Cytoskeleton Structure and Functions
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Questions and Answers

What is the primary function of microtubules in the cytoskeleton?

  • Maintaining cell shape and mechanical support
  • Cell movement and chromosome separation (correct)
  • Muscle contraction
  • Connecting with the Extracellular Matrix

    • What is the approximate diameter of microtubules?

  • 25 nm (correct)
  • 50 nm
  • 7-9 nm
  • 10 nm

    • What is the monomeric unit of actin microfilaments?

  • Tubulin
  • G-actin (correct)
  • Kinesin
  • F-actin

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

    <p>ATP hydrolysis</p> Signup and view all the answers

    What is the percentage of cellular protein comprised of actin microfilaments?

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

    What is the orientation of monomers in the formation of actin microfilaments?

    <p>Head to Tail</p> Signup and view all the answers

    What is the characteristic of actin microfilaments in terms of their structure?

    <p>Dynamic and flexible</p> Signup and view all the answers

    What is the typical structure of actin microfilaments?

    <p>Two tightly wound chains</p> Signup and view all the answers

    What is the fundamental unit of the intermediate filament?

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

    What is the consequence of mutations in keratins?

    <p>Epidermolysis bullosa simplex (EBS)</p> Signup and view all the answers

    What is the structure composed of 13 parallel protofilaments?

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

    How do intermediate filaments interact with the cytoskeleton?

    <p>They link actin microfilaments and microtubules</p> Signup and view all the answers

    What is the consequence of abnormal expression of neurofilamins?

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

    What is a characteristic of subunit exchange in intermediate filaments?

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

    What is the function of villin in microvilli?

    <p>Forms parallel bundles of actin</p> Signup and view all the answers

    What is the role of gelsolin in microvilli?

    <p>Cuts and binds (+) end of actin</p> Signup and view all the answers

    What is the process by which cells move across the extracellular matrix?

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

    What is the function of lamins in cells?

    <p>Supporting nuclear structure</p> Signup and view all the answers

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

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

    What is the function of filamin in microvilli?

    <p>Creates a mesh of actin</p> Signup and view all the answers

    What is the function of thymosin β4 in microvilli?

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

    What is the process by which a ring of actin forms in the centre of a cell?

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

    What is the typical diameter of a microtubule?

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

    What is the function of kinesin motor proteins in the cell?

    <p>To move cargo towards the (+) end of microtubules</p> Signup and view all the answers

    What is the role of the centrosome in spindle formation during mitosis?

    <p>To initiate spindle formation</p> Signup and view all the answers

    What is the function of dynein motor proteins in the cell?

    <p>To move cargo towards the (-) end of microtubules</p> Signup and view all the answers

    Which of the following is a congenital condition characterized by organs in a mirrored position?

    <p>Situs inversus</p> Signup and view all the answers

    What is the purpose of the basal body in cilia?

    <p>To facilitate the movement of components within cilia</p> Signup and view all the answers

    What is the effect of colchicine on microtubules?

    <p>It prevents microtubule formation</p> Signup and view all the answers

    What is the function of microtubules in the structure of organelles?

    <p>To stabilise the structure of organelles</p> Signup and view all the answers

    What is the role of microtubules in the movement of cargo within cells?

    <p>To facilitate the movement of cargo along their length</p> Signup and view all the answers

    What is the role of primary cilia in Autosomal Dominant Polycystic Kidney Disease (ADPKD)?

    <p>Primary cilia are associated with abnormal function of polycystin-1 and -2</p> Signup and view all the answers

    What is the function of microtubules in the cell?

    <p>To dynamically change the structure of cells</p> Signup and view all the answers

    What is the type of junction that connects cells to the extracellular matrix (ECM)?

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

    What is the effect of Taxol on microtubules?

    <p>It stabilizes microtubules</p> Signup and view all the answers

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

    <p>They connect junctions between cells to the cell cytoskeleton</p> Signup and view all the answers

    Study Notes

    Cytoskeleton

    • A network of protein filaments in a cell with a range of functions, including:
      • 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 a globular protein (G-actin) containing an ATP-binding domain
    • G-actin polymerizes to microfilament (F-actin) via ATP hydrolysis
    • Polymerization involves non-covalent interactions, with monomers in Head-to-Tail orientation
    • Each microfilament has a (+) and (-) end, and monomers can be added at both ends
    • Dynamic structure, with length dependent on relative rate of loss and gain of G-actin monomers
    • Actin microfilaments are usually present as two tightly wound chains
    • Comprise approximately 5% of cellular protein

    Function of Actin

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

    Actin Binding Proteins

    • G-actin binding proteins:
      • Thymosin β4: inhibits polymerization
    • Cross-linking proteins:
      • Villin: parallel bundles in microvilli
      • Filamin: joining at angles to create a mesh
    • Severing:
      • Gelsolin: cuts and binds (+) end; the other part depolymerizes – "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, requiring ATP hydrolysis
    • Movement within cells:
      • Cytokinesis: ring of actin forms in the center of the cell, anchors to the plasma membrane, and myosin contraction constricts the cell
    • Movement of cells:
      • Lamellipodia-mediated cell movement across the extracellular matrix (ECM)

    Intermediate Filaments (IF)

    • Polymers of individual IF proteins, 10nm in diameter
    • Different IF proteins in different cell types:
      • Epithelial cells: keratin(s) for physical support and external structures
      • Axons: neurofilamin(s) for structural arrangement of axons
      • Universal (nuclear): lamins A, B, C for supporting nuclear structure
    • Usually stable and not dynamic, except for lamins during mitosis

    Formation of IF Polymer

      1. Intermediate filament protein (monomer)
      1. Helical dimer
      1. Two dimers combine to form a tetramer (the fundamental unit of the IF)
      1. Tetramers link in a staggered formation and end-to-end to form the filament
      1. Subunit exchange is slow but occurs throughout the length of the filament

    Microtubules

    • Tubulin monomer is a heterodimer of α-tubulin and β-tubulin
    • Protofilament has a (-) and (+) end
    • Monomers can be rapidly added and removed from both (+) and (-) ends
    • 13 parallel protofilaments arranged in a hollow tube, approximately 25nm diameter

    Microtubule-Organizing Centre (MTOC)

    • Usually has one end attached to a MTOC
    • One MTOC associated with the nucleus
    • Microtubules "grow out" from MTOC until reaching the destination and then are stabilized

    Function of Microtubules

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

    Mitotic Spindle

    • Spindle consists of microtubules
    • Spindle formation initiated from the centrosome (a type of Microtubule-Organizing Centre)
    • Centrosomes contain a pair of centrioles
    • Centrioles contain stable fused microtubules
    • Centrosomes form at two poles of the cell
    • Kinetochore microtubules attached to the centromere of the chromatid
    • Aster microtubules attach the centrosome to the cell membrane

    Cilia

    • Membrane-bound hair-like extensions
    • Microtubules are the central support
    • MTOC is called Basal Body, located close to the membrane
    • Microtubules facilitate the movement of components up and down within cilia
    • All cells have a single primary cilium
    • Disassembled during mitosis
    • Specialized types:
      • Stereocilia in the inner ear for sound detection
      • Motile cilia in respiratory/lung ciliated epithelia for beating and moving fluid around

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    Description

    This quiz covers the composition and functions of the cytoskeleton, including actin microfilaments, intermediate filaments, and microtubules. Learn about the different types of filaments and their roles in maintaining cell shape, intracellular transport, and more.

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