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Quizzes > Science

Cytoskeleton Quiz: Identify the Smallest Cytoskeletal Elements

Quick, free cell skeleton quiz to test your knowledge. Instant results.

Editorial: Review CompletedCreated By: Shandra MutchieUpdated Aug 24, 2025
Difficulty: Moderate
2-5mins
Learning OutcomesCheat Sheet
Paper art illustration stylized microtubules actin filaments cytoskeleton components coral background quiz text

This quiz helps you spot the smallest parts of the cytoskeleton and tell microtubules, microfilaments, and intermediate filaments apart. To round out your cell biology, check out our organelle functions quiz, explore microfilaments in cell motility, or test division knowledge with the cytokinesis true or false quiz.

What is the basic building block of microtubules?
Alpha-beta tubulin heterodimer
Keratin filament
G-actin monomer
Myosin II filament
Microtubules are polymers of ?- and ?-tubulin subunits that form a heterodimer, which is the smallest building block. These dimers assemble end-to-end to create protofilaments, and 13 protofilaments associate laterally to form a hollow microtubule. The heterodimer structure is essential for GTP binding and dynamic instability. .
What is the monomeric unit of microfilaments?
Tubulin dimer
Dynein motor
G-actin
F-actin
Actin filaments (microfilaments) are polymers of globular actin (G-actin) monomers. G-actin binds ATP and assembles head-to-tail into filamentous actin (F-actin). This assembly underlies many processes like muscle contraction and cell motility. .
Which protein forms the third major component of the cytoskeleton, besides microtubules and microfilaments?
Intermediate filament proteins
Spectrin
Kinesin
Clathrin
Intermediate filaments are polymers of various proteins like vimentin, keratins, and lamins, forming the third major cytoskeleton element. They provide mechanical strength and structural integrity. Unlike microtubules and actin filaments, they are more stable and lack polarity. .
Which component is necessary for actin polymerization nucleation?
Arp2/3 complex
Katanin
Dynein
Formin homology 2 alone
The Arp2/3 complex nucleates branched actin filaments by binding existing filaments and creating a new branch. It imitates actin's barbed end structure for nucleation. This branching is critical for lamellipodia formation and cell motility. .
What nucleotide is bound by tubulin dimers during polymerization?
UTP
GTP
CTP
ATP
Tubulin ?- and ?-subunits bind GTP; the ?-tubulin GTP is hydrolyzed after assembly, driving dynamic instability. GTP-bound tubulin favors polymerization, while GDP-bound favors depolymerization. This GTPase activity underlies microtubule dynamics. .
Which end of an actin filament grows faster?
Barbed (plus) end
Neither end
Both ends equal
Pointed (minus) end
Actin filaments are polarized with a fast-growing barbed (plus) end and a slower pointed (minus) end. ATP-actin adds preferentially at the barbed end, driving elongation. The polarity is critical for directional cell movement. .
What size are individual actin monomers approximately?
8 kDa (~1 nm)
200 kDa (~30 nm)
110 kDa (~25 nm)
42 kDa (~5.5 nm)
G-actin has a molecular mass of ~42 kDa and a diameter of about 5.5 nm. This size allows it to polymerize into helical filaments. Structural analyses confirm these dimensions. .
Which motor protein moves toward the microtubule plus end?
Katanin
Dynein
Myosin V
Kinesin-1
Kinesin-1 is a plus-end - directed microtubule motor that transports vesicles and organelles toward the cell periphery. Dynein moves to the minus end. Directionality is defined by the motor domain structure. .
Which cellular structure nucleates most microtubules in animal cells?
Centrosome
Mitochondrion
Golgi apparatus
Ribosome
The centrosome contains ?-tubulin ring complexes that serve as templates for microtubule nucleation. It organizes microtubules in interphase and mitosis. Other sites can nucleate microtubules, but the centrosome is primary. .
What is the diameter of a microtubule?
10 nm
7 nm
25 nm
50 nm
Microtubules have an external diameter of ~25 nm formed by 13 protofilaments. The hollow lumen has a diameter of ~15 nm. These dimensions contribute to mechanical rigidity. .
Which protein severs actin filaments?
Formin
Arp2/3
Gelsolin
Profilin
Gelsolin binds to the barbed end of actin filaments and severs them in a Ca2+-dependent manner. This activity regulates filament length and turnover. It also caps the new barbed ends. .
What is the fundamental subunit of intermediate filaments?
Actin monomer
Coiled-coil dimer of IF proteins
Tubulin dimer
Spectrin tetramer
Intermediate filament proteins form parallel coiled-coil dimers that associate antiparallel in tetramers, the basic soluble subunit. These tetramers laterally pack into unit-length filaments. This assembly is distinct from microtubule and actin polymerization. .
Which accessory protein promotes actin filament elongation?
Cofilin
Kinesin
Tau
Formin
Formins nucleate and remain associated with growing barbed ends of actin filaments to facilitate rapid elongation. They help bypass the lag in spontaneous nucleation. This activity is crucial in filopodia formation. .
Which vitamin-derived molecule is required for actin polymerization?
ATP
CoA
FAD
NAD+
ATP binds to G-actin and is hydrolyzed after incorporation into F-actin, regulating polymer dynamics and stability. ATP-actin has higher affinity for filament ends. ADP-actin promotes depolymerization. .
Which intermediate filament is found in neuronal axons?
Keratin
Desmin
Vimentin
Neurofilament
Neurofilaments are a class of intermediate filaments specific to neurons, especially abundant in axons, providing tensile strength and regulation of axon diameter. They consist of NF-L, NF-M, and NF-H proteins. Their phosphorylation state affects axonal transport. .
Which protein caps the minus end of microtubules in cells?
EB1
Stu2
Katanin
?-TuRC
The ?-tubulin ring complex (?-TuRC) caps and nucleates the minus end of microtubules at the centrosome. It provides a template for polymerization of ?/?-tubulin dimers. This activity stabilizes the minus end against depolymerization. .
What phenomenon describes simultaneous addition at the barbed end and loss at the pointed end of actin filaments?
Dynamic instability
Branching
Treadmilling
Annealing
Treadmilling occurs when the polymerization rate at the barbed end equals the depolymerization rate at the pointed end, causing subunits to cycle through the filament. It is driven by ATP hydrolysis on actin. This process helps maintain filament length while producing flux. .
Which post-translational modification regulates microtubule stability?
Acetylation of ?-tubulin
Ubiquitination of dynein
Methylation of actin
Phosphorylation of desmin
Acetylation of ?-tubulin on Lys40 in the lumen correlates with stable, long-lived microtubules. It affects motor protein interactions and resistance to mechanical stress. Deacetylation by HDAC6 reverses this mark. .
What is the critical concentration in filament assembly?
Number of nuclei needed
Monomer concentration where assembly and disassembly rates are equal
Concentration of ATP required
Amount of filament per unit area
Critical concentration (Cc) is the monomer concentration at which net polymer growth is zero, as addition and loss rates balance. Below Cc, filaments shrink; above Cc, they grow. Different ends can have distinct Cc values. .
Which domain in formin proteins mediates actin nucleation?
SH3 domain
FH2 domain
PH domain
Kinase domain
The Formin Homology 2 (FH2) domain of formins dimerizes to nucleate and processively elongate actin filaments at the barbed end. It forms a ring-like structure around the filament tip. FH1 domains recruit profilin - actin complexes. .
Which protein tracks growing microtubule plus ends?
Gelsolin
EB1
Profilin
Tau
End-binding protein 1 (EB1) binds the GTP cap region of growing microtubule plus ends, regulating dynamics and recruiting other +TIP proteins. It distinguishes growing versus shrinking ends. EB1 is essential for proper microtubule function. .
Which molecule promotes actin monomer sequestration?
Cofilin
Formin
Arp2/3
Thymosin ?4
Thymosin ?4 binds G-actin and prevents its addition to filament ends, regulating the pool of polymerizable actin. This sequestration maintains proper monomer concentration for controlled polymerization. Release is countered by profilin. .
What structure anchors intermediate filaments to desmosomes?
Plakins
Spectrins
Ankyrins
Cadherins
Plakin family proteins, like desmoplakin, link intermediate filaments to desmosomal cadherins, anchoring the keratin network to cell - cell junctions. This reinforces tissue integrity under stress. Mutations cause skin blistering diseases. .
Which factor accelerates microtubule catastrophe?
Kinesin-13
Tau
CLASP
MAP4
Kinesin-13 family members bind microtubule ends and induce protofilament curling, promoting rapid depolymerization (catastrophe). They do not move along microtubules but destabilize ends. This regulates spindle dynamics. .
Which protein enhances actin filament disassembly?
Formin
Profilin
Arp2/3
Cofilin
Cofilin binds ADP-actin in filaments, inducing twist and fragmentation, which accelerates disassembly and monomer release. It is crucial for actin turnover in motile structures. Regulation occurs via phosphorylation. .
What is the role of profilin in actin dynamics?
Severs filaments
Nucleases DNA
Promotes ADP-ATP exchange on G-actin
Caps barbed ends
Profilin binds G-actin and accelerates ADP-to-ATP exchange, replenishing the pool of polymerization-competent actin. It also delivers ATP-actin to barbed ends in conjunction with formins. Profilin regulates actin assembly spatially. .
Which intermediate filament protein is expressed in muscle cells?
Neurofilament
Vimentin
Desmin
Keratin
Desmin is the muscle-specific intermediate filament that aligns Z-disks and maintains sarcomere integrity in skeletal and cardiac muscle. Mutations in desmin lead to myopathies. It forms a scaffold linking myofibrils. .
Which protein stabilizes microtubule minus ends in non-centrosomal arrays?
CAMSAP
EB3
Tau
MAP2
CAMSAP proteins bind and stabilize microtubule minus ends, promoting non-centrosomal microtubule arrays in differentiated cells. They protect minus ends from depolymerization. This is important in epithelial cell polarity. .
Which lysine residue on ?-tubulin is commonly acetylated in stable microtubules?
Lys167
Lys252
Lys40
Lys311
Lys40 of ?-tubulin, located on the luminal side, is acetylated in long-lived microtubules, correlating with stability and enhanced motor transport. The enzyme MEC-17 (?TAT1) mediates this modification. Deacetylation by HDAC6 reverses it. .
What mechanism underlies microtubule dynamic instability?
Phosphorylation cycles
GTP hydrolysis-induced conformational change
Actin crosslinking
ATP binding
Dynamic instability arises when GTP-bound tubulin adds to microtubule ends, then hydrolysis to GDP triggers protofilament curling and rapid depolymerization. The GTP cap regulates the switch between growth and shrinkage. This generates search-and-capture behavior. .
Which region of actin interacts with myosin in muscle?
Barbed end
ATP-binding cleft
Hydrophobic pocket around subdomains 1 and 3
Pointed end
Myosin motor domains bind a hydrophobic pocket formed between actin subdomains 1 and 3, mediating strong interactions during the power stroke. This interface changes affinity during nucleotide cycles. Structural studies show key contact residues. .
What structural feature distinguishes F-actin from G-actin?
Monomeric globule
Helical polymer with two strands
Triple helix
Sheet-like array
F-actin is a double-helical polymer consisting of head-to-tail G-actin subunits, forming two long strands twisted around each other. This structure underpins filament stability and interactions with binding proteins. G-actin alone is a monomeric, globular protein. .
Which molecular ruler regulates microtubule length in cilia?
Stathmin
Tau
EB1
Kinesin-2/IFT complexes
Intraflagellar transport (IFT) driven by kinesin-2 delivers building blocks to the ciliary tip, balancing assembly and disassembly to control microtubule length. The frequency and rate of IFT trains define the steady-state length. Disruption alters cilium size. .
Which factor directly catalyzes GTP exchange on tubulin heterodimers?
Tubulin exchanger protein
Guanine nucleotide exchange factor
Microtubule-associated exchangease
None; intrinsic exchange upon GDP release
Tubulin heterodimers have intrinsic ability to exchange GDP for GTP in solution without an exchange factor. This nucleotide loading occurs before assembly. No known GEF is required. .
Which subunit arrangement describes the 13-protofilament microtubule?
All A-type lattice
A B-type lattice with a seam
All B-type lattice
No lattice, random assembly
The standard microtubule has a B-type lateral lattice except at one seam where an A-type lattice occurs, due to ?-? lateral contacts. This seam affects dynamics and interactions with MAPs. Cryo-EM has visualized this seam. .
What is described by the term 'G-actin ATPase activity'?
Rapid hydrolysis in monomers
No hydrolysis occurs
Slow hydrolysis after polymerization
Hydrolysis before assembly
G-actin has low intrinsic ATPase activity that is greatly enhanced upon polymerization into F-actin, leading to ATP hydrolysis after incorporation. This delays phosphate release, generating the ADP-Pi cap. The timing influences filament stability. .
Which conformational change occurs in tubulin upon GTP hydrolysis?
Dimerization enhancement
Curvature of protofilaments
Dissociation of ?/? subunits
Straightening of protofilaments
GTP hydrolysis in ?-tubulin induces a conformational change that promotes protofilament bending and outward curling, triggering depolymerization. GTP-bound dimers favor straight protofilaments in the lattice. This switch drives dynamic instability. .
Which cofactor is required for cofilin activity regulation?
Methylation by PRMTs
Acetylation by CBP
Phosphorylation by LIM kinases
Ubiquitination by E3 ligases
Cofilin is inactivated by phosphorylation at serine residues by LIM kinases and reactivated by SSH phosphatases. This regulation controls actin turnover rates. Phospho-cofilin cannot bind F-actin. .
Which tool measures in vivo actin filament dynamics by fluorescence?
X-ray crystallography
Electron tomography
Mass spectrometry
FRAP (Fluorescence Recovery After Photobleaching)
FRAP uses photobleaching of fluorescently labeled actin followed by monitoring recovery, revealing filament turnover rates in living cells. It quantifies monomer exchange and mobility. Other methods measure structure but not live dynamics. .
Which actin nucleator creates unbranched filaments at focal adhesions?
Arp2/3
Cofilin
Capping protein
Formin mDia1
mDia1, a Diaphanous-related formin, nucleates and elongates straight actin filaments at focal adhesions, supporting stress fiber formation. It remains processively attached to barbed ends. Arp2/3, by contrast, creates branched networks. .
Which cryo-EM resolution revealed the microtubule seam structure?
10 Å
50 Å
20 Å
3.5 Å
Cryo-EM studies around 3.5 Å resolution have resolved the seam in microtubules, revealing the A-lattice discontinuity. This high resolution provided insights into lateral contacts. It deepened understanding of dynamic behavior. .
What is the role of septins in cytoskeletal organization?
Nucleate actin branches
Cap microtubule plus ends
Form diffusion barriers and scaffold filaments
Sever intermediate filaments
Septins are GTP-binding proteins that assemble into filaments acting as diffusion barriers and scaffolds, interacting with actin and microtubules. They compartmentalize membranes and support cytokinesis. Their dysfunction links to disease. .
Which advanced technique maps cytoskeletal protein interactions in living cells at nanometer resolution?
Light scattering
Transmission EM
Super-resolution microscopy (STORM/PALM)
Confocal microscopy
STORM and PALM achieve ~20 nm resolution in live or fixed cells, allowing visualization of cytoskeletal protein arrangements and interactions beyond diffraction limits. These techniques have revealed nanodomains and filament organization. Standard fluorescence cannot reach this resolution. .
Which protein family regulates microtubule severing in neurons?
Arp2/3 complex
Formins
Spastin, katanin, and fidgetin
Septins
AAA ATPases such as spastin, katanin, and fidgetin sever microtubules by removing tubulin dimers, remodeling the cytoskeleton during neuronal development and maintenance. Dysregulation leads to neurodegenerative disorders. Their activity is tightly regulated by post-translational modifications. .
Which computational model describes actin network growth at leading edges?
Random coil model
Dendritic nucleation model
Hooke's law chain model
Treadmilling-only model
The dendritic nucleation model posits that Arp2/3 - mediated branches form a dense network at the leading edge, driving membrane protrusion. Actin polymerization at barbed ends pushes the plasma membrane forward. Computational work recapitulates lamellipodial dynamics. .
Which small molecule inhibits actin polymerization by binding barbed ends?
Phalloidin
Cytochalasin D
Taxol
Nocodazole
Cytochalasin D binds to the barbed end of actin filaments, preventing monomer addition and inducing depolymerization. It's widely used to study actin dynamics in cells. Phalloidin, by contrast, stabilizes filaments. .
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Study Outcomes

  1. Identify cytoskeletal filament types -

    Recognize the three major components - microtubules, actin filaments, and intermediate filaments - and identify which are the smallest components of the cytoskeleton by their molecular dimensions.

  2. Differentiate filament structures -

    Compare the architecture and polymerization properties of microtubules versus actin filaments to understand their distinct roles in cell shape and motility.

  3. Analyze cytoskeletal protein functions -

    Answer targeted cytoskeletal proteins questions to reinforce how accessory proteins regulate filament stability, cross-linking, and dynamics.

  4. Apply knowledge in trivia format -

    Engage with cell cytoskeleton trivia that challenges your recall of filament organization, protein interactions, and functional implications.

  5. Solve structure-based quiz scenarios -

    Use insights from the cytoskeleton structure quiz and microtubules and actin quiz prompts to diagnose cellular defects and predict filament behavior in diverse contexts.

Cheat Sheet

  1. Size Hierarchy of Cytoskeletal Components -

    In most textbooks like Alberts et al., microtubules measure ~25 nm, intermediate filaments ~10 nm, and actin filaments ~7 nm, making actin filaments the smallest components of the cytoskeleton. To answer which are the smallest components of the cytoskeleton, learn the "MIC" mnemonic: Microtubules, Intermediate, then actin for descending diameter. Remembering this helps nail cytoskeleton structure quiz and cell cytoskeleton trivia questions.

  2. Actin Monomers and Polymerization -

    G-actin (globular) monomers bind ATP and polymerize head-to-tail into F-actin (filamentous) strands, a key topic in microtubules and actin quiz content. Use the phrase "G-ATP to F-ber" to recall that ATP hydrolysis drives filament growth and turnover in cells. This polymerization dynamic underlies many cytoskeletal proteins questions about assembly rates.

  3. Treadmilling Dynamics -

    Actin filaments undergo treadmilling when the plus end polymerizes faster than the minus end depolymerizes, maintaining a constant length but flux of subunits, a popular concept in cell cytoskeleton trivia. Think of a conveyor belt: new subunits added in front, old ones drop off at the rear. This dynamic process explains how cells move and change shape during migration and division.

  4. Regulatory Proteins Control Architecture -

    Proteins like formins nucleate straight actin filaments while the Arp2/3 complex nucleates branched networks; this regulation is often probed in cytoskeletal proteins questions. A simple mnemonic - "Form Branch" - links formin to linear growth and Arp2/3 to branching. Understanding these regulators is crucial for advanced cell & cytoskeleton quiz rounds.

  5. Functional Roles of Actin Filaments -

    As the smallest components of the cytoskeleton, actin filaments power cell motility, muscle contraction, and cytokinesis by interacting with myosin motors, a favorite topic in cell cytoskeleton trivia. Remember the phrase "ACT in Action" to link actin to active processes like pseudopodia formation. This functional insight reinforces why actin is central in many microtubules and actin quiz questions.

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