Mock Test 3

Cell Biology Mock Test 03 (Advanced / Part-C Level)

120:00

1. Following membrane fusion, the SNARE complex must be disassembled for vesicle recycling. This highly energetically unfavorable process is catalyzed by: A) Rab-GTPases B) NSF and alpha-SNAP using ATP hydrolysis C) Dynamin using GTP hydrolysis D) Clathrin uncoating ATPase (Hsc70)

Ans: B. N-ethylmaleimide-sensitive factor (NSF) is a hexameric AAA-ATPase that, along with its adaptor alpha-SNAP, unravels the tightly coiled SNARE bundles.

2. In the Calnexin/Calreticulin cycle for ER protein quality control, which enzyme serves as the folding sensor by re-glycosylating improperly folded proteins? A) Glucosidase II B) ER Mannosidase I C) UDP-glucose:glycoprotein glucosyltransferase (UGGT) D) Protein disulfide isomerase (PDI)

Ans: C. UGGT recognizes exposed hydrophobic patches on misfolded proteins and adds a single terminal glucose, sending the protein back to Calnexin for another folding attempt.

3. Maturation of early endosomes into late endosomes involves a "Rab cascade." This transition is typically characterized by the replacement of: A) Rab7 by Rab5 B) Rab5 by Rab7 C) Rab1 by Rab11 D) Rab11 by Rab4

Ans: B. Rab5 (marker for early endosomes) recruits the GEF for Rab7. Active Rab7 then recruits a GAP for Rab5, shutting off Rab5 and converting the organelle to a late endosome.

4. During co-translational translocation, the Signal Recognition Particle (SRP) binds to both the signal sequence and the ribosome. SRP release from the SRP receptor requires: A) ATP hydrolysis by the ribosome B) GTP hydrolysis by both SRP and the SRP receptor C) Cleavage of the signal peptide by signal peptidase D) Binding of BiP to the translocating nascent chain

Ans: B. Both SRP and its receptor possess GTPase activity. Their mutual interaction stimulates simultaneous GTP hydrolysis, triggering SRP release.

5. Which of the following mitochondrial protein import complexes is specifically required for the insertion of multipass integral membrane proteins (e.g., metabolite carriers) into the inner mitochondrial membrane? A) TOM complex B) TIM23 complex C) TIM22 complex D) SAM complex

Ans: C. TIM22 specializes in integrating multipass proteins into the inner membrane, whereas TIM23 primarily translocates proteins into the matrix or inserts single-pass proteins.

6. The transport of proteins into the thylakoid lumen of chloroplasts can occur via the Tat (Twin-arginine translocation) pathway. A unique feature of the Tat pathway compared to the Sec pathway is that it: A) Requires ATP hydrolysis for translocation B) Transports fully folded proteins across the membrane C) Depends entirely on a GTP gradient D) Uses the SRP for targeting

Ans: B. The Tat pathway uses the thylakoid pH gradient (PMF) to translocate fully folded, complex proteins (often containing redox cofactors), unlike the Sec pathway which translocates unfolded proteins.

7. In the actin cytoskeleton, formins and the Arp2/3 complex serve different nucleating functions. Formins primarily nucleate: A) Branched actin networks by binding to the minus end B) Unbranched, linear actin filaments by remaining associated with the growing plus end C) Microtubules at the centrosome D) Intermediate filament assembly

Ans: B. Formins generate long, unbranched actin filaments (e.g., in filopodia and stress fibers) and walk along the elongating barbed (+ve) end, whereas Arp2/3 generates branched networks.

8. The mechanochemical cycle of Kinesin-1 dictates that the leading motor head tightly binds to the microtubule when it is: A) Bound to ADP B) Nucleotide-free C) Bound to ATP D) Phosphorylated by a generic kinase

Ans: C. In kinesin, ATP binding to the leading head induces a conformational change (neck linker zippering) that throws the trailing head forward. ATP-bound kinesin has high affinity for microtubules.

9. The Anaphase-Promoting Complex (APC/C) changes its specificity during the cell cycle by associating with different co-activators. During late mitosis and early G1, APC/C associates with: A) Cdc20 B) Cdh1 C) Sic1 D) p27

Ans: B. APC/C-Cdc20 operates at the metaphase-to-anaphase transition. As Cdk levels fall, Cdh1 becomes dephosphorylated, binds APC/C, and maintains low cyclin levels through G1.

10. The SCF ubiquitin ligase complex is responsible for the G1/S transition. How does SCF recognize its targets (e.g., CKIs like p27)? A) The target must be bound to a microtubule B) The target must be unphosphorylated C) The target must be specifically phosphorylated by G1/S-Cdks D) The target must contain a KDEL sequence

Ans: C. The F-box protein of the SCF complex only recognizes its substrates (like p27 or Sic1) after they have been phosphorylated by specific kinases, linking protein degradation to signaling pathways.

11. In the DNA damage response, double-strand breaks activate the ATM kinase. A key downstream effector of ATM that halts the cell cycle is: A) Chk2, which phosphorylates and inactivates Cdc25 B) Chk1, which ubiquitinates p53 C) Aurora B kinase, which detaches microtubules D) Mdm2, which stabilizes p53

Ans: A. ATM phosphorylates Chk2. Chk2 phosphorylates Cdc25 phosphatase, leading to its export or degradation, thus preventing the removal of inhibitory phosphates from Cdks and halting the cell cycle.

12. GPCR desensitization prevents overstimulation. Beta-arrestin binding to a phosphorylated GPCR not only blocks G-protein interaction but also serves as a scaffold to activate: A) The Wnt pathway B) MAP Kinase (ERK) signaling pathways C) The intrinsic apoptotic cascade D) Adenylyl cyclase independently of Gs

Ans: B. Beta-arrestin acts as a signaling scaffold that recruits components of the MAPK cascade (like Raf, MEK, ERK), initiating a G-protein-independent signaling wave often localized to endosomes.

13. In RTK signaling, the Grb2 protein acts as an adaptor. Its SH2 domain binds to phosphotyrosine on the receptor, while its SH3 domains bind to: A) Proline-rich sequences on SOS (a Ras-GEF) B) Phosphoserine residues on Raf C) Inositol phospholipids (PIP3) at the membrane D) The kinase domain of MEK

Ans: A. SH3 domains recognize proline-rich motifs (PXXP). Grb2 uses its SH3 domains to recruit SOS to the plasma membrane, where it can activate Ras.

14. The PTEN tumor suppressor antagonizes the PI3K/Akt pathway. Biochemically, PTEN functions as a: A) Tyrosine kinase inhibitor B) Serine/threonine phosphatase C) Lipid phosphatase that converts PIP3 to PIP2 D) GTPase activating protein (GAP) for Rheb

Ans: C. PTEN removes the 3-phosphate from PI(3,4,5)P3, reverting it to PI(4,5)P2 and thus preventing the PH-domain mediated recruitment of Akt to the membrane.

15. In the absence of a Wnt signal, beta-catenin is kept at low levels by the "destruction complex." Which kinase(s) within this complex phosphorylate beta-catenin to target it for ubiquitination? A) PKA and PKC B) GSK-3 and Casein Kinase 1 (CK1) C) PI3K and Akt D) JNK and p38

Ans: B. Beta-catenin is sequentially phosphorylated by CK1 and then GSK-3 while held in the scaffold of Axin and APC. This phosphorylation creates a degron recognized by the beta-TrCP ubiquitin ligase.

16. The Hedgehog signaling pathway utilizes primary cilia in vertebrates. Binding of Hedgehog to the Patched (Ptc) receptor relieves the inhibition of which multipass transmembrane protein? A) Frizzled B) Smoothened (Smo) C) Dishevelled (Dsh) D) Notch

Ans: B. Patched tonically inhibits Smoothened. Hedgehog binding to Patched internalizes Patched, allowing Smoothened to translocate to the primary cilium and activate the Gli transcription factors.

17. Notch signaling represents an extreme form of short-range communication (contact-dependent). The cleavage that releases the Notch Intracellular Domain (NICD) is catalyzed by: A) Caspase-3 B) Matrix metalloproteinases (MMPs) C) The gamma-secretase complex (Presenilin) D) ADAM10 exclusively

Ans: C. Following an extracellular cleavage by ADAM metalloproteases, the transmembrane domain of Notch is cleaved by the gamma-secretase complex (a process called Regulated Intramembrane Proteolysis).

18. During the induction of apoptosis, the BH3-only proteins (like Bid, Bim, Puma) promote cell death by: A) Directly cleaving executioner caspases B) Inhibiting anti-apoptotic Bcl-2 family members and/or directly activating Bax/Bak C) Forming pores in the plasma membrane D) Binding to and inhibiting the apoptosome

Ans: B. BH3-only proteins act as sensors of cellular stress. They bind to and neutralize anti-apoptotic protectors (like Bcl-2/Bcl-xL) and some can directly activate the pore-formers Bax and Bak.

19. In Extrinsic Apoptosis, the Death-Inducing Signaling Complex (DISC) relies on homotypic interaction domains to recruit Procaspase-8. Which domain mediates the binding between the death receptor (e.g., Fas) and the adaptor protein FADD? A) Death Effector Domain (DED) B) Death Domain (DD) C) CARD domain D) BIR domain

Ans: B. The Death Domain (DD) of the activated receptor binds to the DD of the FADD adaptor. FADD then uses its Death Effector Domain (DED) to recruit Procaspase-8.

20. Necroptosis is a form of programmed necrotic cell death that serves as a viral defense mechanism. It is executed by the oligomerization of which protein in the plasma membrane? A) Gasdermin D B) MLKL (Mixed Lineage Kinase Domain-like pseudokinase) C) RIPK1 D) Cytochrome c

Ans: B. In necroptosis, RIPK3 phosphorylates MLKL. Phospho-MLKL oligomerizes and translocates to the plasma membrane, forming pores that lead to cell lysis.

21. During macroautophagy, the expanding phagophore must be decorated with a ubiquitin-like protein to secure cargo and close the autophagosome. In mammalian cells, this protein is: A) ATG5 B) LC3 (ATG8 homolog) lipidated to phosphatidylethanolamine C) Beclin-1 D) mTORC1

Ans: B. LC3-I is conjugated to phosphatidylethanolamine (PE) to form LC3-II, which localizes to the autophagosomal membrane and is widely used as an autophagy marker.

22. Integrin "inside-out" signaling alters the extracellular conformation of integrins from a bent (inactive) to an extended (active) state. This is typically triggered by the binding of which intracellular protein to the integrin beta tail? A) Fibronectin B) FAK (Focal Adhesion Kinase) C) Talin D) Catenin

Ans: C. Talin (often in concert with Kindlin) binds to the cytoplasmic tail of the beta integrin subunit, separating the alpha and beta tails and inducing the active extracellular conformation.

23. Desmosomes provide strong mechanical attachments between adjacent epithelial cells. The non-classical cadherins that make up the desmosome core are: A) E-cadherin and N-cadherin B) Desmoglein and Desmocollin C) Claudin and Occludin D) Selectins and Mucin-like molecules

Ans: B. Desmogleins and desmocollins are the specialized cadherins found in desmosomes, linking to intermediate filaments via plaque proteins like plakoglobin and desmoplakin.

24. The establishment of apical-basal polarity in epithelial cells relies on tight junctions. Which protein complex is crucial for initially defining the apical domain and establishing the tight junction border? A) The Scribble complex B) The Par complex (Par3/Par6/aPKC) C) The WASH complex D) The Exocyst complex

Ans: B. The Par3/Par6/aPKC complex localizes apically and phosphorylates basolateral proteins (like the Lgl/Scribble/Dlg complex) to exclude them from the apical domain.

25. Hyaluronan (Hyaluronic Acid) is an atypical glycosaminoglycan (GAG) because it: A) Contains a core protein B) Is highly sulfated C) Is synthesized directly at the plasma membrane by a transmembrane synthase D) Is synthesized within the Golgi apparatus

Ans: C. Unlike other GAGs, hyaluronan contains no core protein, is not sulfated, and is synthesized by HA synthases at the plasma membrane, spinning directly into the extracellular space.

26. The "Warburg Effect" in cancer cells refers to their tendency to rely on aerobic glycolysis despite the presence of oxygen. A key molecular mechanism facilitating this is the expression of a specific splice variant of Pyruvate Kinase. Which variant? A) PKM1 B) PKM2 C) PKL D) PKR

Ans: B. Cancer cells highly express PKM2, which exists in a low-activity dimeric state. This creates a bottleneck in glycolysis, allowing upstream intermediates to be diverted for biomass synthesis (nucleotides, lipids).

27. According to Knudson's "Two-Hit Hypothesis" for tumor suppressor genes, an individual with familial retinoblastoma inherits: A) Two mutant alleles of the Rb gene B) One mutant allele, with the second allele frequently lost via Loss of Heterozygosity (LOH) C) A dominant gain-of-function mutation in the Rb gene D) A hyperactive E2F transcription factor

Ans: B. They inherit one defective copy (first hit). A somatic mutation or LOH provides the second hit, completely ablating Rb function and leading to tumors.

28. Most human cancers achieve replicative immortality by reactivating telomerase. What is the most common genetic mechanism for this reactivation in melanoma and glioblastoma? A) Amplification of the TERC RNA gene B) Alternative Lengthening of Telomeres (ALT) via homologous recombination C) Activating point mutations in the TERT gene promoter D) Missense mutations in the telomerase catalytic site

Ans: C. Mutations in the TERT promoter create new binding sites for transcription factors (like ETS), driving massive upregulation of the telomerase reverse transcriptase.

29. Cellular Senescence is an irreversible growth arrest often triggered by telomere attrition or oncogene activation. A hallmark biomarker specifically identifying senescent cells in tissues is: A) High levels of Ki-67 B) Senescence-Associated beta-Galactosidase (SA-β-gal) activity at pH 6.0 C) Cleaved Caspase-3 D) Increased phosphorylation of Rb

Ans: B. The expansion of the lysosomal compartment in senescent cells leads to detectable beta-galactosidase activity at a suboptimal pH of 6.0, serving as a classic marker.

30. When analyzing cell cycle distribution by flow cytometry using Propidium Iodide (PI) DNA staining, cells in the G2/M phase will display: A) Half the fluorescence intensity of G1 cells B) The exact same fluorescence intensity as S phase cells C) Exactly twice the fluorescence intensity of G1 cells (4n DNA content) D) No fluorescence, as PI only stains dead cells

Ans: C. For cell cycle analysis, cells are fixed and permeabilized. Since PI binds stoichiometrically to DNA, G2/M cells (having replicated their DNA) have 2x the PI signal of G1 cells.

31. In a Fluorescence Recovery After Photobleaching (FRAP) experiment on a transmembrane receptor, the final level of fluorescence recovery represents: A) The diffusion coefficient (D) B) The mobile fraction of the receptor C) The rate of endocytosis D) The half-life of the fluorophore

Ans: B. The plateau level (extent of recovery) indicates the percentage of the protein that is free to diffuse (mobile fraction). Proteins anchored to the cytoskeleton form the immobile fraction.

32. FRET (Förster Resonance Energy Transfer) is highly sensitive to the distance between the donor and acceptor fluorophores. The efficiency of energy transfer is inversely proportional to: A) The 2nd power of the distance (1/r²) B) The 4th power of the distance (1/r⁴) C) The 6th power of the distance (1/r⁶) D) The 8th power of the distance (1/r⁸)

Ans: C. FRET efficiency E = 1 / [1 + (r/R0)^6]. This stark 1/r^6 dependence makes it an exquisite "molecular ruler" for detecting protein-protein interactions within 1-10 nm.

33. When utilizing CRISPR-Cas9 to introduce a precise, sequence-specific mutation or tag into the genome, one must provide a repair template to exploit which DNA repair pathway? A) Non-Homologous End Joining (NHEJ) B) Homology-Directed Repair (HDR) C) Base Excision Repair (BER) D) Mismatch Repair (MMR)

Ans: B. While NHEJ creates random indels useful for knockouts, HDR uses a provided homologous template to introduce precise genetic modifications (Knock-in).

34. Lipid asymmetry in the plasma membrane is essential for function (e.g., exposing phosphatidylserine to signal apoptosis). The enzyme that moves specific phospholipids from the outer to the inner leaflet using ATP is called a: A) Floppase (ABC transporter family) B) Scramblase C) Flippase (P-type ATPase family) D) Phospholipase

Ans: C. Flippases move lipids "in" (towards the cytoplasm). Floppases move lipids "out". Scramblases move lipids bidirectionally down their gradients, typically independent of ATP but requiring calcium.

35. Which structural motif is exclusively found in transcription factors that coordinate a central Zinc ion to stabilize an alpha-helix for binding the major groove of DNA? A) Leucine Zipper B) Helix-Loop-Helix (HLH) C) Zinc Finger (e.g., C2H2 motif) D) Homeodomain

Ans: C. The C2H2 zinc finger utilizes two cysteines from a beta-sheet and two histidines from an alpha-helix to coordinate a zinc ion, structurally securing the domain for DNA recognition.

36. In vesicular transport, the coating protein utilized for trafficking from the Trans-Golgi Network (TGN) directly to lysosomes (via endosomes) utilizes an adaptor complex known as: A) AP-1 or AP-3 (with Clathrin) B) AP-2 (with Clathrin) C) COPI D) COPII

Ans: A. Clathrin vesicles from the TGN to endosomes use AP-1 (or GGAs), whereas AP-2 is exclusively used for clathrin-mediated endocytosis at the plasma membrane.

37. The phenomenon of "Treadmilling" vs. "Dynamic Instability" dictates cytoskeletal mechanics. Which statement correctly differentiates them? A) Treadmilling requires GTP; Dynamic instability requires ATP. B) Treadmilling involves growth at one end and loss at the other; Dynamic instability involves rapid switching between growth and shrinkage at the same end. C) Microtubules treadmill; Actin exhibits dynamic instability. D) Treadmilling only happens in vitro, not in vivo.

Ans: B. Actin filaments classically treadmill at steady state. Microtubules are the textbook example of dynamic instability driven by GTP cap loss at the plus end.

38. In the Smad signaling pathway downstream of TGF-beta, which Smad protein acts as the "Co-Smad" that forms a complex with receptor-regulated Smads before translocating to the nucleus? A) Smad2 B) Smad3 C) Smad4 D) Smad7

Ans: C. R-Smads (Smad2/3) are phosphorylated by the receptor, then complex with the Co-Smad (Smad4) to enter the nucleus and regulate transcription. Smad7 is an inhibitory Smad.

39. Imatinib (Gleevec) is a paradigm-shifting targeted cancer therapy. It acts as a competitive inhibitor of ATP binding to the kinase domain of which oncogenic fusion protein? A) HER2/neu B) BCR-ABL C) EML4-ALK D) BRAF V600E

Ans: B. Imatinib revolutionized the treatment of Chronic Myeloid Leukemia (CML) by specifically inhibiting the hyperactive BCR-ABL tyrosine kinase created by the Philadelphia chromosome translocation.

40. Optogenetics allows researchers to control cell behavior with light. The most commonly used tool, Channelrhodopsin-2 (ChR2), upon blue light illumination functions as a: A) Light-gated chloride pump causing hyperpolarization B) Light-gated proton pump C) Light-gated non-specific cation channel causing depolarization D) Light-activated adenylyl cyclase

Ans: C. ChR2 opens in response to blue light, allowing Na+ and Ca2+ to influx, which depolarizes the cell (commonly used to activate neurons). Halorhodopsin is the chloride pump used for hyperpolarization.