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Questions and Answers
What explains the lag phase during polymerization of tubulin?
What explains the lag phase during polymerization of tubulin?
The lag phase is explained by the slow assembly of the nucleus due to the time taken for nucleation.
How can the lag phase of polymerization be affected?
How can the lag phase of polymerization be affected?
The lag phase can be reduced or abolished by adding premade nuclei, such as fragments of polymerized microtubules or actin filaments.
What are the three phases of in vitro assembly of cytoskeletal filaments?
What are the three phases of in vitro assembly of cytoskeletal filaments?
The three phases are the lag phase, growth phase, and equilibrium phase.
What occurs during the growth phase of polymerization?
What occurs during the growth phase of polymerization?
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What are the primary structural components that make up microtubules?
What are the primary structural components that make up microtubules?
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Describe the equilibrium phase in the context of polymerization.
Describe the equilibrium phase in the context of polymerization.
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What distinguishes the plus end of a filament from the minus end?
What distinguishes the plus end of a filament from the minus end?
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What is the outer diameter of microtubules, and how does it compare to the diameter of intermediate filaments?
What is the outer diameter of microtubules, and how does it compare to the diameter of intermediate filaments?
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How do intermediate filaments contribute to the mechanical strength of epithelial tissue?
How do intermediate filaments contribute to the mechanical strength of epithelial tissue?
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What role do conformational changes of subunits play in polymerization?
What role do conformational changes of subunits play in polymerization?
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Why is the nucleus of tubulin described as having a more complicated structure?
Why is the nucleus of tubulin described as having a more complicated structure?
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What is the role of microtubule-organizing centers (MTOCs), and which structure acts as an MTOC?
What is the role of microtubule-organizing centers (MTOCs), and which structure acts as an MTOC?
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Describe the dynamic nature of actin filaments and their structures involved in cell movement.
Describe the dynamic nature of actin filaments and their structures involved in cell movement.
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What type of filament forms the nuclear lamina, and how does it contribute to the cell's architecture?
What type of filament forms the nuclear lamina, and how does it contribute to the cell's architecture?
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Explain the significance of the constant state of flux in macromolecular components of the cytoskeleton.
Explain the significance of the constant state of flux in macromolecular components of the cytoskeleton.
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How do the structures of microtubules and intermediate filaments differ in terms of rigidity?
How do the structures of microtubules and intermediate filaments differ in terms of rigidity?
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What initiates motility in neutrophils as they pursue bacteria?
What initiates motility in neutrophils as they pursue bacteria?
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How does the arrangement of actin filaments affect neutrophil movement?
How does the arrangement of actin filaments affect neutrophil movement?
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Explain the importance of filament nucleation in actin polymerization.
Explain the importance of filament nucleation in actin polymerization.
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What role do lamellipodia and filopodia play in neutrophil behavior?
What role do lamellipodia and filopodia play in neutrophil behavior?
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Describe the relationship between actin monomers and the stability of actin filaments.
Describe the relationship between actin monomers and the stability of actin filaments.
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What visual evidence indicates that neutrophils are effectively pursuing bacteria?
What visual evidence indicates that neutrophils are effectively pursuing bacteria?
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How does the plasma membrane relate to actin filament orientation?
How does the plasma membrane relate to actin filament orientation?
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What is the significance of multiple subunit contacts in actin polymerization?
What is the significance of multiple subunit contacts in actin polymerization?
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What determines the equilibrium constant for the association of polymer subunits?
What determines the equilibrium constant for the association of polymer subunits?
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How does nucleotide hydrolysis affect the dynamics of actin and tubulin polymerization?
How does nucleotide hydrolysis affect the dynamics of actin and tubulin polymerization?
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What happens to the growth rate of the plus and minus ends of a polymer when C is greater than Ca?
What happens to the growth rate of the plus and minus ends of a polymer when C is greater than Ca?
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In the context of polymer dynamics, what form of the monomer typically adds to the filament?
In the context of polymer dynamics, what form of the monomer typically adds to the filament?
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What specific nucleotide do actin molecules carry before they are hydrolyzed?
What specific nucleotide do actin molecules carry before they are hydrolyzed?
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When the concentration of subunits is less than the critical concentration (C < Ca), what is observed at both ends of the polymer?
When the concentration of subunits is less than the critical concentration (C < Ca), what is observed at both ends of the polymer?
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Why must the ratio of $K_{off}$ and $k_{on}$ remain the same at both ends of a simple polymerization reaction?
Why must the ratio of $K_{off}$ and $k_{on}$ remain the same at both ends of a simple polymerization reaction?
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What is the significance of the final state of a subunit after dissociation during polymerization?
What is the significance of the final state of a subunit after dissociation during polymerization?
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Study Notes
The Cytoskeleton
- Cells must be correctly shaped, physically robust, and properly structured internally to function correctly
- Cells must adapt, rearrange, and change their shape to interact with environment and each other
- Spatial and mechanical functions depend on a system of filaments called the cytoskeleton
- The cytoskeleton consists of three main families of protein filaments: actin filaments, microtubules, and intermediate filaments
- Each filament type has unique mechanical properties, dynamics, and biological roles
- Filament systems have to function collectively, giving the cell its needed strength, shape, and movement capabilities
Function and Dynamics of the Cytoskeleton
- Actin filaments determine cell surface shape and are necessary for cell movement (e.g., pushing one cell into two)
- Microtubules organize organelles, direct intracellular transport, and form the mitotic spindle for chromosome segregation
- Intermediate filaments provide mechanical strength to cells
Actin Filaments
- Actin filaments (microfilaments) are helical polymers of the protein actin
- They have a diameter of 8 nm
- Organize into linear bundles, two-dimensional networks, and three-dimensional gels
- Highly concentrated beneath the plasma membrane, called the cortex
- Involved in microvilli, stress fibers, and muscle contraction
Microtubules
- Microtubules are hollow cylinders of the protein tubulin
- Have an outer diameter of 25nm
- Long, straight, and often extend from a microtubule-organizing center (MTOC) called a centrosome
- Involved in intracellular transport, organizing the mitotic spindle during cell division
- Involved in cilia and flagella
Intermediate Filaments
- Intermediate filaments are rope-like fibers with a diameter of about 10 nm
- Consist of a family of intermediate filament proteins
- Form a nuclear lamina beneath the inner nuclear membrane for support
- Enable structural strength across the cytoplasm in tissues like epithelial
- Located in and around the nucleus and are important for maintaining structural integrity, like в the nuclear lamina, and providing mechanical strength to tissues like epithelia
Cytoskeletal Filaments Are Dynamic but Can Nevertheless Form Stable Structures
- Large-scale cytoskeletal structures can change or persist based on cell needs, ranging in duration from temporary to permanent
- Rearrangements in cells require little energy
- Examples of dynamic structures include filopodia, lamellipodia, and pseudopodia, which cells use for exploration and movement
- Cell motility involves an actin network dependent protrusion of the leading edge through lamellipodia and filopodia structures which contain actin filaments with elongating barbed ends oriented toward the plasma membrane
Diagram of Changes in Cytoskeletal Organization Associated with Cell Division
- Cell division involves reorganization of actin filaments and assembly of a bipolar mitotic spindle
- Actin filaments rearrange, the cell becomes spherical
- Bipolar mitotic spindle aligns and segregates duplicated chromosomes
- Actin filaments form a contractile ring which divides the cell into two
- Daughter cells re-establish cytoskeletal organization
A neutrophil in pursuit of bacteria
- Neutrophils in blood quickly reassemble actin to push toward bacteria, a rapid process
- Bacteria movement cause cells to quickly change orientation and direction
The Cytoskeleton Determines Cellular Organization and Polarity
- Stable structures (e.g., microvilli and cilia) in cells like epithelial cells maintain cell surface organization, length, and diameter that persist for long periods.
- Some cells maintain stable organization for entire lifetime of animal
Filaments Assemble from Protein Subunits That Impart Specific Physical and Dynamic Properties
- Filaments are polymers which assemble from small subunits, allowing rapid reorganization of structure when needed
- Subunit composition is either globular (actin and tubulin) or fibrous (intermediate filaments)
Filaments Assemble from Protein Subunits
- Filaments are built by adding subunits—tubulin and actin
- Microtubules are made from 13 protofilaments
- Loss/addition of subunit affects strength and adaptability of filaments
Nucleation
- Formation of actin filaments usually involves spontaneous subunit interaction
- It's important for cell shape and movement control
- Nucleation often involves oligomer formation to stabilize the filament before further elongation
On Rates and Off Rates
- Polymerization and depolymerization are regulated by rate constants for addition (kon) and loss (koff) of monomers at polymer ends
- Critical concentration is when rate of addition equals rate of loss
Nucleation
- Nuclei that are assembled from multiple subunit contacts enable faster polymerization.
- In actin, two subunits bind weakly, but a trimer forms a stable nucleus
- In tubulin, larger nuclei are formed with more subunits, although basic principle is the same; and lag phase is possible
The Critical Concentration
- Critical concentration (Cc) is when the rate of subunit addition equals the rate of subunit loss
- When Cc is reached, the system is in equilibrium
Time Course of Polymerization
- Polymerization has 3 phases: lag, growth, and equilibrium (steady state)
- Polymerization begins with a lag phase until stable nuclei form
- Growth phase involves monomer addition at filament ends
- Steady state is reached when monomer addition equals monomer loss at that concentration
Plus and Minus Ends
- Polymerization rates differ at plus and minus ends (fast and slow, respectively, for example), controlled by subunit interactions
- This creates directionality even though subunits do interact bi-directionally
- Nucleoside triphosphate hydrolysis removes some of this constraint allowing for independent control of rates at ends
- Shape of filament also plays role (e.g., ATP hydrolysis affects ability of filaments to associate/disassociate and allows the cell to control the rate of addition and removal of subunits)
Nucleotide Hydrolysis
- Hydrolyzing nucleotide (e.g., ATP) in subunits alters the subunit's interaction with the polymer, affecting how fast/slow subunits attach and detach
- Allows for dynamic, regulated polymer growth
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Description
This quiz explores the mechanisms of polymerization of tubulin and the formation of cytoskeletal filaments. It covers the phases of assembly, the structural components of microtubules, and their roles in cellular mechanics. Test your understanding of how these processes contribute to cellular function and integrity.
