lecture 20 Flashcards
Salt is suddenly removed from a tubulin‑polymerizing assay. Which outcome best matches the mechanism shown on slide3?;A) Increased lateral bonding;B) Repulsion between dimers rises, slowing polymerization;C) GTP hydrolysis stops;D) MAPs spontaneously bind
B– Slide3: Without cation shielding, negative charges repel and polymerization slows.
On slide6 treadmilling is depicted. If ATP‑actin monomer concentration doubled while ADP‑actin disassembly stayed constant, which change occurs first?;A) Plus‑end growth accelerates;B) Minus‑end shrinkage accelerates;C) Filament shortens overall;D) Treadmilling stops
A– Slide6: More ATP‑actin boosts plus‑end addition before other rates change.
A cytochalasin cap is placed on the plus end after treadmilling has reached steady‑state (slide6). Predict filament behavior.;A) Net growth continues;B) Net shrinkage from minus end;C) Catastrophe at both ends;D) Length remains constant
B– Slide6: Plus‑end addition blocked; minus‑end loss continues so filament shortens.
Slide8 shows pathogens recruiting ARP2/3. If ActA is deleted from Listeria, what is the immediate effect?;A) Microtubule‑based motility increases;B) Actin comet‑tail fails, reducing intracellular propulsion;C) ARP complex nucleates faster;D) Host myosin‑II drags bacteria
B– Slide8: ActA is the ARP2/3 activator; without it the actin tail cannot form.
During neutrophil chase (slide12), which actin‑related event enables rapid directional change?;A) Stabilization of existing filaments;B) Localized disassembly of rear cortex and new polymerization at front;C) Global microtubule reorientation;D) Myosin‑I vesicle transport
B– Slide12: Cell turns by dismantling rear cortex and building new front actin.
Slide9 depicts actin cortex vs. stress fibers. Destroying cross‑linking proteins in the cortex first affects which protrusion type?;A) Stress fiber contraction;B) Lamellipodium expansion;C) Sarcomere shortening;D) Filopodium length
B– Slide9: Lamellipodia rely on cross‑linked cortical networks; loss shrinks them.
According to slide11, high capping‑protein activity at the leading edge would primarily;A) Extend filopodia spikes;B) Shorten lamellipodia breadth;C) Increase myosin‑II filaments;D) Stabilize microtubules
B– Slide11: Excess capping terminates branch elongation, narrowing lamellipodia.
Slide14 lists Myosin‑I and Myosin‑II. Which feature explains why Myosin‑II, not Myosin‑I, forms bipolar contractile filaments?;A) Presence of long tail that self‑associates;B) Plus‑end minus‑end orientation;C) Higher ATPase rate;D) Calcium‑binding light chains
A– Slides14‑15: Myosin‑II tails zipper into bipolar filaments for contraction.
A mutation prevents Myosin‑II tails from zippering (slide15). What cellular contractile process fails first?;A) Sarcomere sliding;B) Vesicle tethering;C) Kinesin cargo movement;D) ARP nucleation
A– Slides15/18: No thick filament → sarcomeres cannot power sliding.
Slide16 shows a bipolar filament sliding two opposite‑polarity actin filaments. If both actin filaments were oriented with plus ends on the same side, the myosin filament would:;A) Generate no net sliding;B) Slide both in same direction;C) Depolymerize filaments;D) Reverse its motor polarity
A– Slide16: Opposite polarity is required; identical polarity cancels movement.
On slide18, plus ends of thin filaments anchor at Z‑discs. Which experimental severing would immediately shorten sarcomere length?;A) Cut at Z‑disc;B) Cut in middle of myosin thick filament;C) Cut near plus ends of actin;D) Cut microtubules in cytoplasm
B– Slide18: Severing thick filament releases tension, so thin filaments retract.
Slide19 explains simultaneous sarcomere activation. If Ca²⁺ release channels opened sequentially down the fiber instead of simultaneously, contraction would be:;A) Stronger and faster;B) Peristaltic and slower;C) Unaffected;D) Prevented entirely
B– Slide19: Desynchronized Ca²⁺ waves yield slow peristaltic contraction.
Referencing slides20‑25, which step is blocked first when ATP analog that cannot be hydrolyzed binds myosin?;A) Detachment from actin;B) Cocking of lever arm;C) Release of Pi during power stroke;D) Re‑attachment in rigor
B– Slide22: Hydrolysis drives lever‑arm cocking; no hydrolysis blocks this step.
Slide26 shows voltage‑gated Ca²⁺ channels. If extracellular Ca²⁺ were chelated, the cytosolic rise would:;A) Persist via SR release;B) Fail because SR channels require external Ca²⁺ trigger;C) Increase two‑fold;D) Close T‑tubules
A– Slides26‑29: Mechanical link lets SR release Ca²⁺ even without external Ca²⁺.
Based on slide28, a drug that destroys T‑tubules but leaves SR intact causes:;A) Normal Ca²⁺ release;B) No depolarization reaches SR, so contraction fails;C) Continuous Ca²⁺ leak;D) Hyper‑contractility
B– Slide28: T‑tubules carry depolarization; without them SR stays closed.
Slide30 details tropomyosin control. If troponin lost Ca²⁺‑binding, muscle would:;A) Contract spontaneously;B) Remain relaxed despite Ca²⁺ spikes;C) Hydrolyze ATP faster;D) Form longer sarcomeres
B– Slide30: Troponin can’t shift tropomyosin, blocking myosin binding.
Slide31 shows Ca²⁺ pumps returning levels to rest. Inhibiting these pumps prolongs contraction because;A) Tropomyosin blocks binding;B) Cytosolic Ca²⁺ stays high, troponin remains active;C) Myosin loses ADP;D) ATP synthesis stops
B– Slide31: Elevated Ca²⁺ keeps troponin active until pumps clear it.
Slide33 compares kinesin & dynein. A vesicle needing movement toward cell periphery would recruit primarily:;A) Dynein;B) Kinesin;C) Myosin‑V;D) ARP2/3
B– Slide33: Kinesin motors walk toward microtubule plus ends at cell edge.
Slide34 states kinesin keeps one head attached. If both heads detached simultaneously each step, expected cargo transport would:;A) Accelerate;B) Become less processive with frequent detachment;C) Reverse direction;D) Shift to minus end
B– Slide34: Losing constant attachment lowers processivity—frequent falls.
Slide35 shows dynactin anchoring dynein to cargo. Removing Arp1 filament first disrupts:;A) Dynein ATPase activity;B) Physical linkage of vesicle to motor;C) Microtubule binding domain;D) Kinesin recruitment
B– Slide35: Arp1 is the cargo‑side scaffold; without it vesicle cannot bind dynein.
From slide5, if ATP‑actin binding site mutated so only ADP binds, initial polymerization rate would:;A) Increase;B) Decline sharply due to reduced plus‑end addition;C) Remain equal;D) Cause filament branching
B– Slide5: ATP‑actin adds fastest at plus ends; ADP‑actin adds slowly.
Slide7 diagram lists severing proteins. Excess severing activity in cortex yields which migration phenotype?;A) Stiffer cortex;B) Rapid retraction and poor protrusion stability;C) Longer filopodia;D) Hyper‑stable lamellipodia
B– Slide7: Too much severing destabilizes protrusions, causing rapid retraction.
Slide10 sequence shows tension‑contraction‑attachment. Blocking integrin focal contacts specifically impairs which step?;A) Protrusion;B) Attachment that anchors traction;C) Myosin contraction;D) Actin depolymerization
B– Slide10: Focal adhesion attachment anchors the traction force.
Slide3 table shows in vivo shrinkage ~20µm/min vs. growth 1–10µm/min. Which factor explains faster shrinkage?;A) ATP abundance;B) GTP‑cap loss exposes GDP lattice that peels rapidly;C) MAP stabilization;D) Tau spacing
B– Slide3: Loss of GTP cap destabilizes GDP lattice → rapid peeling.
