The Cytoskeleton Overview
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Questions and Answers

What is the outer diameter of microtubules?

  • 50 nm
  • 10 nm
  • 100 nm
  • 25 nm (correct)

    • Which of the following best describes the primary structural characteristic of intermediate filaments?

  • They are hollow and rigid.
  • They are short and branched fibers.
  • They are flexible tubes made of actin.
  • They are ropelike fibers with a diameter of about 10 nm. (correct)

    • Where are spindle microtubules primarily anchored in a dividing cell?

  • Cytoplasm around cell junctions
  • Nuclear lamina
  • Microtubule-organizing center (MTOC) (correct)
  • Actin cortex

    • What role do intermediate filaments play in epithelial tissue?

    <p>They provide mechanical strength.</p> Signup and view all the answers

    Which is NOT a type of dynamic actin structure mentioned?

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

    What is the primary protein that makes up microtubules?

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

    What feature distinguishes microtubules from actin filaments regarding their structure?

    <p>Microtubules are hollow cylinders.</p> Signup and view all the answers

    How does the structural rearrangement of large-scale cytoskeletal components occur?

    <p>It requires little extra energy as they are in a constant state of flux.</p> Signup and view all the answers

    What ratio must be identical at both ends of a simple polymerization reaction?

    <p>Koff/kon</p> Signup and view all the answers

    How does the hydrolysis of ATP affect actin polymerization?

    <p>It reduces the binding affinity of the subunits.</p> Signup and view all the answers

    What happens when the concentration of free monomers is greater than the critical concentration?

    <p>Both ends of the polymer grow.</p> Signup and view all the answers

    Which form of the subunit is typically added to the filament during polymerization?

    <p>T form carrying ATP</p> Signup and view all the answers

    What is the expected behavior of subunits at the minus end of the polymer?

    <p>Subunits always dissociate from the minus end.</p> Signup and view all the answers

    What role do bound nucleotide hydrolysis play in polymerization?

    <p>They destabilize the polymer by reducing binding affinity.</p> Signup and view all the answers

    At what point does a polymer grow until?

    <p>C = Ca</p> Signup and view all the answers

    What distinguishes the plus end from the minus end in polymer growth?

    <p>The plus end grows faster due to higher initial concentration.</p> Signup and view all the answers

    What initiates motility in cells?

    <p>An actin-dependent protrusion of the leading edge</p> Signup and view all the answers

    What structures are primarily involved in the protrusion of the leading edge of a neutrophil?

    <p>Filopodia and lamellipodia</p> Signup and view all the answers

    How does a trimer of actin molecules contribute to filament nucleation?

    <p>It forms a nucleus that allows rapid elongation</p> Signup and view all the answers

    What mechanism allows neutrophils to quickly change direction while pursuing bacteria?

    <p>Reassembly of the actin network within a pseudopod</p> Signup and view all the answers

    What occurs during filament nucleation with actin subunits?

    <p>Oligomers form more stable structures enabling rapid addition of monomers</p> Signup and view all the answers

    What role do actin filaments play in the shape and movement of cells?

    <p>They regulate cell shape and support movement through dynamic assembly</p> Signup and view all the answers

    What happens to actin filaments as neutrophils pursue bacteria?

    <p>They undergo rapid disassembly and reassembly at the leading edge</p> Signup and view all the answers

    What stabilizes the nucleation process of actin filament formation?

    <p>The formation of stable oligomers that enable growth</p> Signup and view all the answers

    What is the primary reason for the lag phase during the polymerization of tubulin?

    <p>The time taken for nucleation</p> Signup and view all the answers

    How can the lag phase in the polymerization of tubulin be altered?

    <p>By adding premade nuclei or fragments of polymerized structures</p> Signup and view all the answers

    Which phase of polymerization corresponds to the equilibrium where addition of monomers balances disassembly?

    <p>Equilibrium phase</p> Signup and view all the answers

    What distinguishes the plus end from the minus end of an actin filament or microtubule?

    <p>The plus end grows faster than the minus end</p> Signup and view all the answers

    What occurs during the lag phase of polymerization?

    <p>Nucleation of the filament</p> Signup and view all the answers

    How does the conformation of subunits affect polymerization at the ends of the filament?

    <p>It allows for faster growth at one end compared to the other</p> Signup and view all the answers

    What is the role of premade nuclei in the polymerization of tubulin?

    <p>They act as catalysts for nucleation</p> Signup and view all the answers

    What happens when the polymerization process reaches an equilibrium phase?

    <p>The rate of addition equals the rate of disassembly</p> Signup and view all the answers

    Signup and view all the answers

    Study Notes

    The Cytoskeleton

    • Cells require organization in space and interaction with their environment for proper function.
    • Cells need a correct shape, structural integrity, and the ability to change shape and move.
    • Internal components need to be rearranged as needed (growth, division, adaptation).
    • The cytoskeleton, a system of filaments, facilitates these spatial and mechanical functions.

    Types of Protein Filaments

    • The cytoskeleton's function depends on three main filament families: actin filaments, microtubules, and intermediate filaments.
    • Each filament type has distinct mechanical/biological properties and roles, but all share fundamental features.
    • These filaments work together to provide strength, shape, and movement capabilities to the cell.

    Actin Filaments

    • Actin filaments, also known as microfilaments, are helical polymers of the protein actin.
    • They have a diameter of 8 nm and can form linear bundles, networks, and gels.
    • Actin filaments are highly concentrated in the cell cortex (just beneath the plasma membrane).
    • They are involved in cell surface projections (microvilli, filopodia, lamellipodia, and pseudopodia).
    • They contribute to cell movement and exploration of its environment.

    Microtubules

    • Microtubules are long, hollow cylinders (25 nm diameter) made of the protein tubulin.
    • They are rigid and straight, frequently attached to a microtubule-organizing center (MTOC) called a centrosome.
    • Microtubules play roles in organelle positioning, intracellular transport, and forming the mitotic spindle during cell division.
    • Cilia and flagella are also composed of microtubules.

    Intermediate Filaments

    • Intermediate filaments are rope-like fibers (10 nm diameter) composed of a large family of intermediate filament proteins.
    • They form a meshwork within the nucleus (called the nuclear lamina).
    • They can span across the cytoplasm providing mechanical strength to cells.
    • They can form strong linkages between cells (e.g., skin/epithelium).

    Cytoskeletal Filaments: Dynamic Structures

    • Large-scale cytoskeletal structures can change or persist according to a cell's needs.
    • Macromolecular components of these structures are in a constant state of flux.
    • Rearrangement within a cell requires little energy when conditions change.

    Cell Division and the Cytoskeleton

    • Cells reorganize actin and microtubule filaments during division.
    • Actin filaments form contractile rings pinching the cell in two during cytokinesis.
    • Microtubule arrays are reorganized into a spindle to separate chromosomes.

    Nucleus and Cytoskeleton Interactions

    • The nuclear lamina, a meshwork of intermediate filaments, is located just beneath the inner nuclear membrane.
    • This structure adds strength and stability to the nucleus.

    Nucleation (Rate Limiting Step in Actin Filament Formation)

    • Actin subunit assembly into filaments is regulated by nucleation, which is the formation of an initial oligomer or nucleus of subunits.
    • The stable nucleus then rapidly elongates by adding more subunits.
    • Nucleation is rate limiting, as this initial step determines the overall polymerization rate.

    Polymerization On/Off Rates & Critical Concentration (Cc or Kd)

    • Subunits are added to and removed from the filament ends at specific rates (kon and koff).
    • The rate of addition is proportional to the free subunit concentration (konC).
    • Subunits' removal rate is constant (koff) and independent of free subunit concentration.
    • Equilibrium occurs when the addition and loss rates are equal, at a constant Critical Concentration.

    The Critical Concentration (Cc)

    • The critical concentration (Cc or Kd) is the concentration of free subunits at which the rate of subunit addition equals the rate of subunit loss.
    • The polymerization of a polymer stops when the concentration of free monomers reaches this critical level.

    Time Course of Polymerization

    • Polymerization of filaments like actin or microtubules follows a three-phase pattern.
      • Lag phase: characterized by initial nucleation steps.
      • Growth phase: filaments elongate rapidly.
      • Equilibrium phase: rates of addition and loss balance each other.

    Plus and Minus Ends

    • Filament ends grow at different rates (plus end grows faster than minus end).
    • The different rates at which filaments grow on different ends are due to the conformational change of subunits as they enter polymers.

    Nucleotide Hydrolysis

    • ATP (or GTP) hydrolysis occurs in actin (or tubulin) filaments after monomers associate.
    • This process affects filament stability and growth rates.

    Accessory Proteins and Motors

    • Accessory proteins regulate the length, structure, number, and attachments of filaments and their interactions within the cell.
    • These proteins convert information from signaling pathways to cytoskeletal functions enabling various cellular processes including motion.

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    Cytoskeleton 1-23 PDF

    Description

    Explore the significance of the cytoskeleton in cellular structure and function. This quiz covers the types of protein filaments, including actin filaments, microtubules, and intermediate filaments, as well as their roles in maintaining cell shape and movement. Test your knowledge of how these components interact to enable cellular processes.

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