lecture 20 chapter 17

Question 1

Why does increasing ionic strength (e.g., adding salts like Mg²⁺) significantly enhance microtubule polymerization?;A) It provides energy for polymerization;B) It shields negative charges on tubulin, promoting dimer assembly;C) It hydrolyzes GTP bound to tubulin;D) It cross-links microtubules

Answer 1

B

Question 2

What would likely happen if actin monomers failed to hydrolyze ATP after incorporation into filaments?;A) Rapid depolymerization of filaments;B) Stabilization of filaments without dynamic turnover;C) Accelerated treadmilling;D) Continuous spontaneous disassembly

Answer 2

B

Question 3

If a mutation blocked ARP complex function, which cellular activity would most likely be impaired?;A) Stabilization of intermediate filaments;B) Vesicle transport along microtubules;C) Formation of branched actin networks for cell motility;D) Polymerization of microtubules

Answer 3

C

Question 4

What explains the rapid cellular movement of neutrophils toward bacteria?;A) Stable actin bundles push evenly against the cell membrane;B) Dynamic assembly and disassembly of cortical actin at the leading edge;C) Continuous elongation of microtubules;D) Increased formation of intermediate filaments

Answer 4

B

Question 5

What triggers myosin-II heads to dissociate from actin filaments during muscle contraction cycles?;A) Binding of Ca²⁺ ions;B) Hydrolysis of ATP;C) Binding of ATP;D) Release of inorganic phosphate (Pi)

Answer 5

C

Question 6

During muscle contraction, actin and myosin filaments shorten.;A) True;B) False

Answer 6

B

Question 7

Which scenario would prevent muscle relaxation after contraction?;A) Continuous ATP availability;B) Failure to pump calcium back into the SR;C) Mutation reducing myosin head affinity for actin;D) Rapid tropomyosin movement

Answer 7

B

Question 8

Why must tropomyosin shift its position during muscle contraction?;A) To prevent filament sliding;B) To expose binding sites for myosin on actin;C) To bind directly to calcium;D) To hydrolyze ATP

Answer 8

B

Question 9

What would happen if T-tubules failed to transmit an action potential deep into muscle fibers?;A) Increased muscle contraction force;B) No calcium release from the sarcoplasmic reticulum;C) Continuous muscle contraction;D) Immediate breakdown of actin filaments

Answer 9

B

Question 10

How does the ARP complex facilitate bacterial pathogens like Listeria movement inside host cells?;A) Stabilizing microtubules;B) Inducing actin polymerization to propel bacteria;C) Forming contractile actin rings;D) Binding directly to intermediate filaments

Answer 10

B

Question 11

What ensures rapid contraction of muscle cells throughout the entire muscle simultaneously?;A) Slow ATP hydrolysis by myosin;B) Rapid, simultaneous release of Ca²⁺ via T-tubules;C) Gradual release of neurotransmitter;D) Slow diffusion of calcium through the cytosol

Answer 11

B

Question 12

If ATP hydrolysis by myosin-II was inhibited, what step of the muscle contraction cycle would be most directly impacted?;A) Initial myosin-actin attachment;B) Myosin head detachment from actin;C) Cocking of the myosin head;D) Calcium release from SR

Answer 12

C

Question 13

Which statement correctly compares kinesin and myosin motors?;A) Both maintain constant filament attachment during walking;B) Myosin maintains attachment continuously, kinesin does not;C) Kinesin maintains continuous filament attachment, myosin does not;D) Neither maintains continuous attachment

Answer 13

C

Question 14

Which mutation would selectively disrupt dynein but not kinesin motor activity?;A) Mutation in heavy chains forming microtubule-binding domains;B) Mutation in actin-binding domains;C) Mutation increasing ATP binding affinity;D) Mutation stabilizing actin filaments

Answer 14

A

Question 15

If actin treadmilling was experimentally accelerated, what cellular effect would you observe?;A) Reduced cell motility;B) Stabilized cortical structures;C) Increased protrusion at the leading edge;D) Decreased actin depolymerization

Answer 15

C

Question 16

What effect does myosin-II filament bipolarity have on its function?;A) Enables transport of vesicles in a single direction;B) Allows sliding of actin filaments past each other;C) Increases affinity for microtubules;D) Reduces ATPase activity

Answer 16

B

Question 17

Loss of tropomyosin from actin filaments would result in;A) Permanent muscle relaxation;B) Constantly exposed myosin-binding sites;C) Continuous calcium release;D) Increased ATP hydrolysis

Answer 17

B

Question 18

What is the direct consequence of calcium re-uptake by the sarcoplasmic reticulum after contraction?;A) Continued muscle contraction;B) Relaxation due to myosin dissociation from actin;C) Strengthening of actin filaments;D) Increased ATP availability

Answer 18

B

Question 19

What would happen if kinesins lost their ATP hydrolysis activity?;A) Enhanced vesicle transport;B) Permanent binding to microtubules without movement;C) Increased dynamic instability;D) Faster depolymerization of microtubules

Answer 19

B

Question 20

What distinguishes dyneins structurally from kinesins?;A) Directionality of movement;B) Number and arrangement of heavy chains;C) Affinity for actin;D) Requirement for calcium binding

Answer 20

B

Question 21

What cellular function would be directly impaired if the cortical actin network failed to disassemble at the trailing edge during cell crawling?;A) Enhanced speed of cell migration;B) Reduced directional movement;C) Increased membrane fluidity;D) Improved filament stability

Answer 21

B

Question 22

If a mutation caused actin monomers to permanently bind ATP without hydrolysis, how would actin filament stability change?;A) Filaments would rapidly depolymerize;B) Filaments would stabilize excessively, reducing turnover;C) Filaments would increase in dynamic instability;D) Filaments would become rigid and straight

Answer 22

B

Question 23

What molecular event directly allows tropomyosin to expose myosin-binding sites on actin during muscle contraction?;A) Binding of ATP to myosin heads;B) Binding of Ca²⁺ to troponin;C) ATP hydrolysis by actin filaments;D) Phosphorylation of myosin-II

Answer 23

B

Question 24

If intracellular calcium remained elevated after muscle contraction, what immediate effect would occur?;A) Muscle fibers remain relaxed;B) Troponin and tropomyosin stay displaced, prolonging contraction;C) Myosin heads dissociate from actin;D) Actin filaments depolymerize rapidly

Answer 24

B

Question 25

What explains why muscle contraction strength increases with higher frequency neuronal stimulation?;A) Increased ATP synthesis;B) Persistent elevation of cytosolic Ca²⁺;C) Enhanced actin polymerization;D) Increased tropomyosin rigidity

Answer 25

B

Question 26

If a drug selectively blocked the ARP2/3 complex in cells, what direct outcome would you observe during infection by Listeria monocytogenes?;A) Increased bacterial replication rate;B) Reduced bacterial motility inside host cells;C) Enhanced actin filament polymerization;D) Increased stability of microtubules

Answer 26

B

Question 27

Which characteristic distinguishes the walking mechanisms of kinesins from myosins on their respective filaments?;A) Both spend equal time detached from filaments;B) Kinesins remain continuously attached to their filament during movement, whereas myosins transiently detach;C) Myosins remain continuously attached, kinesins detach frequently;D) Both motors detach frequently during movement

Answer 27

B

Question 28

What functional advantage does myosin-II's bipolar filament arrangement provide during muscle contraction?;A) Allows unidirectional movement of actin;B) Enables actin filaments to slide past each other toward the sarcomere center;C) Stabilizes microtubules;D) Reduces ATP consumption

Answer 28

B

Question 29

During cell migration, the leading-edge protrusions are primarily driven by which actin filament behavior?;A) Permanent stabilization;B) Rapid polymerization at the front;C) Enhanced depolymerization at the front;D) Increased intermediate filament assembly

Answer 29

B

Question 30

If tropomyosin permanently occupied the myosin-binding sites on actin, muscle fibers would;A) Contract continuously;B) Remain unable to contract;C) Rapidly shorten;D) Hydrolyze ATP more efficiently

Answer 30

B

Question 31

Which protein interaction directly allows bacterial pathogens such as Shigella to move intracellularly?;A) Direct binding of actin monomers to bacteria;B) Recruitment of ARP2/3 via host N-WASP proteins;C) Activation of myosin-II;D) Binding to intermediate filaments

Answer 31

B

Question 32

What would happen if calcium pumps in the sarcoplasmic reticulum were inhibited?;A) Permanent muscle relaxation;B) Sustained muscle contraction due to elevated cytosolic calcium;C) Decreased myosin ATPase activity;D) Reduced filament overlap

Answer 32

B

Question 33

What would most directly cause increased treadmilling of actin filaments at the leading edge of a crawling cell?;A) Decreased ATP-actin monomer availability;B) Increased ATP-bound actin monomer availability;C) Stabilization by tropomyosin;D) Increased presence of intermediate filaments

Answer 33

B

Question 34

Why does actin treadmilling maintain filament length constant during cell crawling?;A) Filaments grow equally at both ends;B) Filaments grow at the plus end and lose monomers at the minus end at similar rates;C) Filaments polymerize rapidly without depolymerization;D) Filaments disassemble rapidly at both ends

Answer 34

B

Question 35

What immediate consequence would occur if voltage-gated calcium channels on the T-tubules failed to open during a muscle action potential?;A) Increased muscle contraction strength;B) No calcium release from the SR, preventing muscle contraction;C) Continuous ATP hydrolysis;D) Tropomyosin removal from actin

Answer 35

B

Question 36

What structural feature allows myosin heads to 'walk' along actin filaments during contraction?;A) Continuous ATP binding;B) Cyclical ATP binding, hydrolysis, and ADP release causing conformational changes;C) Permanent calcium binding;D) Rigid actin binding domain

Answer 36

B

Question 37

If dynein motor proteins were inhibited, what type of intracellular transport would be most directly affected?;A) Transport toward microtubule plus ends;B) Transport toward microtubule minus ends;C) Transport along actin filaments;D) Intermediate filament organization

Answer 37

B

Question 38

Why is rapid actin filament disassembly essential at the rear of a migrating cell?;A) To strengthen the rear cortex;B) To allow membrane retraction and forward cell movement;C) To stabilize intermediate filaments;D) To increase cell attachment

Answer 38

B

Question 39

What immediate effect would occur if kinesin motors permanently lost affinity for ATP?;A) Faster vesicle transport along microtubules;B) Kinesins remain attached but immobilized on microtubules;C) Increased dynamic instability of microtubules;D) Permanent detachment from microtubules

Answer 39

B

Question 40

Which protein's structural change directly exposes actin binding sites for myosin during muscle contraction initiation?;A) Myosin-II;B) Tropomyosin regulated by troponin binding calcium;C) ARP2/3 complex;D) Kinesin

Answer 40

B