the Inner Mitochondrial membrane (TIM23) mediates the import of proteins into the mitochondrial matrix.

7. Peroxisomes are the primary cellular sites for the: A) Synthesis of most cellular ATP B) Glycosylation of secretory proteins C) Beta-oxidation of very-long-chain fatty acids D) Degradation of ubiquitin-tagged proteins

Ans: C. Very-long-chain fatty acids (VLCFAs) are initially shortened in peroxisomes before completing beta-oxidation in the mitochondria.

8. In nuclear import, cargo proteins bearing an NLS bind to importin in the cytoplasm. The cargo is released inside the nucleus when importin binds to: A) Ran-GDP B) Ran-GTP C) Exportin D) The nuclear pore complex (NPC)

Ans: B. High concentrations of Ran-GTP in the nucleus bind to importin, causing a conformational change that releases the cargo.

9. "Dynamic instability" of microtubules is highly dependent on the hydrolysis of: A) ATP bound to alpha-tubulin B) ATP bound to beta-tubulin C) GTP bound to alpha-tubulin D) GTP bound to beta-tubulin

Ans: D. The GTP on beta-tubulin is hydrolyzable. Loss of the "GTP cap" leads to rapid depolymerization (catastrophe).

10. Intermediate filaments found specifically in epithelial cells that provide mechanical strength are composed of: A) Vimentin B) Keratin C) Desmin D) Lamins

Ans: B. Keratins are the characteristic intermediate filaments of epithelial cells, attaching to desmosomes and hemidesmosomes.

11. In the cell cycle, the kinase Wee1 acts to: A) Activate M-Cdk by removing inhibitory phosphates B) Inhibit M-Cdk by adding inhibitory phosphates C) Phosphorylate and degrade Cyclin B D) Ubiquitinate securin

Ans: B. Wee1 phosphorylates CDK1 (M-Cdk) at Tyr15, keeping it inactive until the cell is ready to enter mitosis (counteracted by Cdc25).

12. The tumor suppressor p53 arrests the cell cycle in response to DNA damage mainly by inducing the transcription of: A) Cyclin D B) p21 (CIP/KIP family) C) Cdc25 D) E2F

Ans: B. p21 is a CDK inhibitor (CKI) that binds to and inactivates G1/S-Cdk and S-Cdk complexes, halting cell cycle progression.

13. During anaphase, the Anaphase-Promoting Complex (APC/C) is activated by binding to: A) Cyclin B B) Cdc20 C) Mad2 D) Separase

Ans: B. Binding of Cdc20 activates APC/C, allowing it to ubiquitinate securin and trigger chromosome separation.

14. Receptor Tyrosine Kinases (RTKs) are activated upon ligand binding via: A) Cleavage of the extracellular domain B) Receptor dimerization and trans-autophosphorylation C) Binding of G-proteins D) Internalization into clathrin-coated pits

Ans: B. Ligand binding induces receptor dimerization, bringing the kinase domains close enough to phosphorylate each other on tyrosine residues.

15. In the canonical Wnt signaling pathway, the stabilization and nuclear translocation of which protein activates gene transcription? A) GSK-3-beta B) Axin C) Beta-catenin D) APC (Adenomatous Polyposis Coli)

Ans: C. Wnt signaling inhibits the destruction complex, allowing beta-catenin to accumulate and translocate to the nucleus.

16. Which of the following members of the Bcl-2 protein family is strictly ANTI-apoptotic? A) Bax B) Bak C) Bcl-2 D) Bid

Ans: C. Bcl-2 prevents apoptosis by inhibiting pro-apoptotic proteins like Bax and Bak, keeping the mitochondrial membrane intact.

17. Vitamin C deficiency causes scurvy primarily due to its role as a cofactor in the: A) Polymerization of actin filaments B) Hydroxylation of proline and lysine residues in collagen C) Glycosylation of fibronectin D) Cross-linking of elastin

Ans: B. Hydroxylated proline stabilizes the collagen triple helix via hydrogen bonds; the enzyme prolyl hydroxylase requires Vitamin C.

18. Tight junctions, which prevent the leakage of solutes between epithelial cells, are primarily formed by: A) Cadherins B) Integrins C) Claudins and Occludins D) Connexins

Ans: C. Claudins and occludins form the sealing strands of tight junctions.

19. Cadherin-mediated cell-cell adhesion is highly dependent on the presence of extracellular: A) Magnesium ions (Mg2+) B) Calcium ions (Ca2+) C) Potassium ions (K+) D) Sodium ions (Na+)

Ans: B. The name "Cadherin" derives from "Calcium-dependent adhesion". Ca2+ binds to hinge regions to rigidify the extracellular domains.

20. The oncogenic mutation of the Ras protein most commonly results in it being: A) Unable to bind GTP B) Locked in a permanently GDP-bound state C) Unable to hydrolyze GTP, remaining permanently active D) Secreted into the extracellular matrix

Ans: C. Mutations often destroy Ras's intrinsic GTPase activity, locking it in the "on" state and driving continuous cell proliferation.

21. In GPCR desensitization, GRK (G-protein-coupled receptor kinase) phosphorylates the active receptor, which then recruits: A) Beta-arrestin B) Adenylyl cyclase C) Calmodulin D) RGS proteins

Ans: A. Arrestin binding prevents the receptor from interacting with G-proteins and also serves as an adaptor for clathrin-mediated endocytosis.

22. Which cell organelle is characterized by the presence of a double membrane, porins, and its own circular DNA? A) Endoplasmic Reticulum B) Peroxisome C) Golgi Apparatus D) Mitochondrion

Ans: D. Mitochondria (and chloroplasts) have these features, supporting the endosymbiotic theory of their origin.

23. In the context of cell signaling, Nitric Oxide (NO) exerts its physiological effect (e.g., smooth muscle relaxation) by directly binding to and activating: A) Protein Kinase G (PKG) B) Adenylyl cyclase C) Soluble Guanylyl Cyclase D) Endothelial Nitric Oxide Synthase (eNOS)

Ans: C. NO diffuses into target cells and binds the heme group of soluble guanylyl cyclase, generating cGMP from GTP.

24. The nucleolus is the primary site of: A) mRNA splicing B) Ribosome biogenesis and rRNA transcription C) Nuclear envelope assembly D) DNA replication

Ans: B. The nucleolus organizes around tandem repeats of rDNA, transcribing rRNA and assembling ribosomal subunits.

25. Proteins destined for degradation by the proteasome are tagged with ubiquitin on which specific amino acid residue? A) Serine B) Threonine C) Lysine D) Tyrosine

Ans: C. Ubiquitin is covalently attached via an isopeptide bond to the epsilon-amino group of lysine residues on the target protein.