Lecture 20 - Cytoskeleton II (Ch. 17)

What is the approximate diameter of an actin filament (microfilament)?

A. 2 nm

B. 7 nm

C. 25 nm

D. 37 nm

B

Actin filaments are composed of monomers of which protein?

A. Tubulin

B. Keratin

C. Globular actin (G-actin)

D. Myosin

C

Which molecule is typically found in the deep cleft of an actin monomer?

A. GTP

B. ATP or ADP

C. Calcium

D. cAMP

B

To which end of the actin filament are monomers more likely to be added?

A. Minus end

B. Plus end

C. Both ends equally

D. The Z-disc end

B

What is the name of the process where an actin filament maintains a constant length while monomers are added at one end and lost at the other?

A. Dynamic instability

B. Catastrophe

C. Treadmilling

D. Nucleation

C

How many protofilaments typically make up an actin filament?

A. Two

B. Nine

C. Thirteen

D. Thirty-two

A

What event occurs shortly after an ATP-bound actin monomer is incorporated into a filament?

A. It is phosphorylated to become ATP-actin

B. ATP is hydrolyzed to ADP

C. It immediately dissociates from the plus end

D. It binds to a microtubule

B

Which of the following is a stable actin-based structure found in animal cells?

A. Lamellipodia

B. Microvilli

C. Contractile ring

D. Filopodia

B

Which drug binds to and stabilizes actin filaments, preventing their disassembly?

A. Cytochalasin

B. Latrunculin

C. Taxol

D. Phalloidin

D

What is the action of the drug Latrunculin on actin?

A. It caps the plus ends

B. It binds actin monomers and prevents their polymerization

C. It stabilizes the filament

D. It severs existing filaments

B

Which actin-binding protein is responsible for promoting the formation of branched actin networks in lamellipodia?

A. Formin

B. ARP complex

C. Tropomyosin

D. Myosin II

B

What is the function of monomer-sequestering proteins?

A. They promote rapid polymerization

B. They bind monomers to reduce the chance of spontaneous polymerization

C. They stabilize the minus end of the filament

D. They hydrolyze ATP into ADP

B

Which protein is often found in filopodia to help organize actin into parallel bundles?

A. Nucleating protein

B. Bundling protein

C. Severing protein

D. Capping protein

B

Approximately what percentage of total protein in animal cells is represented by actin?

A. 1%

B. 5%

C. 20%

D. 50%

B

Which structure is a thin, sheet-like projection used by a crawling cell to adhere to a substrate?

A. Filopodium

B. Lamellipodium

C. Microvillus

D. Stress fiber

B

Which transmembrane proteins are responsible for attaching a crawling cell to the extracellular matrix?

A. Cadherins

B. Integrins

C. Claudins

D. Myosins

B

What is the primary energy source that drives the protrusion of the leading edge in cell crawling?

A. ATP hydrolysis by myosin

B. Actin polymerization

C. Glucose fermentation

D. Sodium-potassium pump

B

Which member of the Rho GTPase family promotes the formation of lamellipodia?

A. Rho

B. Rac

C. Cdc42

D. Ras

B

Activation of the Cdc42 protein primarily leads to the formation of which structure?

A. Contractile bundles

B. Lamellipodia

C. Filopodia

D. The nuclear envelope

C

Which Rho GTPase is associated with the formation of contractile bundles?

A. Rho

B. Rac

C. Cdc42

D. Rab

A

In which direction do most myosin motor proteins (like Myosin I and II) move along actin filaments?

A. Toward the minus end

B. Toward the plus end

C. Randomly in both directions

D. They remain stationary

B

Which type of myosin is responsible for vesicle movement and consists of a single head and a tail?

A. Myosin I

B. Myosin II

C. Myosin V

D. Myosin X

A

What structural arrangement do Myosin II molecules form to facilitate muscle contraction?

A. Long single-stranded polymers

B. Bipolar myosin filaments

C. Circular rings

D. Branched networks

B

What is the basic contractile unit of a muscle cell?

A. Myofibril

B. Sarcomere

C. Z-disc

D. T-tubule

B

During muscle contraction, which of the following structures actually shortens?

A. Actin filaments

B. Myosin filaments

C. Sarcomeres

D. None of the above

C

To which part of the sarcomere are the plus ends of actin filaments attached?

A. The M-line

B. The Z-disc

C. The sarcoplasmic reticulum

D. The T-tubule

B

What is the role of Calcium (Ca²⁺) in muscle contraction?

A. It provides the energy for the power stroke

B. It binds to troponin, causing a conformational change that moves tropomyosin

C. It hydrolyzes ATP on the myosin head

D. It stabilizes the actin filament plus ends

B

Where is Calcium stored within a muscle cell until it is needed for contraction?

A. Nucleus

B. Mitochondria

C. Sarcoplasmic reticulum

D. Golgi apparatus

C

In the cross-bridge cycle, what causes the myosin head to reduce its affinity for actin and detach?

A. ATP hydrolysis

B. Binding of a new ATP molecule

C. Release of Phosphate (Pi)

D. Binding of Calcium

B

Which of the following correctly describes the "rigor" configuration of myosin?

A. Myosin is bound to ATP and detached from actin

B. Myosin is tightly bound to actin in the absence of ATP

C. Myosin is hydrolyzed and "cocked"

D. Myosin is bound to tropomyosin

B