Cell Biology: Cytoskeleton Structure and Function
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Questions and Answers

What is the approximate diameter of actin microfilaments?

  • 15-17 nm
  • 7-9 nm (correct)
  • 10-12 nm
  • 5-7 nm

    • What is the process involved in the polymerization of G-actin to microfilament (F-actin)?

  • ATP hydrolysis (correct)
  • GTP hydrolysis
  • ATP synthesis
  • Phosphorylation

    • What is the orientation of monomers in the head-to-tail arrangement during actin microfilament formation?

  • Head-to-tail (correct)
  • Head-to-head
  • Side-by-side
  • Tail-to-tail

    • What percentage of cellular protein is composed of actin microfilaments?

    <p>5%</p> Signup and view all the answers

    What is the function of actin microfilaments in muscle contraction?

    <p>Covered in 'Skeletal Muscle' lecture</p> Signup and view all the answers

    What is the role of intermediate filaments in the cytoskeleton?

    <p>Tissue-specific function</p> Signup and view all the answers

    What is the diameter of microtubules?

    <p>25 nm</p> Signup and view all the answers

    What is the function of cytoskeleton in cell structure?

    <p>Maintaining cell shape</p> Signup and view all the answers

    What is the fundamental unit of an intermediate filament?

    <p>Tetramer</p> Signup and view all the answers

    What is a characteristic of subunit exchange in intermediate filaments?

    <p>It occurs slowly but throughout the length of the filament</p> Signup and view all the answers

    Which protein is involved in linking intermediate filaments to actin microfilaments and microtubules?

    <p>Plectin</p> Signup and view all the answers

    What is a disease caused by mutations in neurofilamins?

    <p>Amyotrophic lateral sclerosis</p> Signup and view all the answers

    What is a characteristic of microtubules?

    <p>They are composed of 13 parallel protofilaments</p> Signup and view all the answers

    What is the term for the ends of a protofilament?

    <p>Plus and minus ends</p> Signup and view all the answers

    What is the function of thymosin β4 in actin binding proteins?

    <p>Inhibits polymerisation</p> Signup and view all the answers

    Which protein is responsible for creating a mesh in actin microfilaments?

    <p>Filamin</p> Signup and view all the answers

    What is the result of gelsolin cutting and binding to the (+) end of actin microfilaments?

    <p>Depolymerisation of the other part</p> Signup and view all the answers

    What is the function of myosin in non-muscle cells?

    <p>Allowing cell movement</p> Signup and view all the answers

    What is the function of lamellipodia in cell movement?

    <p>Mediating cell movement across the extracellular matrix</p> Signup and view all the answers

    What is the function of intermediate filaments in epithelial cells?

    <p>Providing physical support and external structures</p> Signup and view all the answers

    What is unique about lamins compared to other intermediate filaments?

    <p>They are exceptional in being stable and dynamic</p> Signup and view all the answers

    What is the diameter of intermediate filaments?

    <p>10nm</p> Signup and view all the answers

    What is the underlying cause of situs inversus?

    <p>Abnormal function of primary cilia in the embryo</p> Signup and view all the answers

    Which of the following is a consequence of disrupted microtubule formation?

    <p>Interference with cell division</p> Signup and view all the answers

    What is the role of actin microfilaments in cell-cell adhesion?

    <p>Connection to the cell cytoskeleton through desmosomes</p> Signup and view all the answers

    Which anti-cancer chemotherapeutic agent stabilizes microtubules?

    <p>Taxol</p> Signup and view all the answers

    What is the consequence of mutated proteins associated with primary cilia function?

    <p>Formation of kidney cysts</p> Signup and view all the answers

    What is the role of cytoskeleton in cell-cell adherence?

    <p>Formation of desmosomes</p> Signup and view all the answers

    What is the primary function of the Microtubule-Organising Centre (MTOC) in a cell?

    <p>To facilitate the assembly of microtubules at the spindle poles</p> Signup and view all the answers

    Which of the following motor proteins is responsible for moving cargo towards the cell periphery?

    <p>Kinesin</p> Signup and view all the answers

    What is the function of the centrioles in the centrosome?

    <p>To contain stable fused microtubules</p> Signup and view all the answers

    What is the primary function of microtubules in the structure of cilia?

    <p>To provide a central internal support for the cilium</p> Signup and view all the answers

    What is the function of the kinetochore microtubules in the spindle?

    <p>To attach to the centromere of chromatids</p> Signup and view all the answers

    What is the rate of movement of vesicles along microtubules in axons?

    <p>10 cm per day</p> Signup and view all the answers

    What is the function of the Basal Body in cilia?

    <p>To act as a Microtubule-Organising Centre (MTOC)</p> Signup and view all the answers

    What is the function of actin filaments in stereocilia?

    <p>To keep the stereocilia rigid</p> Signup and view all the answers

    Study Notes

    Cytoskeleton

    • Network of protein filaments with various functions:
      • Connection with Extracellular Matrix (ECM)
      • Maintaining cell shape
      • Intracellular transport
      • Cytokinesis
      • Chromosome separation
      • Cell movement

    Composition of Cytoskeleton

    • Actin microfilaments:
      • Polymers of actin, 7-9 nm diameter
    • Intermediate filaments:
      • Tissue-specific proteins, 10 nm diameter
    • Microtubules:
      • Polymers of tubulin, 25 nm diameter

    Actin Microfilaments

    • Monomer is globular protein (G-actin) containing ATP-binding domain
    • G-actin polymerises to microfilament (F-actin), process involves ATP hydrolysis
    • Polymerisation via non-covalent interactions, monomers in Head to Tail orientation, each microfilament has a (+) and (-) end
    • Dynamic structure: length depends on relative rate of loss and gain of G-actin monomers
    • Actin microfilaments are usually present as two tightly wound chains
    • Comprises ~5% of cellular protein

    Function of Actin

    • Muscle contraction
    • Mechanical support (e.g., in microvilli)
    • Maintaining cell shape
    • Cell movement

    Actin Binding Proteins

    • G-actin binding proteins (e.g., thymosin β4: inhibits polymerisation)
    • Cross-linking proteins (e.g., villin: parallel bundles in microvilli, filamin: joining at angles to create a mesh)
    • Severing proteins (e.g., gelsolin: cuts and binds (+) end; the other part depolymerises – “gel to sol”)

    Contraction in Non-Muscle Cells

    • Muscle myosin is also present in non-muscle cells
    • Interaction between myosin and actin microfilaments allows movement, requires ATP hydrolysis
    • Movement within cells (e.g., cytokinesis, lamellipodia-mediated cell movement)

    Intermediate Filaments (IF)

    • Polymers of individual IF proteins, 10 nm in diameter
    • Different IF proteins in different cell types (e.g., epithelial cells: keratin(s), axons: neurofilamin(s), universal (nuclear): lamins A, B, C)
    • Usually stable and not dynamic (exception: lamins are dynamic during mitosis)
    • Formation of IF polymer involves helical dimer, tetramer, and staggered formation of protofilaments
    • Interaction between IFs and cytoskeleton (e.g., plectin links actin microfilaments and microtubules)

    Microtubules

    • Protofilament has a (-) and (+) end
    • Monomers can be added and removed from both (+) and (-) ends
    • 13 parallel protofilaments arranged in a hollow tube, microtubules approx. 25 nm diameter
    • Microtubule-Organising Centre (MTOC): usually has one end attached to MTOC, microtubules “grow out” from MTOC until reach destination and then are stabilised
    • Assembly and disassembly of microtubule: GTP bound monomers assemble onto filament, GDP bound monomers dissociate rapidly

    Function of Microtubules

    • Dynamic scaffold
    • Spindle for chromatid separation during mitosis
    • Movement of cargo to specific locations in cell
    • Central internal support of cilia
    • Stabilise structure of cells (e.g., platelets)
    • Organise the structure of organelles (e.g., endoplasmic reticulum)

    Mitotic Spindle

    • Spindle consists of microtubules, initiated from the centrosome (type of Microtubule-Organising Centre)
    • Centrosomes contain a pair of centrioles with stable fused microtubules
    • Kinetochore microtubules attached to the centromere of chromatid, aster microtubules attach centrosome to cell membrane

    Cilia

    • Membrane-bound hair-like extensions
    • Microtubules are central support
    • MTOC is called Basal Body, located close to membrane
    • Microtubules facilitate movement of components up and down within cilia
    • All cells have single primary cilia, disassembled during mitosis
    • Specialised types: stereocilia in the inner ear – sound detection, motile cilia in respiratory/lung ciliated epithelia – beat and move fluid around

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    Description

    This quiz covers the three main components of the cytoskeleton, their assembly and disassembly, and their roles in cell structure and motility. Understand the importance of the cytoskeleton in various cellular processes.

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