📝 Actin filaments structure and function (13 MCQs)
📖 From Principles of Biochemistry • 1. The Foundations of Biochemistry • 13 questions available
What is Actin filaments structure and function?
Definition:
Actin filaments, also known as microfilaments, are thin, flexible protein fibers with a diameter of about 7 nm, composed of polymerized globular actin (G-actin) monomers that assemble into two helical strands (F-actin), and they play essential roles in maintaining cell shape, enabling cell motility, muscle contraction, cytokinesis, and intracellular transport by providing structural support and generating force through polymerization and depolymerization.
Working:
Actin filaments work by undergoing dynamic assembly and disassembly, where ATP-bound G-actin monomers add to the plus end faster than to the minus end, creating treadmilling, and they interact with myosin motor proteins to generate contractile forces, with the rate of polymerization described by , where is the number of subunits, and they also form networks with cross-linking proteins like filamin to create meshworks that support the plasma membrane.
Example:
A simple example is muscle contraction, where actin filaments in sarcomeres interact with myosin filaments to slide past each other, shortening the muscle fiber, and in non-muscle cells, actin filaments form the leading edge of lamellipodia during cell migration, allowing cells to crawl and explore their environment.
Reason:
Understanding actin filaments is crucial for cell biology, as they are involved in nearly all aspects of cell movement, division, and shape, and their dysregulation leads to diseases like cancer metastasis, muscular dystrophies, and immune deficiencies, making them targets for therapeutic intervention.
📝 All Actin filaments structure and function MCQs
Q1. A cell contains a population of actin filaments that rapidly changes shape while maintaining a relatively stable overall cell boundary. Which property best explains this behavior?
📖 Explanation: Actin filaments are dynamic polymers whose subunits can be added or removed in response to cellular signals. This allows cells to reorganize their internal architecture while preserving or rapidly changing their overall shape.
Q2. Which molecular building block directly polymerizes to form an actin filament?
📖 Explanation: Actin filaments are polymers assembled from individual actin protein subunits. Tubulin forms microtubules, collagen forms extracellular structural materials, and phospholipids primarily form biological membranes rather than actin polymers.
Q3. Two cells contain equal amounts of actin protein. Cell X has most of its actin incorporated into filaments, whereas Cell Y has most actin in a soluble pool. What is the strongest conclusion?
📖 Explanation: Equal total actin does not imply equal polymerization state. Cell X has more actin incorporated into filaments, while Cell Y retains more soluble subunits, indicating different balances between filament assembly and disassembly.
Q4. A migrating cell must extend its leading edge while retracting its rear. Which strategy involving actin filaments would best support this coordinated movement?
📖 Explanation: Directed movement requires spatially controlled cytoskeletal remodeling. Actin assembly can support protrusion at the leading edge, while regulated disassembly and rearrangement elsewhere permits contraction and rear retraction.
Q5. A laboratory treatment greatly reduces the availability of free actin subunits but does not directly destroy existing filaments. Which immediate cellular change is most likely?
📖 Explanation: Reducing the soluble actin pool limits the subunits available for polymerization. Existing filaments may remain temporarily, but processes requiring rapid filament assembly would be impaired before all pre-existing filaments necessarily disappear.
Q6. A researcher observes that blocking actin polymerization prevents a cell from forming a narrow protrusion, but the cell still contains many pre-existing actin filaments. What does this observation most strongly indicate?
📖 Explanation: The persistence of pre-existing filaments shows that simply having actin present is insufficient. The experiment specifically implicates filament remodeling or new assembly as an important step in protrusion formation.
Q7. A drug causes actin filaments to become unusually stable. A cell initially appears structurally supported but later loses its ability to rapidly change shape. Which explanation best connects these observations?
📖 Explanation: Actin networks provide both structural support and dynamic remodeling. Excessive stabilization can preserve existing structures but interfere with the controlled assembly and disassembly required for rapid changes in cell shape.
Q8. A student claims, "Because actin filaments provide mechanical support, they must remain permanently assembled to work." Which criticism is most accurate?
📖 Explanation: Actin filaments can provide mechanical strength while also undergoing regulated remodeling. Their biological usefulness comes partly from balancing persistence with dynamic rearrangement rather than maintaining every filament permanently.
Q9. In an experiment, the amount of polymerized actin is measured after increasing concentrations of a regulatory compound. The results are: 0 units → 20%, 2 units → 35%, 4 units → 51%, 6 units → 68%, 8 units → 69%. What is the best interpretation?
📖 Explanation: The measured polymerized fraction increases substantially as compound concentration rises and then changes very little between 6 and 8 units. This pattern suggests promotion of polymerization followed by an apparent saturation or limiting effect.
Q10. A researcher measures filament abundance under four conditions: Control = 100 units, Treatment A = 140 units, Treatment B = 55 units, Treatment C = 98 units. If the goal is to identify a treatment that most strongly promotes filament formation, which should be selected?
📖 Explanation: Treatment A produces the greatest measured amount of filament-associated actin, increasing from 100 to 140 units. Therefore, among the tested conditions, it provides the strongest evidence for enhanced filament formation.
Q11. A mutation increases actin filament assembly in one region of a cell but simultaneously prevents normal disassembly elsewhere. Which outcome is most plausible?
📖 Explanation: Actin organization depends on spatially coordinated assembly and disassembly. Enhancing assembly in one region while blocking disassembly elsewhere could create abnormal filament accumulation and reduce the flexibility required for coordinated cellular remodeling.
Q12. A scientist compares two cells. Cell A rapidly changes shape after stimulation, whereas Cell B changes shape slowly despite having similar total actin levels. Measurements show Cell A has faster turnover between soluble and filamentous actin. Which inference is best?
📖 Explanation: Similar total actin amounts can support different cellular behaviors because polymerization dynamics matter. Faster exchange between soluble and filamentous pools can allow Cell A to reorganize its actin network more rapidly after stimulation.
Q13. A hypothetical actin network gains a small number of additional filament ends while the concentration of available actin subunits remains sufficient for assembly. If regulatory conditions favor growth, what is the most reasonable prediction?
📖 Explanation: Additional filament ends provide more potential locations for incorporation of actin subunits. When sufficient subunits and favorable regulatory conditions are present, increasing available ends can enhance the capacity for filament growth and network remodeling.