Lec-5-Cytoskeleton. EASY WORD.OSR

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Questions and Answers

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?

Providing tensile strength Signup and view all the answers

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

Dimers Signup and view all the answers

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

Actin filaments Signup and view all the answers

What type of intermediate filaments are found in epithelial cells?

Keratin filaments Signup and view all the answers

Which type of filaments are known to support nerve cells?

Neurofilaments Signup and view all the answers

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

Tetramers line up and assemble side by side Signup and view all the answers

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

Intermediate filaments are made from different protein subunits Signup and view all the answers

What direction do kinesins typically move along microtubules?

Toward the plus end Signup and view all the answers

What is the main function of dyneins in a cell?

Moving toward the minus end of microtubules Signup and view all the answers

What structure allows cilia to create movement?

Bending of their core Signup and view all the answers

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

Cilia and flagella Signup and view all the answers

What role do actin binding proteins play in cell function?

They stabilize actin filaments Signup and view all the answers

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

Treadmilling Signup and view all the answers

What facilitates the bending motion of cilia and flagella?

Sliding forces between microtubules Signup and view all the answers

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

By the binding of their tails to cell components Signup and view all the answers

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

A whipping motion Signup and view all the answers

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

A network of fibrous proteins Signup and view all the answers

What happens to the nuclear lamina during cell division?

It breaks down and reforms. Signup and view all the answers

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

Phosphorylation and dephosphorylation. Signup and view all the answers

What are tubulin dimers primarily composed of?

Alpha and beta tubulin. Signup and view all the answers

What is the structural polarity of protofilaments in microtubules?

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

During dynamic instability, what happens to microtubules?

They can switch between polymerization and depolymerization. Signup and view all the answers

How does GTP hydrolysis affect microtubule stability?

It causes instability and promotes depolymerization. Signup and view all the answers

What anchors the minus end of microtubules in animal cells?

Centrosome. Signup and view all the answers

What can stabilize a growing microtubule?

Attaching to another molecule or cell structure. Signup and view all the answers

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

Independent microtubule organization in each end. Signup and view all the answers

What is the primary function of motor proteins along microtubules?

To transport organelles, vesicles, and molecules. Signup and view all the answers

What initiates the process of muscle contraction in skeletal muscles?

Release of neurotransmitters from the motor nerve Signup and view all the answers

What role do Ca2+ ions play in muscle contraction?

They bind to troponin, allowing myosin to attach to actin Signup and view all the answers

How does myosin II facilitate muscle contraction?

By attaching to both actin filaments and moving them Signup and view all the answers

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

Z disc Signup and view all the answers

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

To assist in the assembly and disassembly of actin networks Signup and view all the answers

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

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

What is the composition of a sarcomere?

Actin and myosin filaments Signup and view all the answers

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

Ca2+ is taken back into the sarcoplasmic reticulum Signup and view all the answers

What structural characteristic gives vertebrate myofibrils a striped appearance?

Repetitive pattern of sarcomeres Signup and view all the answers

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

Phosphorylation and dephosphorylation of myosin heads 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.