📝 Microtubules structure and function (13 MCQs)
📖 From Principles of Biochemistry • 1. The Foundations of Biochemistry • 13 questions available
What is Microtubules structure and function?
Definition:
Microtubules are cylindrical, hollow protein polymers with an outer diameter of about 25 nm, composed of α-tubulin and β-tubulin heterodimers that assemble into protofilaments arranged in a helical lattice, typically 13 protofilaments forming the microtubule wall, and they function in maintaining cell shape, intracellular transport, chromosome segregation during mitosis and meiosis, and as the core of cilia and flagella, providing tracks for motor proteins.
Working:
Microtubules work by polymerizing from γ-tubulin ring complexes at centrosomes, with growth and shrinkage occurring at the plus end in a phenomenon called dynamic instability, where GTP-bound tubulin adds to the growing end, and upon hydrolysis to GDP, the microtubule becomes unstable and can undergo catastrophes, and they serve as tracks for kinesin and dynein motors that transport vesicles and organelles, with the force generated by these motors described by the equation , where is energy and is the step size.
Example:
A simple example is the mitotic spindle during cell division, where microtubules attach to chromosomes via kinetochores and separate sister chromatids into daughter cells, and in neurons, microtubules transport synaptic vesicle precursors along axons, demonstrating their role in intracellular trafficking.
Reason:
Microtubules are essential for cell division and intracellular organization, and their dysfunction is linked to neurodegenerative diseases like Alzheimer's, cancer, and ciliopathies, making them important targets for chemotherapy drugs like taxol and colchicine, and for understanding cellular mechanics and diseases.
📝 All Microtubules structure and function MCQs
Q1. A cell is treated with a compound that prevents tubulin subunits from assembling into microtubules. Which cellular process would be most directly disrupted first?
📖 Explanation: Microtubules form the spindle structures that organize and separate chromosomes during cell division. Blocking tubulin assembly prevents proper spindle formation, so chromosome segregation is directly impaired. DNA replication, translation, and glycolysis depend on different molecular machinery.
Q2. Which statement best explains why microtubules can function as intracellular transport tracks rather than merely structural elements?
📖 Explanation: Microtubules possess structural polarity, creating distinct ends that help motor proteins move directionally. Kinesins and dyneins use these tracks to transport vesicles and other cargo. Thus, polarity and motor-protein interaction explain their transport role.
Q3. A researcher observes that a cell has normal actin organization but severely reduced microtubule formation. Which observation would provide the strongest evidence that the defect specifically affects microtubule-dependent transport?
📖 Explanation: Long-range vesicle movement commonly depends on microtubule tracks and associated motor proteins. If microtubules are disrupted, vesicles may accumulate near their origin because directional intracellular transport is impaired, while actin-dependent processes can remain relatively normal.
Q4. A drug-treated cell contains many free tubulin subunits but very few long microtubule polymers. Which interpretation is most reasonable?
📖 Explanation: The presence of abundant free tubulin indicates that tubulin production can still occur, but polymer formation is failing. A compound that blocks polymerization or destabilizes existing microtubules would therefore produce this pattern without requiring reduced tubulin synthesis.
Q5. A mutation makes microtubules unusually stable so that they rarely undergo disassembly. A dividing cell nevertheless fails to segregate chromosomes efficiently. Why can excessive stability cause this outcome?
📖 Explanation: Chromosome segregation requires dynamic remodeling of spindle microtubules so that attachments can form, correct errors, and generate appropriate movements. Excessive stability can restrict this remodeling, causing faulty chromosome attachments or impaired spindle behavior despite abundant microtubules.
Q6. A scientist compares two cells. Cell X has normal tubulin concentration but defective motor proteins, whereas Cell Y has reduced tubulin polymerization but normal motors. Which result would best distinguish the two defects?
📖 Explanation: Cell X can still assemble microtubule tracks because tubulin polymerization is normal, but defective motors prevent effective cargo movement. Cell Y has impaired track formation, so even functional motors have fewer microtubule pathways available for long-range transport.
Q7. A graph shows that microtubule polymer mass rises rapidly during the first 10 minutes, remains nearly constant for 15 minutes, and then decreases sharply. Which interpretation best matches this pattern?
📖 Explanation: An increase in polymer mass indicates net assembly, while a plateau indicates approximate balance between assembly and disassembly. The later decrease indicates that disassembly exceeds assembly. This interpretation connects the graph to the dynamic behavior of microtubule polymers.
Q8. A cell suddenly loses functional microtubules but retains intact plasma membranes and actin filaments. Which combination of effects is most plausible?
📖 Explanation: Microtubules participate in chromosome movement through the mitotic spindle and provide tracks for long-distance intracellular transport. Their loss therefore affects both processes. Glycolysis, DNA synthesis, membrane integrity, and ribosomal peptide formation rely primarily on other systems.
Q9. A student claims, 'If a cell contains more tubulin protein, it must necessarily contain more microtubule polymers.' Which observation most strongly challenges this reasoning?
📖 Explanation: Total tubulin concentration does not automatically equal polymerized tubulin concentration. Tubulin can exist as soluble subunits, and polymer formation depends on cellular conditions and regulatory mechanisms. Therefore, increased tubulin protein does not necessarily mean increased microtubule mass.
Q10. A researcher increases the concentration of free tubulin while maintaining conditions that strongly favor microtubule disassembly. What is the most defensible prediction?
📖 Explanation: Polymer formation depends not only on the amount of available tubulin but also on conditions controlling assembly and disassembly. If the cellular environment strongly favors disassembly, increasing tubulin alone may not produce a sustained increase in microtubule polymers.
Q11. A cell shows normal microtubule formation, but vesicles fail to move efficiently toward a particular cellular region. Further testing shows that one motor-protein class is defective. What conclusion is best supported?
📖 Explanation: Microtubules provide tracks, but motor proteins generate directed movement of cargo along those tracks. Therefore, normal microtubule formation does not guarantee normal transport. A defective motor can selectively disrupt movement toward regions served by that motor.
Q12. Two drugs are tested. Drug A prevents microtubule assembly, while Drug B causes excessive stabilization of existing microtubules. Both reduce successful cell division. What shared principle explains the result?
📖 Explanation: Both insufficient assembly and excessive stabilization can disrupt cell division because spindle microtubules must be dynamically organized. Successful chromosome segregation requires appropriate formation, attachment, remodeling, and movement rather than merely having either many or few microtubules.
Q13. A hypothetical cell has a microtubule network whose assembly rate is twice its disassembly rate, but a mutation later makes the disassembly rate four times the original value while assembly remains unchanged. What is the most likely consequence after the mutation?
📖 Explanation: Initially, assembly is greater than disassembly, producing net polymer growth. After the mutation, disassembly becomes four times its original rate while assembly remains unchanged, making disassembly greater than assembly. The polymer population therefore tends to shrink over time.