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25 Case-Based MCQs on Cytoskeleton: Test Your Medical Knowledge!
1. A 45-year-old male presents with persistent respiratory infections and infertility. Further testing reveals defective ciliary movement due to a mutation affecting a cytoskeletal component. Which of the following structures is primarily affected?
A) Actin filaments
B) Intermediate filaments
C) Microtubules
D) Myosin
E) Spectrin
Answer: C) Microtubules
Explanation: The patient’s symptoms suggest primary ciliary dyskinesia (Kartagener’s syndrome), caused by defective dynein arms within microtubules. Microtubules form the core of cilia and flagella, and their dysfunction leads to impaired mucociliary clearance (causing recurrent infections) and defective sperm motility (causing infertility).
Incorrect Options:
A) Actin filaments: Actin filaments are crucial for cell motility and shape but are not involved in ciliary movement.
B) Intermediate filaments: These provide structural support to cells but do not contribute to ciliary function.
D) Myosin: Myosin is a motor protein associated with actin filaments, not microtubules.
E) Spectrin: Spectrin stabilizes the plasma membrane, especially in erythrocytes, but is not involved in ciliary movement.
2. A researcher studying cell motility in fibroblasts treats cultured cells with a drug that disrupts actin polymerization. Which of the following processes will most likely be inhibited?
A) Ciliary beating
B) Chromosome segregation
C) Lamellipodia formation
D) Mitochondrial transport
E) Nuclear envelope disassembly
Answer: C) Lamellipodia formation
Explanation: Actin filaments are essential for cell migration, particularly in lamellipodia (sheet-like protrusions at the leading edge of motile cells). Disrupting actin polymerization (e.g., using cytochalasin) impairs cell movement.
Incorrect Options:
A) Ciliary beating: Cilia rely on microtubules, not actin filaments.
B) Chromosome segregation: This process depends on microtubules forming the mitotic spindle.
D) Mitochondrial transport: Mitochondria move along microtubules via motor proteins (dynein and kinesin).
E) Nuclear envelope disassembly: This process involves intermediate filaments (lamins), not actin.
3. A 30-year-old woman presents with multiple blisters on her skin after minor trauma. Genetic testing reveals a mutation in the keratin genes. Which of the following cytoskeletal components is most likely defective?
A) Actin filaments
B) Desmin filaments
C) Intermediate filaments
D) Microtubules
E) Tubulin
Answer: C) Intermediate filaments
Explanation: Keratin is a type of intermediate filament found in epithelial cells. Mutations in keratin genes (e.g., KRT5, KRT14) lead to epidermolysis bullosa simplex, a condition characterized by skin fragility and blister formation.
Incorrect Options:
A) Actin filaments: Actin is involved in cell shape and movement but does not provide the mechanical strength necessary for epithelial integrity.
B) Desmin filaments: Desmin is found in muscle cells, not epithelial cells.
D) Microtubules: Microtubules are involved in intracellular transport and mitosis but are not the main structural component of epithelial cells.
E) Tubulin: Tubulin is the building block of microtubules, which are not the primary structural support for the skin.
4. A scientist is studying the transport of vesicles in neurons. She uses a drug that inhibits dynein motor proteins. Which of the following intracellular transport processes will be most affected?
A) Anterograde transport of synaptic vesicles
B) Endocytosis at the plasma membrane
C) Mitochondrial fusion
D) Retrograde transport along microtubules
E) Sarcomere contraction
Answer: D) Retrograde transport along microtubules
Explanation: Dynein is a motor protein responsible for retrograde transport, moving materials toward the minus end of microtubules (toward the nucleus). In neurons, it transports endocytic vesicles and damaged organelles from the synapse to the cell body.
Incorrect Options:
A) Anterograde transport of synaptic vesicles: This process is mediated by kinesin, not dynein.
B) Endocytosis at the plasma membrane: Endocytosis relies on actin filaments, not microtubule-based motors.
C) Mitochondrial fusion: Mitochondrial dynamics involve different proteins (e.g., mitofusins) rather than microtubule-based transport.
E) Sarcomere contraction: This process depends on actin and myosin interactions, not microtubules.
5. A pathologist examines a cardiac muscle biopsy from a patient with a history of dilated cardiomyopathy. Immunohistochemistry shows abnormal staining for a cytoskeletal protein essential for muscle integrity and force transmission. Which of the following proteins is most likely defective?
A) Actin
B) Desmin
C) Lamin A
D) Tubulin
E) Vimentin
Answer: B) Desmin
Explanation: Desmin is an intermediate filament found in muscle cells, linking Z-discs of sarcomeres and maintaining structural integrity. Mutations in desmin lead to myopathies and cardiomyopathies.
Incorrect Options:
A) Actin: Actin is involved in contraction but does not provide the primary structural support affected in desminopathies.
C) Lamin A: Lamin A is found in the nuclear lamina, supporting the nucleus, not the sarcomere.
D) Tubulin: Tubulin forms microtubules, which are more involved in intracellular transport rather than muscle structural integrity.
E) Vimentin: Vimentin is an intermediate filament found in mesenchymal cells but is not a major component of muscle cells.
6. A 28-year-old woman presents with muscle weakness and difficulty standing up from a seated position. Genetic testing reveals a mutation in the dystrophin gene. Which of the following cytoskeletal components is primarily disrupted in this disorder?
A) Actin filaments
B) Intermediate filaments
C) Microtubules
D) Myosin
E) Tubulin
Answer: A) Actin filaments
Explanation: Duchenne muscular dystrophy (DMD) is caused by mutations in the dystrophin gene, which anchors actin filaments to the sarcolemma. The absence of dystrophin leads to muscle fiber damage and progressive weakness.
Incorrect Options:
B) Intermediate filaments: These provide mechanical support but are not directly involved in dystrophin function.
C) Microtubules: These assist in intracellular transport but do not play a primary role in muscle fiber stability.
D) Myosin: Myosin interacts with actin for muscle contraction but is not affected in DMD.
E) Tubulin: Tubulin forms microtubules, which do not anchor the cytoskeleton to the extracellular matrix.
7. A 52-year-old woman with breast cancer is treated with paclitaxel, a chemotherapy drug that stabilizes microtubules and prevents their depolymerization. Which of the following processes will be most directly affected by this drug?
A) Actin polymerization
B) Chromosome segregation
C) Endocytosis
D) Lamellipodia formation
E) Sarcomere contraction
Answer: B) Chromosome segregation
Explanation: Paclitaxel stabilizes microtubules, preventing them from depolymerizing. This interferes with the mitotic spindle, blocking cell division and leading to apoptosis in rapidly dividing cancer cells.
Incorrect Options:
A) Actin polymerization: Actin is involved in cell shape and motility but is not the target of paclitaxel.
C) Endocytosis: This process involves actin filaments, not microtubules.
D) Lamellipodia formation: Lamellipodia require dynamic actin remodeling, which paclitaxel does not affect.
E) Sarcomere contraction: Muscle contraction relies on actin-myosin interactions, not microtubules.
8. A 60-year-old man presents with progressive difficulty in swallowing and muscle weakness. Muscle biopsy reveals inclusion bodies containing abnormal accumulations of desmin. Which of the following diseases is most likely?
A) Alzheimer’s disease
B) Desmin-related myopathy
C) Duchenne muscular dystrophy
D) Huntington’s disease
E) Multiple sclerosis
Answer: B) Desmin-related myopathy
Explanation: Desmin-related myopathy (DRM) is a disease where desmin, an intermediate filament in muscle cells, aggregates abnormally, disrupting muscle function and leading to progressive weakness.
Incorrect Options:
A) Alzheimer’s disease: This neurodegenerative disease involves tau protein, not desmin.
C) Duchenne muscular dystrophy: DMD is caused by dystrophin mutations, not desmin abnormalities.
D) Huntington’s disease: A neurodegenerative disorder caused by CAG repeat expansion in the huntingtin gene.
E) Multiple sclerosis: MS is an autoimmune disease affecting myelin, not the cytoskeleton.
9. A 34-year-old man undergoes a liver biopsy for suspected cirrhosis. Histopathology reveals Mallory bodies composed of aggregated intermediate filaments. Which of the following proteins is most likely accumulating?
A) Actin
B) Desmin
C) Keratin
D) Tubulin
E) Vimentin
Answer: C) Keratin
Explanation: Mallory bodies, seen in alcoholic liver disease and other chronic liver conditions, contain aggregated keratin intermediate filaments. These contribute to hepatocyte damage.
Incorrect Options:
A) Actin: Actin filaments are involved in cell movement and shape but do not form Mallory bodies.
B) Desmin: Desmin is found in muscle cells, not liver cells.
D) Tubulin: Tubulin forms microtubules but is not involved in Mallory body formation.
E) Vimentin: Vimentin is an intermediate filament in mesenchymal cells, not hepatocytes.
10. A 40-year-old woman is diagnosed with progeria, a premature aging disorder. Genetic analysis reveals a mutation affecting the nuclear lamina. Which of the following cytoskeletal components is defective?
A) Actin filaments
B) Desmin filaments
C) Intermediate filaments
D) Microtubules
E) Spectrin
Answer: C) Intermediate filaments
Explanation: Progeria (Hutchinson-Gilford Progeria Syndrome) is caused by mutations in the LMNA gene, which encodes lamin A, an intermediate filament protein that supports the nuclear envelope. Defective lamin A leads to nuclear instability and premature cell aging.
Incorrect Options:
A) Actin filaments: Actin maintains cell shape but does not form the nuclear lamina.
B) Desmin filaments: Desmin is muscle-specific and unrelated to progeria.
D) Microtubules: Microtubules aid intracellular transport but do not support the nuclear envelope.
E) Spectrin: Spectrin stabilizes the plasma membrane, especially in red blood cells, but is not related to nuclear lamina defects.
11. A researcher is studying a cell line and observes that a particular cytoskeletal component requires ATP for its polymerization and demonstrates treadmilling, a process where monomers add at one end and dissociate from the other. Which cytoskeletal component is most likely being observed?
A) Intermediate filament
B) Keratin
C) Microtubule
D) Tubulin
E) Actin filament
Answer: E) Actin filament
Explanation: Actin filaments exhibit treadmilling, a dynamic process where actin monomers (G-actin) polymerize at the plus (+) end while depolymerizing at the minus (-) end. This process is ATP-dependent, as actin monomers must bind ATP to polymerize efficiently. Actin is crucial for cellular movement, endocytosis, and structural integrity.
Incorrect Options:
A) Intermediate filament: These filaments provide mechanical support but do not undergo treadmilling or require ATP for polymerization.
B) Keratin: Keratin is a type of intermediate filament primarily found in epithelial cells, and it does not exhibit treadmilling or ATP-dependent assembly.
C) Microtubule: Microtubules undergo dynamic instability rather than treadmilling, and their polymerization relies on GTP, not ATP.
D) Tubulin: Tubulin is the building block of microtubules and requires GTP, not ATP, for polymerization.
12. An 8-month-old female patient presents with jaundice and splenomegaly. Her hemoglobin level is below the reference range, indicating anemia, and her serum lactate dehydrogenase is markedly elevated, suggesting cell lysis. A peripheral blood smear reveals small, spherical erythrocytes lacking central pallor. These findings are best explained by an erythrocyte deficiency of which of the following cytoskeletal components?
A) Actin
B) Collagen
C) Elastin
D) Glycosaminoglycans
E) Spectrin
Answer: E) Spectrin
Explanation: The patient’s symptoms and spherocytes on blood smear suggest hereditary spherocytosis (HS), a disorder caused by a deficiency or dysfunction of spectrin, an essential cytoskeletal protein in red blood cells. Spectrin provides structural integrity by maintaining the biconcave shape of erythrocytes. Its deficiency leads to a loss of membrane flexibility, making RBCs more prone to destruction in the spleen (extravascular hemolysis), leading to anemia, jaundice, and splenomegaly.
Incorrect Options:
A) Actin: Although actin is part of the erythrocyte cytoskeleton, the primary defect in HS is in spectrin, which interacts with actin to maintain cell shape.
B) Collagen: Collagen is a structural protein found in connective tissues, not in the erythrocyte membrane.
C) Elastin: Elastin provides elasticity to tissues like blood vessels and lungs but is not involved in erythrocyte structure.
D) Glycosaminoglycans: These are large polysaccharides found in the extracellular matrix and cartilage, not in red blood cell membranes.
13. A laboratory technician adds the drug Taxol to a culture of rapidly dividing tumor cells. The technician observes that cell division is inhibited. Which of the following effects exerted by Taxol on the cytoskeleton of these tumor cells is the most likely cause?
A) Conversion from the gel to the sol state of the cytosol
B) Disruption of mitotic spindle formation by microtubules
C) Inhibition of treadmilling by F-actin assembling from G-actins
D) Prevention of microtubule motor protein hydrolysis of ATP
E) Promotion of intermediate filament disassembly
Answer: B) Disruption of mitotic spindle formation by microtubules
Explanation: Taxol (Paclitaxel) stabilizes microtubules by preventing their depolymerization. Normally, dynamic instability of microtubules is required for proper assembly and disassembly of the mitotic spindle during cell division. By locking microtubules in a polymerized state, Taxol disrupts spindle dynamics, leading to mitotic arrest and inhibition of tumor cell proliferation.
Incorrect Options:
A) Conversion from the gel to the sol state of the cytosol: The sol-gel transition of the cytosol is controlled by actin dynamics, not microtubules, and is not the mechanism of Taxol action.
C) Inhibition of treadmilling by F-actin assembling from G-actins: Treadmilling is an actin-dependent process, whereas Taxol specifically affects microtubules.
D) Prevention of microtubule motor protein hydrolysis of ATP: Taxol does not inhibit ATP hydrolysis by motor proteins (e.g., kinesin or dynein), but rather stabilizes microtubules themselves.
E) Promotion of intermediate filament disassembly: Intermediate filaments provide structural integrity, but their disassembly is not affected by Taxol.
14. A vesicle within a cell must be transported to another region of the cell along the network of cytoskeletal microtubules. Which of the following proteins may be involved in catalyzing this transport?
A) Dystrophin
B) Kinesin
C) Myosin
D) Spectrin
E) Vimentin
Answer: B) Kinesin
Explanation: Kinesin is a microtubule-associated motor protein responsible for anterograde transport of vesicles, organelles, and other cargo toward the plus (+) end of microtubules, usually directed toward the cell periphery. Kinesin hydrolyzes ATP to drive this movement, making it essential for intracellular transport processes.
Incorrect Options:
A) Dystrophin: A structural protein that links the cytoskeleton (actin filaments) to the extracellular matrix in muscle cells; it does not transport vesicles.
C) Myosin: A motor protein that moves along actin filaments, not microtubules, and is primarily involved in muscle contraction and intracellular transport.
D) Spectrin: A cytoskeletal protein that maintains the biconcave shape of erythrocytes but does not function in vesicular transport.
E) Vimentin: An intermediate filament protein providing structural support in mesenchymal cells, not involved in microtubule-based transport.
15. A person mistakenly consumes poisonous mushrooms containing phalloidin. This toxin disrupts normal cell function by binding tightly to an ATP-dependent component of the cytoskeleton, leading to the stabilization of filaments and inhibition of their disassembly. The toxin therefore binds to:
A) Acidic keratins
B) Cadherins
C) Desmosomes
D) F-actin polymers
E) Tubulin heterodimers
Answer: D) F-actin polymers
Explanation: Phalloidin, found in Amanita mushrooms, specifically binds F-actin (filamentous actin) and prevents its depolymerization, leading to cytoskeletal rigidity and loss of normal cellular function. Actin filaments are ATP-dependent, meaning actin monomers (G-actin) must bind ATP for polymerization into F-actin. This toxin disrupts cell motility, endocytosis, and intracellular transport, ultimately leading to hepatotoxicity and organ failure.
Incorrect Options:
A) Acidic keratins: Keratins are intermediate filaments found in epithelial cells; they do not polymerize via ATP and are not targeted by phalloidin.
B) Cadherins: Cadherins are cell adhesion proteins in adherens junctions and desmosomes, not cytoskeletal components.
C) Desmosomes: Desmosomes are intercellular junctions composed of cadherins and intermediate filaments (keratins), not actin filaments.
E) Tubulin heterodimers: Tubulin forms microtubules, which polymerize using GTP, not ATP. Phalloidin does not affect microtubules.
16. A 17-year-old male is evaluated for slowly progressive muscle weakness in his pelvis and legs. Genetic analysis reveals large deletions in a gene encoding an actin-binding protein, leading to a partially functional protein. This patient is most likely affected by:
A) Becker muscular dystrophy
B) Ehlers-Danlos syndrome
C) Hereditary spherocytosis
D) Marfan syndrome
E) Pemphigus vulgaris
Answer: A) Becker muscular dystrophy
Explanation: Becker muscular dystrophy (BMD) is caused by partially functional dystrophin, a cytoskeletal actin-binding protein that stabilizes muscle fibers. Unlike Duchenne muscular dystrophy (DMD), which results from complete loss of dystrophin, BMD occurs due to in-frame deletions, allowing some functional dystrophin production. This leads to milder and later-onset muscle weakness, primarily affecting proximal muscles (pelvis, thighs) before progressing to other areas.
Incorrect Options:
B) Ehlers-Danlos syndrome: A connective tissue disorder caused by defects in collagen synthesis, leading to hypermobile joints and fragile skin, but not progressive muscle weakness.
C) Hereditary spherocytosis: A red blood cell membrane disorder due to defective spectrin or ankyrin, leading to hemolytic anemia, not muscle weakness.
D) Marfan syndrome: Caused by mutations in fibrillin-1, affecting connective tissue and leading to tall stature, lens dislocation, and aortic aneurysms, but not muscle dystrophy.
E) Pemphigus vulgaris: An autoimmune blistering disorder caused by antibodies against desmosomal proteins, leading to skin and mucosal blistering, not muscle weakness.
17. A cell biologist is examining the dynamics of actin filaments in a cell undergoing changes in shape. She notices an increased rate of disassembly at the minus (-) end of the filaments. Which of the following molecules is likely enriched at the minus (-) end, promoting depolymerization?
A) ATP-bound G-actin
B) ADP-bound G-actinÂ
C) Profilin
D) Thymosin-β4
E) Arp2/3 complex
Answer: B) ADP-bound G-actin
Explanation: Actin filaments undergo a process called treadmilling, where polymerization occurs at the plus (+) end, while depolymerization happens at the minus (-) end. When actin monomers polymerize, they bind ATP, but over time, ATP is hydrolyzed to ADP, weakening actin-actin interactions. ADP-bound G-actin is more prone to dissociating from the minus end, promoting filament disassembly.
Incorrect Options:
A) ATP-bound G-actin: ATP-actin is preferentially incorporated at the plus (+) end, not the minus (-) end, and promotes polymerization rather than disassembly.
C) Profilin: Profilin facilitates actin polymerization by converting ADP-actin to ATP-actin, but it does not promote disassembly.
D) Thymosin-β4: This protein sequesters actin monomers, preventing polymerization but not directly enhancing depolymerization at the minus end.
E) Arp2/3 complex: Arp2/3 nucleates branched actin networks and promotes actin polymerization, particularly at the leading edge of migrating cells, rather than driving filament breakdown.
18. A scientist is studying the effects of a drug on cell morphology. After drug treatment, she observes that the cells have lost their ability to maintain a firm, structured shape, and the cytosol appears more fluid-like. Which cytoskeletal element is most likely affected by this drug?
A) Intermediate filaments
B) Actin filaments
C) Microtubules
D) Vimentin
E) Lamin
Answer: B) Actin filaments
Explanation: Actin filaments (microfilaments) are crucial for maintaining cell shape, mechanical stability, and cytoplasmic viscosity. Disrupting actin dynamics with drugs like cytochalasins or latrunculin leads to cell softening and a more fluid-like cytosol due to the breakdown of cortical actin networks. Actin also plays a key role in cell motility, intracellular trafficking, and maintaining the cell’s structural integrity.
Incorrect Options:
A) Intermediate filaments: These provide mechanical strength and resistance to stress, but their disruption does not make the cytoplasm fluid-like.
C) Microtubules: Microtubules are involved in intracellular transport and mitotic spindle formation; their disruption affects organelle positioning but does not significantly alter the cytosol’s viscosity.
D) Vimentin: Vimentin is a type of intermediate filament found in mesenchymal cells that provides structural integrity but does not control cytoplasmic viscosity.
E) Lamin: Lamins are nuclear intermediate filaments that provide structural support to the nuclear envelope but do not affect overall cell shape and cytoplasmic fluidity.
19. During mitosis, a researcher observes that chromosomes are not properly segregating into the newly forming daughter cells. Upon closer examination, the mitotic spindle appears disorganized. Which cytoskeletal component is most likely malfunctioning?
A) Actin filaments
B) Intermediate filaments
C) Microtubules
D) Spectrin
E) Dystrophin
Answer: C) Microtubules
Explanation: Microtubules are the primary cytoskeletal components that form the mitotic spindle, which is essential for chromosome alignment and segregation during cell division. The spindle consists of dynamic microtubules that attach to kinetochores on chromosomes and pull them toward opposite poles. Disruption of microtubule function—whether by mutation, drugs like colchicine or vinblastine, or improper polymerization—leads to spindle disorganization, causing defective chromosome segregation and aneuploidy.
Incorrect Options:
A) Actin filaments: Actin plays a role in cytokinesis (cell cleavage) but is not responsible for chromosome segregation during mitosis.
B) Intermediate filaments: These provide structural integrity to cells and nuclei but do not participate in mitotic spindle formation.
D) Spectrin: Spectrin maintains the shape of erythrocytes and supports the plasma membrane but is not involved in mitosis.
E) Dystrophin: Dystrophin links actin to the extracellular matrix in muscle cells and is unrelated to chromosome segregation or spindle function.
20. A researcher is studying a cell that has an issue with its structural integrity and support. Upon further investigation, it is found that the cell has a compromised network of rope-like structures. Which of the following cytoskeletal components is most likely affected?
A) Actin filaments
B) Microtubules
C) Intermediate filaments
D) Myosin
E) Tubulin
Answer: C) Intermediate filaments
Explanation: Intermediate filaments are rope-like, durable cytoskeletal components that provide mechanical strength and maintain cellular structure. Unlike actin filaments (which are thin and dynamic) and microtubules (which are hollow and rigid), intermediate filaments form a stable, supportive network that resists mechanical stress. Examples include keratin (in epithelial cells), desmin (in muscle cells), vimentin (in mesenchymal cells), and lamins (in the nucleus).
Incorrect Options:
A) Actin filaments: Actin filaments are involved in cell motility and shape changes, but they are not the primary structural support system.
B) Microtubules: Microtubules provide a scaffolding for intracellular transport and mitosis, but they are not rope-like or primarily responsible for mechanical stability.
D) Myosin: Myosin is a motor protein that interacts with actin, primarily for contraction and movement, not for structural support.
E) Tubulin: Tubulin is the building block of microtubules, which are involved in intracellular transport and spindle formation rather than structural integrity.
21. A scientist is observing a cell undergoing cell division and notices that the contractile ring is not forming correctly, leading to a failure in cytokinesis. Which of the following cytoskeletal components is most likely involved in this issue?
A) Microtubules
B) Intermediate filaments
C) Actin filaments
D) Keratins
E) Tubulin
Answer: C) Actin filaments
Explanation: Actin filaments play a crucial role in cytokinesis, the final step of cell division, by forming the contractile ring. This ring, composed of actin filaments and myosin II, contracts at the cleavage furrow, leading to the separation of the daughter cells. Disruption of actin filaments prevents proper contractile ring formation, resulting in cytokinesis failure and multinucleated cells.
Incorrect Options:
A) Microtubules: Microtubules form the mitotic spindle and are important for chromosome segregation but are not responsible for contractile ring formation.
B) Intermediate filaments: These provide structural support but do not play a direct role in cytokinesis.
D) Keratins: Keratins are intermediate filaments found in epithelial cells, involved in mechanical stability rather than cytokinesis.
E) Tubulin: Tubulin is a component of microtubules, which aid in mitotic spindle formation but do not contribute to the contractile ring.
22. A hematologist is examining a blood sample from a patient with hereditary spherocytosis. The erythrocytes appear spherical and fragile. A deficiency in which of the following proteins is the most likely cause of this condition?
A) Actin
B) Tubulin
C) Vimentin
D) Spectrin
E) Keratin
Answer: D) Spectrin
Explanation: Hereditary spherocytosis (HS) is caused by defects in spectrin, a cytoskeletal protein that maintains the biconcave shape and flexibility of erythrocytes. Spectrin interacts with ankyrin, band 3, and protein 4.2 to form a structural network supporting the plasma membrane. A deficiency or dysfunction of spectrin weakens the cytoskeleton, leading to membrane instability, loss of surface area, and a spherical shape, making RBCs more prone to destruction in the spleen (hemolysis).
Incorrect Options:
A) Actin: While actin interacts with spectrin in erythrocytes, actin deficiency does not cause hereditary spherocytosis.
B) Tubulin: Tubulin forms microtubules, which are essential for intracellular transport and mitotic spindle formation, not erythrocyte shape maintenance.
C) Vimentin: Vimentin is an intermediate filament found in mesenchymal cells, not erythrocytes.
E) Keratin: Keratin is an intermediate filament found in epithelial cells, not in red blood cells.
23. A researcher is studying a cell undergoing rapid changes in morphology and motility. They observe dynamic assembly and disassembly of cytoskeletal filaments, with GTP hydrolysis playing a crucial role in destabilizing the structure. Which cytoskeletal component is most likely involved in this process?
A) Actin filaments
B) Intermediate filaments
C) Microtubules
D) Keratins
E) Spectrin
Answer: C) Microtubules
Explanation: Microtubules undergo dynamic instability, a process where they rapidly polymerize and depolymerize, allowing for cell shape changes, intracellular transport, and mitotic spindle formation. Tubulin heterodimers (α-tubulin and β-tubulin) polymerize using GTP, but after incorporation into the growing microtubule, GTP is hydrolyzed to GDP, making the structure unstable and prone to depolymerization. This dynamic nature allows microtubules to quickly reorganize in response to cellular needs.
Incorrect Options:
A) Actin filaments: Actin filaments also undergo remodeling but rely on ATP hydrolysis, not GTP hydrolysis.
B) Intermediate filaments: These provide structural stability and do not rapidly assemble/disassemble or use nucleotide hydrolysis for polymerization.
D) Keratins: Keratins are a type of intermediate filament that provide mechanical support and are not involved in rapid cytoskeletal dynamics.
E) Spectrin: Spectrin is a cytoskeletal protein that helps maintain cell shape, particularly in erythrocytes, but does not undergo rapid polymerization or GTP hydrolysis.
24. A cell biologist is investigating intracellular transport mechanisms. They observe that organelles are being transported along cytoskeletal tracks by motor proteins that hydrolyze ATP. Which of the following cytoskeletal components is most likely facilitating this transport?
A) Actin filaments
B) Intermediate filaments
C) Microtubules
D) Spectrin
E) Keratin
Answer: C) Microtubules
Explanation: Microtubules serve as tracks for intracellular transport of organelles, vesicles, and other cellular components. This transport is mediated by motor proteins that hydrolyze ATP to generate movement:
- Kinesin moves cargo toward the plus (+) end (anterograde transport, usually toward the cell periphery).
- Dynein moves cargo toward the minus (-) end (retrograde transport, usually toward the nucleus).
This transport system is crucial for proper cellular function, including neuronal signaling, mitosis, and organelle positioning.
Incorrect Options:
A) Actin filaments: Actin filaments are involved in short-range intracellular transport, primarily mediated by myosin, but long-range transport is microtubule-dependent.
B) Intermediate filaments: These provide mechanical strength but do not function as transport tracks.
D) Spectrin: Spectrin maintains cell membrane integrity, particularly in erythrocytes, but does not participate in intracellular transport.
E) Keratin: Keratin is a type of intermediate filament found in epithelial cells that provides structural support but does not assist in intracellular transport.
25. A researcher is investigating a fungal toxin that interferes with actin polymerization. They observe that cells treated with the toxin exhibit excessive actin polymerization, preventing normal depolymerization. Which of the following toxins is most likely being studied?
A) Colchicine
B) Taxol
C) Phalloidin
D) Gelsolin
E) Cytochalasin
Answer: C) Phalloidin
Explanation: Phalloidin, a toxin derived from Amanita mushrooms, binds F-actin and prevents its depolymerization, leading to excessive actin stabilization. This disrupts normal cell motility, endocytosis, and cytoskeletal dynamics, ultimately impairing cellular function and causing toxicity, particularly in liver cells.
Incorrect Options:
A) Colchicine: This drug binds tubulin, preventing microtubule polymerization, not actin filament stabilization.
B) Taxol: Taxol stabilizes microtubules, preventing their depolymerization, but it does not affect actin filaments.
D) Gelsolin: Gelsolin is an actin-severing protein that enhances actin depolymerization, opposite to phalloidin’s effect.
E) Cytochalasin: Cytochalasin inhibits actin polymerization by blocking the plus (+) end, preventing filament growth rather than promoting excessive polymerization.
