Cytoskeleton and motility Practice Questions
21 free Cytoskeleton and motility practice questions for the USMLE Step 1. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
Which of the following is NOT one of the three principal filament types of the eukaryotic cytoskeleton?
- A Microfilaments (actin filaments)
- B Intermediate filaments
- C Microtubules
- D Collagen fibrils
Correct answer: Collagen fibrils
The cytoskeleton in eukaryotic cells is composed of microfilaments, intermediate filaments, and microtubules. Collagen fibrils are extracellular structural proteins, not cytoskeletal filaments.
Which cytoskeletal filament type is primarily responsible for resisting compressive forces within the cell and serving as tracks for intracellular vesicle transport?
- A Actin (microfilaments)
- B Intermediate filaments
- C Microtubules
- D Myosin filaments
Correct answer: Microtubules
Microtubules form hollow tubules comprised of tubulin that resist compression and act as tracks for motor proteins (e.g., kinesin and dynein) to transport vesicles and organelles.
Which cytoskeletal component is most directly involved in generating protrusive structures like lamellipodia and filopodia during cell migration?
- A Intermediate filaments
- B Microtubules
- C Actin (microfilaments)
- D Collagen fibers
Correct answer: Actin (microfilaments)
Actin filaments assemble at the leading edge of motile cells to form lamellipodia and filopodia, generating protrusive force needed for cell migration.
Which cytoskeletal element is most important for maintaining tensile strength and anchoring organelles, rather than rapid remodeling like microfilaments or microtubules?
- A Microtubules
- B Intermediate filaments
- C Actin microfilaments
- D Myosin thick filaments
Correct answer: Intermediate filaments
Intermediate filaments provide mechanical stability, resist tension, and help anchor organelles and the nucleus — functions that require structural support rather than dynamic remodeling.
During mitosis, which cytoskeletal structure is primarily responsible for separating sister chromatids and segregating chromosomes to opposite poles?
- A Actin filaments forming the contractile ring
- B Intermediate filaments organizing nuclear lamina
- C Microtubules forming the mitotic spindle
- D Desmin intermediate filaments
Correct answer: Microtubules forming the mitotic spindle
Microtubules reorganize into the mitotic spindle that attaches to chromosomes and pulls sister chromatids to opposite poles during cell division.
What is the approximate diameter of actin (microfilament) filaments in the cytoskeleton?
- A ~7 nm
- B ~10–12 nm
- C ~24 nm
- D ~100 nm
Correct answer: ~7 nm
Actin filaments (microfilaments) are the thinnest cytoskeletal fibers, with a diameter around 7 nm.
Which motor protein moves cargo toward the minus end of microtubules — i.e., toward the cell center (centrosome)?
- A Kinesin
- B Myosin II
- C Dynein
- D Actin-dependent ATPase
Correct answer: Dynein
Cytoplasmic dynein is a microtubule motor protein that moves cargo toward the minus end of microtubules (toward the centrosome), while kinesin typically moves toward the plus end.
Which cytoskeletal filament is least dynamic (i.e., most stable) under normal conditions, contributing to long-term structural integrity of tissues?
- A Actin microfilaments
- B Intermediate filaments
- C Microtubules
- D Actin stress fibers
Correct answer: Intermediate filaments
Intermediate filaments are less dynamic compared to actin filaments or microtubules; they are stable and provide durable structural support and resistance to mechanical stress.
Which step is NOT directly dependent on actin and myosin-mediated cytoskeletal contraction during cell motility?
- A Protrusion of the leading edge (lamellipodium)
- B Adhesion of leading edge to substrate
- C Release and retraction of the trailing edge
- D Polymerization of microtubules during mitosis
Correct answer: Polymerization of microtubules during mitosis
Polymerization of microtubules during mitosis is mediated by tubulin and microtubule dynamics, not actin-myosin contractility, which is responsible for protrusion, adhesion, and trailing-edge retraction in cell motility.
Which cytoskeletal filament is the main structural element of eukaryotic cilia and flagella, enabling their motility?
- A Actin filaments
- B Intermediate filaments
- C Microtubules
- D Myosin filaments
Correct answer: Microtubules
Cilia and flagella are composed of microtubule-based axonemes; microtubules are the core structural filaments that support their beating and motility.
What is the role of actin polymerization at the leading edge of a migrating cell?
- A Microtubules
- B Actin microfilaments
- C Intermediate filaments
- D Centrosomes
Correct answer: Intermediate filaments
Polymerization of actin filaments at the leading edge causes expansion of the actin network, pushing the plasma membrane outward and forming structures such as lamellipodia and filopodia needed for cell movement.
Which cytoskeletal element would most likely be disrupted in a disease causing fragility of skin epithelial cells under mechanical stress?
- A To form stable intermediate-filament scaffolds
- B To generate protrusive force pushing the membrane outward
- C To assemble spindle fibers during mitosis
- D To generate compressive force for intracellular transport
Correct answer: To generate protrusive force pushing the membrane outward
Intermediate filaments (e.g., keratin in epithelial cells) provide tensile strength to resist mechanical stress. Mutation or disruption in keratin IFs weakens structural integrity and leads to fragility in skin cells.
Which cytoskeletal filaments are most directly involved in cytokinesis (cell division) during cleavage of a dividing cell in animal cells?
- A Microtubules forming the mitotic spindle
- B Intermediate filaments forming nuclear lamina
- C Actin microfilaments forming the contractile ring
- D Collagen fibers facilitating separation
Correct answer: Actin microfilaments forming the contractile ring
During cytokinesis, actin microfilaments (with myosin) assemble a contractile ring at the cleavage furrow, which constricts to divide the cell into two daughter cells.
Which statement best captures the dynamic nature of the cytoskeleton in a typical eukaryotic cell?
- A Cytoskeletal filaments remain rigid and static once assembled
- B Only intermediate filaments are dynamic; actin and microtubules are static
- C Filaments continually grow and shrink, altering cell shape, motility, and division
- D Cytoskeleton does not participate in signal transduction or intracellular transport
Correct answer: Filaments continually grow and shrink, altering cell shape, motility, and division
The cytoskeleton is a dynamic network whose filaments continuously polymerize and depolymerize, rearranging the cell’s shape, enabling motility, organelle transport, and cell division.
Which of the following cytoskeletal processes would most likely be impaired by a toxin that inhibits tubulin polymerization?
- A Muscle contraction mediated by actin-myosin
- B Cell migration via lamellipodia formation
- C Chromosome segregation during mitosis
- D Intermediate filament-based cell support
Correct answer: Chromosome segregation during mitosis
Inhibition of tubulin polymerization prevents formation of microtubules, thereby disrupting the mitotic spindle necessary for chromosome segregation during mitosis.
Which class of proteins links cytoskeletal filaments to the plasma membrane or to other cytoskeletal elements to coordinate cellular architecture and signal transduction?
- A Motor proteins only (e.g. kinesin, dynein)
- B Cytoskeleton-binding (linker/scaffold) proteins
- C Ribosomal proteins
- D DNA polymerases
Correct answer: Cytoskeleton-binding (linker/scaffold) proteins
Cytoskeleton-binding (linker or scaffold) proteins interconnect different filaments, anchor them to membranes or organelles, and coordinate the cytoskeletal network’s organization — crucial for structure, signaling, and movement.
Which of the following motor proteins utilizes ATP to slide along actin filaments and generate contractile force in muscle and nonmuscle cells?
- A Kinesin
- B Dynein
- C Myosin II
- D Tubulin kinase
Correct answer: Myosin II
Myosin II interacts with actin filaments, hydrolyzes ATP, and produces contractile force — fundamental for muscle contraction, cell locomotion, and cytokinesis.
During directed cell migration, which cytoskeletal filament system and associated dynamics are mainly responsible for establishing cell polarity (front–rear axis)?
- A Intermediate filaments providing structural rigidity to the whole cell
- B Leading-edge actin protrusions with rear microtubule reorientation
- C Microtubule polymerization at the cell cortex only
- D A static actin cortex that does not undergo remodeling
Correct answer: Leading-edge actin protrusions with rear microtubule reorientation
Cell polarity during migration depends on actin-driven protrusive structures at the leading edge and coordinated microtubule reorientation that helps organize intracellular trafficking and rear retraction — both cytoskeletal networks are critical.
Which cytoskeletal alteration would most likely impair endocytosis and exocytosis processes dependent on vesicle transport?
- A Loss of intermediate filament proteins from the cytoplasm
- B Stabilizing the actin cortex so it can no longer remodel
- C Excessive microtubule polymerization throughout cytoplasm
- D Complete absence of nuclear lamins from the envelope
Correct answer: Stabilizing the actin cortex so it can no longer remodel
Dynamic remodeling of actin filaments near the plasma membrane is essential for endocytosis and exocytosis; if the actin cortex becomes overly stabilized and rigid, vesicle docking and membrane remodeling are hindered.
Which cytoskeletal element and associated molecular motors are primarily responsible for retrograde (toward nucleus) vesicular transport in neurons?
- A Actin filaments with myosin I
- B Microtubules with dynein
- C Intermediate filaments with keratin
- D Collagen fibers with adhesion proteins
Correct answer: Microtubules with dynein
Microtubules serve as tracks for retrograde transport in neurons, with dynein motor proteins carrying vesicles from distal axon toward the nucleus/cell body.
Which of the following best describes how cytoskeletal dysfunction could contribute to neurodegenerative or muscular diseases?
- A By preventing receptor-mediated endocytosis of nutrients only
- B By weakening cells' tensile strength and causing structural failure
- C By dramatically increasing the rate of uncontrolled cell proliferation
- D By enhancing excessive microtubule polymerization
Correct answer: By weakening cells' tensile strength and causing structural failure
Defects in intermediate filaments or associated linker proteins impair cellular structural integrity, making cells — especially in muscle or nerve tissue — more susceptible to mechanical stress and damage, contributing to disease pathology.