lecture-19 chapter-17 molecular bio Flashcards
A keratinocyte expressing a dominant‑negative keratin mutant experiences minor friction. Which immediate cellular outcome is most likely?;A) Myosin‑II detaches from actin;B) Epidermal cell ruptures forming a blister;C) MAP2 lattices collapse;D) Microtubules undergo catastrophe
B
A scientist adds pre‑formed γ‑tubulin ring complexes to cytosol lacking centrosomes. What happens to microtubule organization?;A) Plus ends point randomly;B) Minus ends nucleate from new MTOC‑like foci;C) Dynamic instability ceases;D) MAP binding is prevented
B
Loss of Tau spacing but retention of MAP2 cross‑linking in a neuron would most directly affect which property of microtubule bundles?;A) Lattice regularity;B) Inter‑filament distance;C) Minus‑end stability;D) GTP‑cap size
B
Phalloidin is applied to motile fibroblasts. Predict the effect on lamellipodia dynamics.;A) Enhanced protrusion and retraction cycles;B) Frozen actin filaments preventing edge advancement;C) Increased treadmilling rate;D) Rapid depolymerization of actin cortex
B
A centrosome is surgically removed from a cultured animal cell fragment (nucleus remains). After 4 h, what arrangement of microtubules is expected?;A) No microtubules present;B) A star of plus‑ends radiating from a new acentriolar MTOC;C) Parallel cortical bundles only;D) Random short fragments with no polarity
B
Why does adding salt (Mg²⁺/K⁺) to purified tubulin accelerate polymerization?;A) Provides ATP for energy;B) Shields repulsive negative charges, lowering nucleation lag;C) Phosphorylates β‑tubulin;D) Creates new MAP‑binding sites
B
Intermediate filaments lack polarity. Which assembly feature explains their superior tensile strength?;A) Head‑to‑tail parallel dimers;B) Staggered antiparallel tetramers eliminating free ends;C) GTP‑cap at both ends;D) Double helical protofilaments
B
A researcher fluorescently marks a tubulin dimer halfway along a microtubule undergoing treadmilling. After 10 min the mark is near the minus‑end. What does this demonstrate?;A) Microtubule sliding;B) Net addition at plus‑end and loss at minus‑end;C) Capping proteins removed;D) GTP hydrolysis absent
B
A cell treated with low doses of vinblastine shows frequent rescue events but rare catastrophes. Which protein is likely over‑expressed to compensate?;A) Catastrophin;B) MAP2;C) Tau;D) γ‑tubulin
B
In the lag phase of actin assembly, what rate‑limiting step predominates?;A) ATP hydrolysis on existing filaments;B) Nucleation of actin oligomers reaching critical size;C) Addition of monomers to plus‑end;D) Treadmilling
B
A fungal cell lacking intermediate filaments relies on actin and microtubules only. Under shear stress it ruptures more easily than a mammalian epithelial cell. Which property is most directly missing?;A) Dynamic instability;B) Rope‑like flexible networks distributing force;C) Motor‑driven transport;D) GTP‑dependent polymerization
B
Dynamic instability depends on the GTP‑cap. Which experimental observation best confirms this?;A) Microtubule shrinkage begins immediately after β‑tubulin GTP hydrolysis;B) Actin filaments depolymerize when ADP bound;C) Intermediate filaments lack nucleotides;D) Tau binding increases catastrophe
A
A neurite displays tightly packed microtubules with short spacing loops. Which accessory protein profile fits best?;A) High MAP2, low Tau;B) Low MAP2, high Tau;C) High catastrophin;D) High γ‑tubulin
B
Knocking out γ‑tubulin in plant cells (no centrioles) primarily disrupts which step?;A) Plus‑end rescue;B) Minus‑end nucleation at the nuclear envelope MTOC;C) Actin treadmilling;D) Keratin dimer formation
B
Which scenario illustrates dynamic instability rather than treadmilling?;A) A filament’s plus‑end drifts forward while minus‑end retracts at equal speed;B) Sudden rapid shortening of a previously growing plus‑end;C) Steady‑state filament length;D) Monomer addition equals disassembly
B
In a keratinocyte, plectin mutation leads to skin blistering AND muscular dystrophy. What shared mechanism explains both symptoms?;A) Actin nucleation defect;B) Failure to link intermediate filaments to other cytoskeletal systems;C) Excess Tau spacing;D) Permanent GTP‑caps
B
In cells expressing only α‑tubulin‑GDP (no GTP), microtubules rarely form. Which principle does this highlight?;A) Hydrolysis energy drives nucleation;B) GTP‑bound subunits stabilize lateral interactions needed for polymerization;C) GDP accelerates plus‑end growth;D) Catastrophe suppressed
B
Which toxin would best test whether growth at both ends of actin filaments contributes to motility?;A) Cytochalasin (caps plus‑ends);B) Latrunculin (sequesters monomers);C) Phalloidin (stabilizes filaments);D) Taxol (targets microtubules)
A
A neuronal MAP2 gene is replaced with a shorter‑loop Tau construct. Predict microtubule arrangement in dendrites.;A) Lattice widens;B) Bundles pack tighter, altering cargo routes;C) Dynamic instability stops;D) Intermediate filaments collapse
B
Why are intermediate‑filament defects often tissue‑specific (e.g., skin vs. neurons)?;A) Same IF proteins in all cells;B) IF gene families are expressed in distinct cell types providing tailored mechanical support;C) IFs hydrolyze ATP differently;D) MAP proteins compensate universally
B
Dynamic instability supplies subunits for remodeling elsewhere in the cell.
t
Loss of the GTP‑cap invariably initiates microtubule catastrophe.
t
MAP2 and Tau produce identical inter‑microtubule spacing in axons.
f
Treadmilling requires net ATP hydrolysis on actin monomers.
t
