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Questions and Answers
Which function is primarily associated with myosin II in eukaryotic cells?
Which function is primarily associated with myosin II in eukaryotic cells?
What is the primary role of filamin in actin networks?
What is the primary role of filamin in actin networks?
Which of the following stages is NOT part of the cell crawling process?
Which of the following stages is NOT part of the cell crawling process?
How does myosin convert chemical energy into mechanical energy?
How does myosin convert chemical energy into mechanical energy?
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In cell crawling, what is primarily involved in the extension of the leading edge?
In cell crawling, what is primarily involved in the extension of the leading edge?
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What is the main role of kinesins in relation to microtubules?
What is the main role of kinesins in relation to microtubules?
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Which characteristic differentiates flagella from cilia?
Which characteristic differentiates flagella from cilia?
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What effect does colcemid have on microtubules?
What effect does colcemid have on microtubules?
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During cell recovery after colcemid treatment, what is observed?
During cell recovery after colcemid treatment, what is observed?
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What is the primary function of dyneins in the context of microtubules?
What is the primary function of dyneins in the context of microtubules?
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Which of the following statements about the centrosome is correct?
Which of the following statements about the centrosome is correct?
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What is meant by dynamic instability in microtubules?
What is meant by dynamic instability in microtubules?
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How do cilia function in the respiratory tract?
How do cilia function in the respiratory tract?
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What role does GTP play in the growth of microtubules?
What role does GTP play in the growth of microtubules?
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What is a primary role of intermediate filaments in eukaryotic cells?
What is a primary role of intermediate filaments in eukaryotic cells?
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Which statement accurately describes the composition of intermediate filaments?
Which statement accurately describes the composition of intermediate filaments?
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How do actin filaments most directly contribute to cell movement?
How do actin filaments most directly contribute to cell movement?
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What distinguishes microfilaments from intermediate filaments in terms of structure?
What distinguishes microfilaments from intermediate filaments in terms of structure?
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What role do accessory proteins play in the cytoskeleton?
What role do accessory proteins play in the cytoskeleton?
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Which of the following cellular structures is associated with microtubules rather than microfilaments?
Which of the following cellular structures is associated with microtubules rather than microfilaments?
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What is the main reason for the dynamic nature of the cytoskeleton?
What is the main reason for the dynamic nature of the cytoskeleton?
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In which way do microtubules contribute to organelle transport?
In which way do microtubules contribute to organelle transport?
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Which of the following functions is primarily performed by actin filaments?
Which of the following functions is primarily performed by actin filaments?
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What characteristic of microtubules affects their dynamic instability?
What characteristic of microtubules affects their dynamic instability?
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What structural feature allows intermediate filaments to provide stability to cells under mechanical stress?
What structural feature allows intermediate filaments to provide stability to cells under mechanical stress?
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Which of the following statements about intermediate filaments is true?
Which of the following statements about intermediate filaments is true?
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How do intermediate filaments regulate their assembly and disassembly within the cell?
How do intermediate filaments regulate their assembly and disassembly within the cell?
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What is the diameter comparison between intermediate filaments, microtubules, and actin filaments?
What is the diameter comparison between intermediate filaments, microtubules, and actin filaments?
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What is the consequence of phosphorylation of nuclear lamins during mitosis?
What is the consequence of phosphorylation of nuclear lamins during mitosis?
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What role do actin filaments play in cellular activities?
What role do actin filaments play in cellular activities?
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Which statement about the composition of intermediate filaments is accurate?
Which statement about the composition of intermediate filaments is accurate?
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What is the primary role of actin filaments beneath the plasma membrane?
What is the primary role of actin filaments beneath the plasma membrane?
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What is the primary characteristic of actin monomers in forming filaments?
What is the primary characteristic of actin monomers in forming filaments?
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What role does the distinct polarity of actin filaments play?
What role does the distinct polarity of actin filaments play?
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Which type of structure forms when actin filaments are crosslinked into loosely organized arrays?
Which type of structure forms when actin filaments are crosslinked into loosely organized arrays?
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What is the function of microvilli in intestinal epithelial cells?
What is the function of microvilli in intestinal epithelial cells?
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What is the primary composition of the contractile ring during cell division?
What is the primary composition of the contractile ring during cell division?
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In what way do actin filaments demonstrate dynamic behavior?
In what way do actin filaments demonstrate dynamic behavior?
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What property distinguishes the plus end of actin filaments from the minus end?
What property distinguishes the plus end of actin filaments from the minus end?
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How does the cell cortex contribute to cellular functions?
How does the cell cortex contribute to cellular functions?
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What mechanism allows actin filaments to be broken down when necessary?
What mechanism allows actin filaments to be broken down when necessary?
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Study Notes
Cytoskeleton Overview
- The cytoskeleton is a network of protein filaments throughout the cytoplasm of eukaryotic cells.
- It plays a structural role, providing support and organization to the cell.
Cytoskeleton Functions
- Maintaining cell shape
- Organizing the cytoplasm
- Facilitating cell movements and motility
- Supporting organelle transport
- Enabling cell division (mitotic chromosomes and cytokinesis)
- Underpinning muscle contraction
Cytoskeleton Structure
- Dynamic structure, continually reorganizing as cells change shape or divide
- Composed of three types of protein filaments: actin filaments, intermediate filaments and microtubules.
- Ordered by increasing diameter: actin, intermediate, and microtubules.
Microtubules
- Composed mainly of the globular protein tubulin.
- Tubulin is a dimer consisting of two closely related polypeptides: α-tubulin and β-tubulin.
- The tubulin dimers arrange in head-to-tail arrays called protofilaments.
- Microtubules consist of 13 linear protofilaments assembled around a hollow core.
- Microtubules are polar structures with two distinct ends: a fast-growing plus end and a slow-growing minus end.
- The centrosome is the major microtubule-organizing center, located near the nucleus.
- Colcemid is a drug that binds tubulin and inhibits microtubule polymerization.
- Taxol is a drug that stabilizes microtubules rather than inhibiting their assembly.
- Treatment with colcemid & then removal allows growth of microtubules outward from the centrosome, showing dynamic instability.
- Microtubules show dynamic instability, where tubulin dimers can depolymerize and polymerize, resulting in a continual and rapid turnover of microtubules.
- Growth of microtubules continues as long as there is a high concentration of tubulin bound to GTP.
- Microtubules are responsible for intracellular transport of organelles (such as secretory vesicles), separation of chromosomes during cell division, and cell locomotion (cilia and flagella).
- Kinesins move cargo toward the (+) end of microtubules (anterograde transport).
- Dyneins transport cargo toward the (-) end (retrograde transport).
- Cilia beat in a coordinated back-and-forth motion, moving cells through fluid or moving fluid over the cell surface.
- Flagella move cells, and differ from cilia in length and wavelike pattern of beating.
- Cilia and flagella have similar structures.
- A cilium/flagellum contains a core of microtubules with a pair in the center surrounded by nine doublet of microtubules.
Intermediate Filaments
- Elaborate network in the cytoplasm of most cells.
- Extends from a ring surrounding the nucleus to the plasma membrane.
- Found in parts of cells subjected to mechanical stress; essential for stabilizing the position of organelles and attaching cells.
- Intermediate filaments contain approximately eight protofilaments wound together in a ropelike structure.
- Do not exhibit dynamic behavior.
- Frequently modified by phosphorylation to regulate assembly and disassembly within the cell.
- Example: Phosphorylation of nuclear lamins during mitosis leads to disassembly of the nuclear envelope.
- Intermediate filaments underlying the nuclear membrane are composed of fibrous proteins, called lamins, which associate to form filaments, providing mechanical support to the nuclear envelope.
Actin Filaments
- The major cytoskeletal protein of most cells.
- Highly abundant beneath the plasma membrane where they form a network.
- Provides mechanical support; determines cell shape; allows for surface movements, migration and engulfment of particles.
- Involved in cell division and muscle contraction.
- Actin molecules are globular proteins.
- Actin monomers polymerize to form filaments.
- Actin filaments are oriented in the same direction (with distinct plus and minus ends).
- The plus end of actin filaments elongates faster than the minus end, showing dynamic behavior.
- Filaments disassemble when necessary by actin subunits dissociating.
- Actin filaments in cells are often bundled or networked, regulated by actin-binding proteins.
- Structures made from bundled actin filaments include microvilli (which increase the surface area for absorption in epithelial cells), stress fibers, filopodia, and contractile rings (important for cell division).
- The cell cortex is a three-dimensional network, mainly of actin filaments and associated proteins, beneath the plasma membrane. Supporting cell shape and surface activities including movement.
- Actin filaments, often working with myosin, are responsible for many types of cell movements.
- Myosin is a motor protein responsible for driving filament sliding and muscle contraction.
- Cell crawling, involves a coordinated cycle of: protrusions, attachment and retraction.
Muscle Contraction
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Skeletal muscle is responsible for voluntary movements.
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Smooth muscle is responsible for involuntary organ movements.
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Myoepithelial cells are responsible for releasing substances like saliva, sweat and milk.
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Skeletal muscles are bundles of muscle fibres, which are single large cells formed by fusion of many individual cells.
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Skeletal muscle cytoplasm largely consists of myofibrils.
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Myofibrils are cylindrical bundles of two types of filaments: thick (myosin) and thin (actin).
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Myofibrils are organized into contractile units called sarcomeres.
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During muscle contraction, sarcomeres shorten, bringing Z discs closer together.
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Actin filaments slide past myosin filaments toward the center of the sarcomere, shortening the sarcomere without changing filament lengths.
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Myosin are the motor that drives the filament sliding.
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Sarcomere structure is symmetrical on its two sides.
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Regulatory proteins (tropomyosin and troponin) regulate muscle contraction; tropomyosin covers the myosin-binding sites on actin molecules, whereas troponin binds to tropomyosin and calcium ions.
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Low calcium levels block the interaction of actin and myosin, preventing muscle contraction.
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High calcium levels shift tropomyosin's position, allowing actin and myosin to interact & contract.
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Muscle contraction is triggered by nerve impulses, leading to calcium release for contraction to occur.
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The sarcoplasmic reticulum is a specialized network that stores calcium ions and releases them in response to nerve impulses.
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Description
Explore the fundamental aspects of the cytoskeleton, including its structure, functions, and components. This quiz covers the role of protein filaments in maintaining cell shape, enabling movements, and supporting organelle transport.
