Questions: The Cytoskeleton: Cellular Architecture

Question 1

A researcher treats cells with a drug that specifically prevents actin polymerization. Which process would be MOST directly impaired?

• A. Cytokinesis — the contractile ring that pinches daughter cells apart is assembled from actin filaments
• B. Chromosome segregation — the mitotic spindle that pulls chromosomes to the poles is made of actin
• C. Long-distance organelle transport — cargo vesicles move along actin tracks toward the cell periphery
• D. Nuclear structural integrity — actin forms the lamins that line the nuclear envelope

Answer: (A)

The contractile ring responsible for cytokinesis is an actin-myosin structure — myosin walks along actin filaments to generate the contractile force that physically pinches the cell in two. Disrupting actin polymerization directly prevents this ring from forming. Option B describes the role of microtubules (not actin) in the mitotic spindle. Long-distance transport (option C) is performed by kinesin and dynein along microtubules. Nuclear lamins (option D) are intermediate filaments, not actin.

Question 2

Which pairing correctly matches each cytoskeletal filament type with its primary function?

• A. Microfilaments → contractile force and cell shape; microtubules → intracellular transport and mitotic spindle; intermediate filaments → mechanical resistance to tensile stress
• B. Microtubules → contractile force; microfilaments → intracellular transport; intermediate filaments → mitotic spindle formation
• C. Intermediate filaments → intracellular transport; microtubules → contractile force; microfilaments → mechanical resistance
• D. All three filament types equally support motor protein movement but differ only in diameter

Answer: (A)

The three types have distinct structures and roles. Microfilaments (actin, ~7 nm) drive contraction and form the cell cortex. Microtubules (tubulin, ~25 nm, hollow) are tracks for kinesin and dynein and form the mitotic spindle. Intermediate filaments (~10 nm) have no associated motor proteins and provide tensile mechanical strength. Option D is incorrect — only microfilaments and microtubules support motor protein movement; intermediate filaments do not.

Question 3

The cytoskeleton is a rigid, permanent scaffold that maintains cell shape, analogous to the bones of a vertebrate skeleton.

• T. True
• F. False

Answer: False

The cytoskeleton is highly dynamic — its filaments constantly polymerize (assemble from protein subunits) and depolymerize (disassemble back into subunits). Microtubules, in particular, undergo rapid cycles of growth and shrinkage called dynamic instability. This constant remodeling allows the cell to reorganize its internal architecture during division, migration, and response to signals. A rigid, permanent scaffold would prevent cells from dividing or changing shape — the opposite of what cells actually do.

Question 4

Intermediate filaments are the most mechanically stable cytoskeletal filaments and have no associated motor proteins.

• T. True
• F. False

Answer: True

Unlike microfilaments and microtubules, intermediate filaments are not tracks for kinesin, dynein, or myosin, and they do not generate motility. Their role is structural: they resist tensile (stretching) forces and distribute mechanical stress across the cell. While actin filaments and microtubules are highly dynamic, intermediate filaments are comparatively stable and provide the cell's baseline mechanical integrity. This makes them well suited for tissues subject to physical stress, such as skin and nerve fibers.

Question 5

What distinguishes the role of microtubules from the role of intermediate filaments in the cytoskeleton, and why does the cell need both?

Model answer: Microtubules are dynamic structures that serve as tracks for long-distance intracellular transport (via kinesin and dynein) and form the mitotic spindle for chromosome segregation. Intermediate filaments are more stable and resist tensile forces, providing mechanical integrity without supporting motility. The cell needs both because transport and division require dynamic, rapidly reorganizable structures, while the physical demands of tissues — resisting stretching and tearing — require stable load-bearing filaments that would be too rigid if they were also the transport network.

The division of labor among cytoskeletal components reflects the cell's simultaneous need for adaptability and robustness. Disrupting microtubules impairs transport and division; disrupting intermediate filaments weakens the cell mechanically. Neither can substitute for the other. The three systems also cooperate — actin provides contractile force, microtubules organize space and transport, and intermediate filaments hold it all together under stress.