Cytoskeleton and Microtubules in Cell Biology

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Questions and Answers

What role do CLASP proteins serve in relation to microtubules?

  • They initiate microtubule catastrophe.
  • They enhance microtubule disassembly.
  • They rescue microtubules from catastrophe. (correct)
  • They stabilize microtubules by preventing polymerization.

Where are the minus ends of microtubules typically anchored?

  • At the nucleus.
  • In the centrosome. (correct)
  • In the cell membrane.
  • At the cell periphery.

What distinctive feature do stable microtubules in nerve cells possess?

  • They terminate in the cytoplasm rather than being anchored. (correct)
  • They have orientation that varies in direction.
  • They are anchored at both ends in the centrosome.
  • They are exclusively formed during mitosis.

What is the primary direction of vesicle transport mediated by plus-end–directed kinesins?

<p>Toward the cell periphery. (B)</p> Signup and view all the answers

Which statement accurately describes the function of dynein and kinesin I?

<p>They serve opposite transport functions on microtubules. (A)</p> Signup and view all the answers

What is the primary role of the cytoskeleton in the context of cell movement?

<p>Generating forces for movement and positioning organelles (C)</p> Signup and view all the answers

Which process indicates that GTP hydrolysis affects microtubule stability?

<p>Dynamic instability of microtubules (A)</p> Signup and view all the answers

What occurs at the plus end of the microtubule during its growth phase?

<p>GTP-bound tubulin dimers associate (D)</p> Signup and view all the answers

Which statement best describes the dynamic instability of microtubules?

<p>Growth and shrinkage are dependent on GTP cap retention. (C)</p> Signup and view all the answers

What is the effect of depolymerases on microtubules?

<p>They initiate shrinkage by removing GTP-tubulin from the plus end. (C)</p> Signup and view all the answers

What structural component is unique to motile cilia compared to primary cilia?

<p>The presence of a central pair of microtubules (D)</p> Signup and view all the answers

Which of the following components is responsible for the sliding motion of interpolar microtubules during anaphase B?

<p>Plus-end-directed motor proteins (C)</p> Signup and view all the answers

How are the outer microtubule doublets in cilia juxtaposed to the central pair of microtubules?

<p>Joined by nexin links and radial spokes (D)</p> Signup and view all the answers

Which type of microtubule is directly responsible for attaching to kinetochores during cell division?

<p>Kinetochore microtubules (B)</p> Signup and view all the answers

What mechanism drives chromosome movement towards the spindle poles during anaphase A?

<p>Depolymerization and shortening of microtubules (D)</p> Signup and view all the answers

What is the primary role of astral microtubules in cell division?

<p>Driving the separation of spindle poles (A)</p> Signup and view all the answers

Which of the following correctly describes intermediate filaments (IFs)?

<p>They are flexible and help cells endure physical stress. (C)</p> Signup and view all the answers

How do dynein and kinesin I differ in their functional directionality?

<p>Dynein moves towards the minus end, while kinesin I moves towards the plus end. (A)</p> Signup and view all the answers

What type of cellular structure do desmosomes involve, and how do they relate to intermediate filaments?

<p>They serve as contact points between epithelial cells linked via intermediate filaments. (D)</p> Signup and view all the answers

What is the structural assembly order of intermediate filaments starting from the polypeptide level?

<p>Polypeptides assemble to form tetramers, then protofilaments, followed by filaments. (D)</p> Signup and view all the answers

Flashcards

Microtubule Rescue

Proteins like CLASP rescue microtubules from catastrophe by stopping disassembly and restarting growth.

Centrosome Function

The centrosome is a microtubule organizing center (MTOC) where microtubule minus ends are anchored.

Microtubule Organization During Mitosis

During mitosis, duplicated centrosomes separate and microtubules reorganize to form the mitotic spindle.

Microtubule Orientation in Axons and Dendrites

Stable microtubules in axons and dendrites are not anchored to the centrosome and have distinct orientation.

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Microtubule Motor Proteins: Dynein and Kinesin I

Dynein carries cargo towards the microtubule minus end (center of the cell), while kinesin I carries cargo towards the plus end (cell periphery).

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Cilia and Flagella

Microtubule-based structures that extend from the cell surface, responsible for movement.

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Mitotic Spindle

A specialized arrangement of microtubules that forms during cell division, responsible for separating chromosomes.

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Kinetochore Microtubules

Microtubules that attach to chromosomes during mitosis, pulling them apart.

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Interpolar Microtubules

Microtubules that overlap in the center of the cell during mitosis, pushing the poles apart.

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Astral Microtubules

Microtubules extending from the spindle poles towards the cell periphery, helping to anchor the spindle.

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Intermediate Filaments

A type of cytoskeletal filament that provides mechanical strength to cells, especially those subjected to physical stress. They are found in muscle cells, neurons, and epithelial cells.

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Desmosomal Cadherins

A protein that helps connect intermediate filaments to desmosomes, forming a strong link between adjacent epithelial cells.

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Hemidesmosomes

A type of cell junction that anchors intermediate filaments to the extracellular matrix (ECM), providing strength and stability.

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Plectin

A protein that links intermediate filaments to integrins, which in turn connect to the ECM.

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Keratin

A structural protein that forms intermediate filaments, particularly in epithelial cells. Mutations in keratin can lead to skin fragility and other disorders.

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What is the structure of a microtubule?

Microtubules are long, hollow cylinders formed by the polymerization of α- and β-tubulin dimers. They are essential for cell division, intracellular transport, and maintaining cell shape.

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What is the role of GTP in microtubule polymerization?

GTP (guanosine triphosphate) is a molecule that provides energy for microtubule polymerization. GTP-bound tubulin dimers are more stable and promote microtubule growth. When GTP is hydrolyzed to GDP, the tubulin dimer becomes less stable, leading to microtubule shrinkage. This dynamic instability allows microtubules to quickly assemble and disassemble, adapting to the changing needs of the cell.

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What is dynamic instability of microtubules?

Dynamic instability refers to the continuous cycle of growth and shrinkage that microtubules undergo. It is driven by the hydrolysis of GTP bound to β-tubulin after polymerization. GTP-bound tubulin promotes growth, while GDP-bound tubulin promotes shrinkage.

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How do microtubule-associated proteins (MAPs) influence dynamic instability?

Microtubule-associated proteins (MAPs) regulate microtubule dynamics. Polymerases promote growth by increasing the incorporation of GTP-bound tubulin. Depolymerases promote shrinkage by removing GTP-bound tubulin from the plus end of the microtubule.

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What are the key functions of microtubules in the cell?

Microtubules are involved in various cellular processes, including cell division, intracellular transport, and maintaining cell shape. They serve as tracks for motor proteins, which transport organelles and vesicles. Microtubules also form the spindle fibers that separate chromosomes during cell division.

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Study Notes

Cytoskeleton and Cell Motility

  • The cytoskeleton is a dynamic scaffold providing structural support to the cell.
  • It acts as a framework for positioning organelles.
  • It's a network of tracks directing the movement of materials (organelles, vesicles, mRNA).
  • It's involved in generating force for cell movement.
  • The cytoskeleton is essential for cell division machinery.
  • The cytoskeleton is composed of microfilaments, microtubules, and intermediate filaments.

Microtubules: Structure and Dynamic Instability

  • Microtubules are hollow tubes made of tubulin dimers (α-tubulin and β-tubulin).
  • Microtubules exhibit dynamic instability, alternating between growth and shrinkage phases.
  • Microtubule growth is initiated by the addition of tubulin dimers to the plus end.
  • GTP hydrolysis regulates microtubule stability; GTP-bound tubulin is more stable than GDP-bound tubulin.
  • Microtubule-associated proteins (MAPs) regulate microtubule organization and stability.
  • Polymerase increases the incorporation of GTP-bound tubulin.
  • Depolymerases remove GTP-bound tubulin.
  • CLASP proteins rescue microtubules from catastrophe.

Microtubule Organization

  • Minus ends of microtubules are anchored in the centrosome (an MTOC).
  • In interphase cells, the centrosome is located near the nucleus, and microtubules extend outward to the cell periphery.
  • During mitosis, duplicated centrosomes separate and microtubules reorganize to form the mitotic spindle.
  • Centrosomes consist of pericentriolar material surrounding a pair of centrioles. Microtubules radiate from this material.

Microtubule-Based Cilia and Flagella

  • Cilia and flagella are microtubule-based structures involved in cell movement.
  • Primary cilia and motile cilia are anchored in basal bodies, containing nine triplets of microtubules.
  • The axoneme of motile cilia includes an additional central pair of microtubules, radial spokes, and dynein arms.

Microtubule Motor Proteins

  • Dynein and kinesin move along microtubules in opposite directions.
  • The globular head domains of the heavy chains bind microtubules, and serve as motor domains of the molecule.
  • Light chains bind to cargo (e.g., vesicles).

Transport of Vesicles

  • Kinesin I moves vesicles and organelles along microtubule plus ends, while dynein moves them along minus ends.
  • The direction of transport corresponds to the direction of microtubule extension.

Association of Endoplasmic Reticulum

  • The endoplasmic reticulum (ER) appears closely correlated with microtubules in epithelial cells.

Intermediate Filaments (IFs)

  • IFs are flexible, rope-like fibers providing mechanical strength.
  • IFs include keratin, desmin, vimentin, and lamins.
  • IFs are a diverse family of proteins.
  • IFs are crucial in providing support and resisting mechanical stress, especially in muscle cells, neurons, and epithelia.

Attachment to Desmosomes and Hemidesmosomes

  • Desmosomes are cell-to-cell junctions involving intermediate filaments.
  • Hemidesmosomes are cell-to-ECM junctions involving intermediate filaments.

Extracellular Matrix (ECM)

  • The ECM has three main components: structural proteins (collagen), polysaccharides (GAGs and Proteoglycans), and adhesion proteins (fibronectin, laminin).
  • The ECM plays a vital role in cells.

Collagen Structure and Function

  • Collagen is a triple helix composed primarily of glycine, proline, and hydroxyproline
  • Protein cross-links are crucial in the strength and stability of collagen.
  • Type IV collagen is a key component of basal laminae

Type IV Collagen and Basal Lamina

  • Type IV collagen forms networks, unlike other collagens which form fibrils.
  • Crucial component of basal lamina.

Proteoglycans

  • Proteoglycans are GAGs covalently linked to a core protein molecule.

Integrins

  • Integrins are major receptors responsible for cell attachment to the ECM.
  • Connect the ECM to the cytoskeletal filaments.

Cell-Matrix Junctions

  • Integrins mediate focal adhesions and hemidesmosomes, crucial for cell attachment.

Functions of Cytoskeleton

  • Golgi localization.

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