Lec-5-Cytoskeleton. EASY WORD.OSR
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What are the three types of protein filaments in the cytoskeleton?

  • Intermediate filaments, Glycoproteins, Collagen
  • Microtubules, Lipid rafts, Actin filaments
  • Actin filaments, Myofilaments, Amyloid fibers
  • Intermediate filaments, Microtubules, Actin filaments (correct)

    • What is the structural composition of intermediate filaments?

  • Rigid rods composed of carbohydrates
  • Short protein segments
  • Rope-like structures made from twisted strands (correct)
  • Flexible chains of nucleic acids

    • How do protein subunits assemble into intermediate filaments?

  • Using beta-sheets to form flat sheets
  • Through a coiled coil shape of dimers and tetramers (correct)
  • By forming monomers that aggregate randomly
  • By binding with lipids to create a membrane

    • What is the primary function of intermediate filaments in cells?

    <p>Providing tensile strength</p> Signup and view all the answers

    What term describes the stable dimers formed by protein subunits in intermediate filaments?

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

    Which of the following is NOT a class of intermediate filaments?

    <p>Actin filaments</p> Signup and view all the answers

    What type of intermediate filaments are found in epithelial cells?

    <p>Keratin filaments</p> Signup and view all the answers

    Which type of filaments are known to support nerve cells?

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

    What is the formation process of the rope-like structure of intermediate filaments?

    <p>Tetramers line up and assemble side by side</p> Signup and view all the answers

    Which of the following statements is true regarding the filament structure?

    <p>Intermediate filaments are made from different protein subunits</p> Signup and view all the answers

    What direction do kinesins typically move along microtubules?

    <p>Toward the plus end</p> Signup and view all the answers

    What is the main function of dyneins in a cell?

    <p>Moving toward the minus end of microtubules</p> Signup and view all the answers

    What structure allows cilia to create movement?

    <p>Bending of their core</p> Signup and view all the answers

    The 9 + 2 arrangement of microtubules is characteristic of which structures?

    <p>Cilia and flagella</p> Signup and view all the answers

    What role do actin binding proteins play in cell function?

    <p>They stabilize actin filaments</p> Signup and view all the answers

    What is the process called where actin monomers move through the filament from plus end to minus end?

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

    What facilitates the bending motion of cilia and flagella?

    <p>Sliding forces between microtubules</p> Signup and view all the answers

    How do motor proteins determine the type of cargo they can transport?

    <p>By the binding of their tails to cell components</p> Signup and view all the answers

    What type of motion does a cilium perform during its cycle?

    <p>A whipping motion</p> Signup and view all the answers

    What is formed by actin filaments in human blood cells to support their shape?

    <p>A network of fibrous proteins</p> Signup and view all the answers

    What happens to the nuclear lamina during cell division?

    <p>It breaks down and reforms.</p> Signup and view all the answers

    What process is responsible for the disassembly and reassembly of the nuclear lamina?

    <p>Phosphorylation and dephosphorylation.</p> Signup and view all the answers

    What are tubulin dimers primarily composed of?

    <p>Alpha and beta tubulin.</p> Signup and view all the answers

    What is the structural polarity of protofilaments in microtubules?

    <p>One end is plus (beta tubulin) and the other is minus (alpha tubulin).</p> Signup and view all the answers

    During dynamic instability, what happens to microtubules?

    <p>They can switch between polymerization and depolymerization.</p> Signup and view all the answers

    How does GTP hydrolysis affect microtubule stability?

    <p>It causes instability and promotes depolymerization.</p> Signup and view all the answers

    What anchors the minus end of microtubules in animal cells?

    <p>Centrosome.</p> Signup and view all the answers

    What can stabilize a growing microtubule?

    <p>Attaching to another molecule or cell structure.</p> Signup and view all the answers

    Which of the following is NOT a characteristic of polarized animal cells?

    <p>Independent microtubule organization in each end.</p> Signup and view all the answers

    What is the primary function of motor proteins along microtubules?

    <p>To transport organelles, vesicles, and molecules.</p> Signup and view all the answers

    What initiates the process of muscle contraction in skeletal muscles?

    <p>Release of neurotransmitters from the motor nerve</p> Signup and view all the answers

    What role do Ca2+ ions play in muscle contraction?

    <p>They bind to troponin, allowing myosin to attach to actin</p> Signup and view all the answers

    How does myosin II facilitate muscle contraction?

    <p>By attaching to both actin filaments and moving them</p> Signup and view all the answers

    What structure anchors the plus ends of actin filaments in a sarcomere?

    <p>Z disc</p> Signup and view all the answers

    What is the primary function of actin-binding proteins in muscle cells?

    <p>To assist in the assembly and disassembly of actin networks</p> Signup and view all the answers

    What occurs when a muscle cell receives an electrical signal from a nerve?

    <p>Action potential causes a release of Ca2+ from the sarcoplasmic reticulum</p> Signup and view all the answers

    What is the composition of a sarcomere?

    <p>Actin and myosin filaments</p> Signup and view all the answers

    What happens to Ca2+ levels after the stimulation of a skeletal muscle by a motor nerve ends?

    <p>Ca2+ is taken back into the sarcoplasmic reticulum</p> Signup and view all the answers

    What structural characteristic gives vertebrate myofibrils a striped appearance?

    <p>Repetitive pattern of sarcomeres</p> Signup and view all the answers

    In smooth muscle, what must occur before contraction can be triggered?

    <p>Phosphorylation and dephosphorylation of myosin heads</p> Signup and view all the answers

    Study Notes

    Cytoskeleton Overview

    • Composed of three protein filament types: intermediate filaments, microtubules, and actin filaments.
    • Each filament has distinct mechanical properties and is constructed from different proteins.
    • Present in epithelial cells and almost all animal cells.

    Intermediate Filaments

    • Strong, rope-like structures providing tensile strength through twisted strands.
    • Composed of protein subunits featuring a central rod domain with unstructured ends.
    • Rod domains contain a helical structure, allowing proteins to form stable dimers via coiled-coil formations.
    • Dimers associate to form staggered tetramers, which align side by side to create the filament structure.

    Classes of Intermediate Filaments

    • Four classes exist:
      • Keratin filaments in epithelial cells.
      • Vimentin and related filaments in connective tissue, muscle, and glial cells.
      • Neurofilaments in nerve cells.
      • Nuclear lamins in the nuclear envelope.
    • Cytoplasmic types include keratin, vimentin, and neurofilaments; nuclear lamins are restricted to the nucleus.

    Nuclear Lamina Dynamics

    • Composed of lamins that disassemble during cell division, reforming afterward.
    • Controlled by the phosphorylation and dephosphorylation of lamins, affecting stability.
    • Phosphorylation alters lamin shape, weakening bonds leading to filament disassembly; dephosphorylation allows for reassembly.

    Microtubule Structure

    • Formed from tubulin dimers, consisting of alpha and beta tubulin.
    • Dimeric structure tightly bound by noncovalent interactions to create the microtubule's wall.
    • Composed of 13 protofilaments, each exhibiting polarity (plus end at beta tubulin, minus end at alpha tubulin).

    Centrosome Function

    • The organizing center for microtubules, containing centrioles and a protein matrix composed of gamma tubulin rings.
    • Microtubules grow from gamma tubulin rings, with alpha and beta tubulin dimers adding to the plus end.

    Dynamic Instability of Microtubules

    • Microtubules exhibit dynamic instability, alternating between phases of growth and rapid shrinkage.
    • Stabilization occurs when the plus end interacts with cellular structures, preventing depolymerization.
    • Growth rate is influenced by GTP hydrolysis in tubulin dimers; GTP-bound dimers promote stability, while GDP-bound ones contribute to instability.

    Cell Polarization

    • Most animal cells are polarized with distinct structural and functional differences at each end.
    • In neurons, axons and dendrites show directed microtubule polarity aiding in transport.

    Motor Proteins

    • Kinesins and dyneins are motor proteins that navigate microtubules; kinesins move toward the plus end while dyneins move toward the minus end.
    • Each motor protein possesses a tail for cargo binding and ATP binding heads for directional movement.

    Cilia and Flagella Motility

    • Cilia perform whip-like motions to transport fluids or propel cells; beat cycles consist of power and recovery strokes.
    • Flagella, longer than cilia, propel entire cells, such as sperm, through fluid.
    • Both structures feature a unique 9+2 arrangement of microtubules.

    Microtubule Bending Mechanism

    • Force generated by dynein molecules enables bending, as they interact with neighboring microtubules.
    • Flexible linkages between microtubules transform sliding forces into bending movement.

    Actin Filaments

    • Interact with actin-binding proteins, forming stable structures such as microvilli or creating temporary structures for cell movement.
    • Myosin motor proteins facilitate actin-based movements.

    Actin Dynamics

    • Actin filaments are inherently unstable; treadmilling occurs when monomer addition exceeds hydrolysis, causing filament movement.
    • Hydrolysis of bound nucleoside triphosphates regulates filament length, similar to microtubule dynamics.

    Structural Support in Blood Cells

    • Human blood cells contain a network of fibrous proteins like actin and spectrin, providing structural support to maintain their disc-like shape.

    Cell Crawling Mechanisms

    • Involves coordinated changes across different cell regions, relying on three key processes essential for effective cell movement.### Cell Movement
    • Protrusions at the leading edge allow cell crawling by anchoring to surfaces.
    • Actin polymerization drives forward movement through the plasma membrane, establishing new actin cortex regions.
    • Old anchorage points detach, enabling a repetitive stepwise movement.

    Actin and Myosin Dynamics

    • Actin-binding proteins facilitate filament growth at leading edges and disassembly at the rear.
    • Myosin I interacts with actin filaments, utilizing ATP for movement and vesicle transport along the filaments.
    • Head domains of myosin I always progress towards the plus ends of actin filaments.

    Myosin II Structure

    • Muscle myosin belongs to the myosin II subfamily, consisting of dimers with two ATPase heads and coiled-coil tails.
    • Clustering forms bipolar filaments that enable sliding actions on actin filaments.

    Sarcomeres and Muscle Fibers

    • Skeletal muscle fibers are multinucleated, formed by smaller cell fusion, with most cytoplasm composed of myofibrils.
    • Myofibrils consist of repeating structural units (sarcomeres) approximately 2.5 µm in length, giving muscles their striped appearance.

    Sarcomeres Composition

    • Actin (thin) and myosin (thick) filaments make up sarcomeres, with actin anchored to Z discs.
    • Myosin filaments are centrally located, with actin filaments extending from each end.

    Muscle Contraction Mechanism

    • Muscle contraction results from synchronized shortening of sarcomeres through the sliding of actin past myosin without filament length alteration.
    • Myosin heads engage with actin via a cycle of attachment, movement, and detachment triggered by nerve signals.

    Role of Calcium Ions (Ca2+)

    • Muscle contraction is initiated when motor nerve signals promote the release of Ca2+ from the sarcoplasmic reticulum into the cytosol.
    • Ca2+ levels rise trigger further interactions between actin and myosin by relieving the inhibitory effect of tropomyosin.

    Troponin and Tropomyosin Interaction

    • Tropomyosin and troponin regulate myosin binding to actin; tropomyosin prevents binding until Ca2+ alters troponin configuration.
    • Increased cytosolic Ca2+ causes these protein movements, facilitating muscle contraction initiation.

    Signal Propagation and Reversal

    • The electrical signal from the plasma membrane reaches all sarcomeres rapidly, leading to simultaneous contraction of myofibrils.
    • Elevated Ca2+ levels are transient; pumps in the sarcoplasmic reticulum restore normal levels swiftly, halting muscle contraction.

    Smooth Muscle Function

    • Smooth muscle, found in various organs, contracts involuntarily and more slowly than skeletal muscle.
    • Activation mechanisms involve enzyme-mediated phosphorylation and dephosphorylation of myosin heads, responsive to multiple signaling molecules.

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    Description

    This quiz explores the three types of protein filaments in the cytoskeleton: intermediate filaments, microtubules, and actin filaments. Learn about their mechanical properties, composition, and presence in animal cells. Test your knowledge of these essential cellular structures.

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