Cell Biology: Cytoskeleton Structure and Function
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Questions and Answers

What is the fundamental unit of an intermediate filament?

  • Monomer
  • Filament
  • Tetramer (correct)
  • Helical dimer

    • What is the result of mutations in keratins?

  • Amyotrophic lateral sclerosis (ALS)
  • Neurodegenerative disease
  • Skin cancer
  • Epidermolysis bullosa simplex (EBS) (correct)

    • What is the structure composed of 13 parallel protofilaments?

  • Intermediate filament
  • Microfilament
  • Microtubule-associated protein
  • Microtubule (correct)

    • What is the function of plectin?

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

    What is the characteristic of subunit exchange in intermediate filaments?

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

    What is the result of abnormal expression of neurofilamins?

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

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

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

    What is the role of villin in microvilli?

    <p>Cross-links actin filaments</p> Signup and view all the answers

    What is the function of gelsolin in actin filaments?

    <p>Cuts and binds (+) end, causing depolymerization</p> Signup and view all the answers

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

    <p>Interacts with actin microfilaments to allow movement</p> Signup and view all the answers

    What is the function of lamellipodia in cell movement?

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

    What is the function of intermediate filaments (IF) in epithelial cells?

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

    What is unique about lamins compared to other intermediate filaments?

    <p>They reform during mitosis</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 approximate diameter of microtubules in the cytoskeleton?

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

    What is the primary function of actin microfilaments in muscle cells?

    <p>Muscle contraction</p> Signup and view all the answers

    Which of the following is NOT a function of the cytoskeleton?

    <p>Cell signaling</p> Signup and view all the answers

    What is the name of the globular protein that monomers polymerize to form actin microfilaments?

    <p>G-actin</p> Signup and view all the answers

    What is the process involved in the polymerization of actin microfilaments?

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

    What is the orientation of monomers in an actin microfilament?

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

    What is the percentage of cellular protein that actin microfilaments comprise?

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

    What is the name of the intermediate filaments' diameter?

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

    What is the main cause of autosomal dominant polycystic kidney disease?

    <p>Mutated proteins associated with abnormal function of primary 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 hemidesmosomes?

    <p>To connect cells to the ECM</p> Signup and view all the answers

    What is the result of defective cilia-mediated movement of growth factors in the embryo?

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

    What is the function of desmosomes?

    <p>To connect cells to each other</p> Signup and view all the answers

    What is the effect of taxol on microtubules?

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

    What is the typical diameter of a microtubule?

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

    Which motor protein moves cargo towards the (-) end of a microtubule?

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

    What is the primary function of the centrioles in the centrosome?

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

    What is the main function of the mitotic spindle?

    <p>To separate chromatids during mitosis</p> Signup and view all the answers

    Which type of cilia is involved in sound detection in the inner ear?

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

    What is the role of the Basal Body in cilia?

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

    What is the function of GTP-bound monomers in microtubule assembly?

    <p>To assemble onto the microtubule filament</p> Signup and view all the answers

    What is the function of the aster microtubules in the mitotic spindle?

    <p>To attach the centrosome to the cell membrane</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 basics of cytoskeleton including its structure, assembly, and disassembly, as well as its importance in cell structure and motility. It's suitable for students of medical science and biochemistry.

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