Question 1

Which of the following statements best describes the polarity of actin microfilaments as demonstrated by the binding of myosin S1 fragments?

Accepted Answer

The plus (barbed) end grows faster than the minus (pointed) end, and S1 fragments bind in an arrowhead pattern indicating polarity.

Question 2

Which of the following drugs would most likely inhibit the addition of new actin monomers to a growing microfilament, and what is its mechanism?

Accepted Answer

Cytochalasin; it caps the plus end of microfilaments, preventing polymerization.

Question 3

A patient presents with blistering of the skin after minor trauma. A biopsy reveals a defect in keratin filaments. Which class of intermediate filament proteins is most likely affected?

Accepted Answer

Class I and II (acidic and basic keratins)

Question 4

Which of the following best explains why intermediate filaments are considered the most stable cytoskeletal elements?

Accepted Answer

They are not polarized and can withstand significant tensile forces without breaking.

Question 5

Which protein is responsible for cross-linking actin filaments into tight bundles, such as those found in microvilli?

Accepted Answer

Fimbrin

Question 6

During muscle contraction, what is the role of calcium ions ($$\mathrm{Ca}^{2+}) in $$the regulation of the thin filament?

Accepted Answer

Calcium binds to TnC, causing tropomyosin to shift and expose myosin-binding sites on actin.

Question 7

Which of the following best describes the process of actin treadmilling?

Accepted Answer

Actin monomers are added at the plus end and removed from the minus end, resulting in a net flux of subunits through the filament.

Question 8

A mutation in the gene encoding plectin would most likely disrupt which cellular function?

Accepted Answer

Linking intermediate filaments to microfilaments and microtubules.

Question 9

Which of the following best explains why muscle relaxation requires ATP?

Accepted Answer

ATP is needed to detach myosin heads from actin and to pump $$\mathrm{Ca}^{2+}$$ back into the sarcoplasmic reticulum.

Question 10

Which of the following intermediate filament proteins is found in the nuclear lamina and is essential for nuclear envelope structure?

Accepted Answer

Lamin A

Question 11

Which actin-binding protein is responsible for severing actin filaments and capping the newly exposed ends?

Accepted Answer

Gelsolin

Question 12

A scientist is studying a cell type that lacks functional spectrin. Which cellular structure or function would most likely be affected?

Accepted Answer

The mechanical stability and shape of the plasma membrane, especially in red blood cells.

Question 13

Which of the following best describes the role of myosin II in cytokinesis?

Accepted Answer

Myosin II moves along actin filaments to generate the contractile force required for cleavage furrow formation.

Question 14

Which of the following is a unique property of intermediate filaments compared to microfilaments and microtubules?

Accepted Answer

They are not polarized and do not serve as tracks for motor proteins.

Question 15

Which of the following best explains the effect of phalloidin on actin filaments?

Accepted Answer

It stabilizes actin filaments by preventing their depolymerization.

Question 16

Which of the following best describes the function of the protein desmin?

Accepted Answer

It provides structural support for contractile machinery in muscle cells.

Question 17

Which of the following is true regarding the assembly of intermediate filaments?

Accepted Answer

Assembly involves the formation of coiled-coil dimers, which then associate into staggered tetramers and finally into rope-like filaments.

Question 18

Which of the following best explains why animal cells can be distinguished based on the types of intermediate filament proteins they contain?

Accepted Answer

Intermediate filament proteins are highly tissue-specific, with different classes expressed in different cell types.

Question 19

Which of the following best describes the role of tropomyosin in muscle contraction?

Accepted Answer

Tropomyosin blocks myosin-binding sites on actin in the absence of calcium.

Cytoskeletal Systems: Microfilaments, Intermediate Filaments, and Muscle Contraction

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