Cytoskeleton Practice Questions

Correct answer: Actin monomers

Microfilaments, also known as actin filaments, are composed of globular actin (G-actin) monomers that polymerize to form filamentous actin (F-actin). They are the thinnest components of the cytoskeleton.

Correct answer: GTP

Microtubules grow by adding GTP-bound tubulin dimers; however, hydrolysis of GTP to GDP by beta-tubulin weakens the affinity between dimers. This leads to rapid depolymerization, or 'catastrophe,' if the GTP cap is lost.

Correct answer: Microfilaments

The contractile ring is composed of actin microfilaments and myosin II motor proteins. As the actin filaments slide past each other, the ring tightens, eventually pinching the parent cell into two daughter cells.

Correct answer: Nuclear lamins

Nuclear lamins form a fibrous meshwork called the nuclear lamina on the inner surface of the nuclear envelope. They help organize chromatin and maintain the shape of the nucleus.

Correct answer: Dynein

Cytoplasmic dyneins are minus-end directed motors that transport vesicles and organelles toward the cell center. Kinesins, conversely, are typically plus-end directed motors that move cargo toward the cell periphery.

Correct answer: Cilia and flagella

Eukaryotic cilia and flagella contain an axoneme with nine peripheral doublet microtubules surrounding two central singlet microtubules (9+2 arrangement). Centrioles and basal bodies instead follow a 9+0 triplet arrangement and lack the central pair. Microvilli are supported by actin microfilaments, not a 9+2 microtubule array.

Correct answer: Myosin, which binds actin and uses ATP

Myosin is the motor protein that directly interacts with actin filaments. In muscle cells, myosin II heads bind to actin and undergo ATP-driven conformational changes that generate the sliding force responsible for sarcomere shortening. Dynein and kinesin are microtubule-based motors, while troponin regulates actin-myosin interaction but is not itself a motor protein.

Correct answer: Intermediate filaments do not require ATP or GTP for assembly

Unlike microtubules (GTP-dependent) and microfilaments (ATP-dependent), intermediate filaments lack structural polarity and do not require nucleotide hydrolysis for polymerization. Their primary role is providing mechanical tensile strength to the cell, not facilitating transport or motility.

Correct answer: Tubulin polymerization

Colchicine binds to tubulin dimers and prevents their assembly into microtubules. This disrupts the formation of the mitotic spindle, arresting cells in metaphase.

Correct answer: Nexin links, which connect adjacent doublets

Nexin links connect adjacent outer doublet microtubules in the axoneme. By providing elastic resistance to the sliding generated by dynein arms, nexin converts sliding motion into the bending motion characteristic of ciliary and flagellar beating.

Correct answer: Epithelial tissue

Keratins are the characteristic intermediate filaments of epithelial cells, providing mechanical integrity to the skin, hair, and nails. Mutations in keratin genes can lead to skin blistering diseases.

Correct answer: It templates microtubule nucleation at the centrosome

The gamma-tubulin ring complex (gamma-TuRC) is concentrated at the centrosome and serves as the nucleation template from which alpha/beta-tubulin dimers begin to polymerize. It defines the minus end of nascent microtubules and ensures they grow outward toward the cell periphery.

Correct answer: Microfilaments

The core of each intestinal microvillus is a bundle of parallel actin microfilaments cross-linked by villin and fimbrin. This rigid actin bundle extends from the tip of the microvillus down into the terminal web, providing structural support for nutrient absorption.

Correct answer: Subunit loss at the minus end matches subunit gain at the plus end

Treadmilling occurs when the rate of G-actin addition at the barbed (plus) end exactly equals the rate of loss at the pointed (minus) end. The filament maintains a constant length while effectively translocating, a process important for cell motility and cytoplasmic reorganization.

Correct answer: Basal body

Basal bodies are structures located at the base of cilia and flagella. They are derived from centrioles and serve as the nucleation site for the growth of axonemal microtubules.

Correct answer: Class IV (Neurofilaments)

Neurofilaments (NF-L, NF-M, and NF-H subunits) belong to Class IV intermediate filaments. They are the most abundant structural components of the axon and their density determines axon diameter, which in turn influences the speed of nerve impulse conduction.

Correct answer: Actin, undergoing sol-gel polymerization

Amoeboid movement depends on the sol-gel transformation of the cytoplasm driven by actin dynamics. Actin polymerizes into a rigid gel network at the leading edge (ectoplasm) and depolymerizes into a fluid sol at the trailing end (endoplasm), enabling the characteristic flowing movement.

Correct answer: The plus end exposes beta-tubulin; the minus end exposes alpha-tubulin

Microtubules are structurally polar. The faster-growing plus end exposes beta-tubulin subunits, while the slower-growing (or anchored) minus end exposes alpha-tubulin. This polarity dictates the direction of motor protein movement: kinesins move toward the plus end and dyneins toward the minus end.

Correct answer: Gelsolin

Gelsolin is an actin-binding protein that severs F-actin filaments in a calcium-dependent manner. By breaking the filaments, it caps the newly formed plus ends and helps transition the cytoplasm from a gel state to a sol state.

Correct answer: Intermediate filaments

Intermediate filaments, such as keratins in epithelial cells, are anchored to the plasma membrane at desmosomes (linking adjacent cells) and hemidesmosomes (linking cells to the basal lamina). This cytoskeletal attachment provides epithelial tissues with the tensile strength to resist mechanical stress.

Correct answer: A linear chain of alpha- and beta-tubulin heterodimers

Microtubules are composed of 13 protofilaments arranged in a hollow cylinder. Each protofilament is a head-to-tail polymer of alpha-tubulin and beta-tubulin heterodimers.

Correct answer: Axonemal dynein

Axonemal dynein arms attached to the A-tubule of one doublet walk along the B-tubule of the adjacent doublet. Because the doublets are anchored by flexible protein links, this sliding is converted into bending.

Correct answer: Intermediate filaments

Unlike actin filaments and microtubules, which have distinct plus and minus ends due to the orientation of their subunits, intermediate filaments are assembled from antiparallel tetramers, resulting in a non-polar fiber.

Correct answer: Catastrophe

Catastrophe occurs when the GTP cap at the plus end of a microtubule is lost due to hydrolysis, leading to rapid depolymerization. The reverse process, where a shrinking microtubule starts growing again, is called rescue.

Correct answer: Centrosome

The centrosome is the main MTOC in animal cells. It consists of two centrioles surrounded by pericentriolar material (PCM), which contains the gamma-tubulin ring complexes that nucleate microtubule growth.

Correct answer: Arp2/3 complex

The Arp2/3 complex binds to the side of an existing actin filament and initiates the growth of a new branch at a 70-degree angle. This is essential for creating the branched actin networks seen in lamellipodia.

Correct answer: Mesenchymal cells

Vimentin is the most widely distributed intermediate filament protein and is a characteristic marker of mesenchymal cells, including fibroblasts, endothelial cells, and white blood cells.

Correct answer: Kinesin

Kinesins are motor proteins that typically move toward the plus end of microtubules. In axons, microtubule plus ends point toward the synaptic terminal, making kinesin responsible for anterograde transport.

Correct answer: Paclitaxel

Paclitaxel (marketed as Taxol) binds to microtubules and stabilizes them, preventing the dynamic changes necessary for spindle function during mitosis. In contrast, Vincristine and Colchicine cause microtubule depolymerization.

Correct answer: Keratin

Keratins form dense networks in epithelial cells (keratinocytes). These filaments anchor to desmosomes, distributing mechanical stress across the tissue and preventing cellular rupture.

Correct answer: Inorganic phosphate is released from the myosin head

The release of inorganic phosphate (Pi) triggers the conformational change in the myosin head known as the power stroke, which pulls the actin filament toward the center of the sarcomere.

Correct answer: It severs actin filaments and remains capped on the plus end

Gelsolin is a calcium-dependent protein that severs actin filaments, effectively liquefying actin gels. It stays bound to the plus end of the newly severed filament to prevent further growth.

Correct answer: ~7 nm

Microfilaments (actin filaments) are the thinnest cytoskeletal elements at ~7 nm. Intermediate filaments are ~10 nm, and microtubules are the thickest at ~25 nm.

Correct answer: Desmin

Desmin is the primary intermediate filament of muscle cells. It forms a scaffold around the Z-discs, connecting them to each other and to the sarcolemma, ensuring coordinated contraction.

Correct answer: The rate of subunit addition equals the rate of loss

At the critical concentration, the system is at a steady state where the rate of polymerization and depolymerization are equal. Above Cc, filaments grow; below Cc, they shrink.

Correct answer: Nebulin

Nebulin is a giant protein that wraps around the actin filament in the sarcomere. Its length corresponds exactly to the length of the thin filament, acting as a structural template.

Correct answer: They stabilize microtubules in the axon

Tau is a Microtubule-Associated Protein (MAP) that stabilizes axonal microtubules. Hyperphosphorylation of Tau leads to the formation of neurofibrillary tangles, a hallmark of Alzheimer's disease.

Correct answer: It has a higher affinity for ATP-bound actin monomers

The plus end (barbed end) of actin grows 5-10 times faster than the minus end (pointed end) because it has a significantly higher affinity for ATP-actin subunits.

Correct answer: Laminopathies

Laminopathies are a group of rare genetic disorders caused by mutations in nuclear lamins. Examples include Hutchinson-Gilford Progeria Syndrome (premature aging) and certain types of cardiomyopathy.

Correct answer: Spindle microtubules

Kinetochore microtubules attach to the chromosomes and shorten during anaphase A, while polar microtubules push the spindle poles apart during anaphase B, both facilitating chromosome segregation.