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38 Cards in this Set
- Front
- Back
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What is the cytoskeleton?
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The cytoskeleton is the basic structural component of the cell that has a variety of functions. It is the major source of structural integrity and provides individual organelles with the ability to move.
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What are the three major types of filaments (smallest to largest)?
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Microfilaments (actin), intermediate filaments and microtubules
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What are intermediate filaments involved in?
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Structural components and providing stability (not involved with any motor proteins).
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What is the structure of the subunits of intermediate filaments?
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The subunits are polar (N terminus positive end and C terminus negative end) and have an alpha-helical region.
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How do the subunits form the intermediate filaments?
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1. Two subunits become a coiled dimer (still has two poles)
2. The coiled dimer binds laterally to another coiled dimer to form an antiparallel tetramer. Each end is identical - therefore there is no polarity. 3. These tetramers form protofilaments 4. Eight tetramers can twist into a rope like structure making the intermediate filaments very strong. |
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What is actin?
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Actin is involved in cell structure, cell locomotion and movement of organelles inside cells. It can be involved in motility as it is associated with motor proteins.
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What are the subunits of action?
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The major unit is globular (long and somewhat like a string). The free form is G-actin which as ATP bound. When it is associated with a filament it is F-action and has ADP bound.
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How do the filaments form protofilaments in actin?
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A protofilament comprises of two parallel filaments of actin monomers assembled end to end (therefore it has polarity). The protofilaments can then coil around each other making a rope like structure (with polarity).
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How are cross linking proteins involved in cross linking proteins?
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Filaments are organised into assemblies with the aid of cross-linking proteins. The type of cross-linking protein affects the type of assembly.
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What are the different types of actin assemblies?
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Contractile rings - gives organs/tissues circular shape.
Bundles - in the processes of migrating cells, tight bundles in filopedia allow extension and the gel-like network that supports plasma membrane and allows broad extensions of the cell. |
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What is polymerisation in actin and what is the rate constant?
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F-actin self assembles from actin subunits. This is dependent on the concentration of actin subunits and time. Thus the rate constant is Kon (/m/sec)
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What is depolymerisation in actin and what is the rate constant?
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It is the shortening of F-actin. It is only dependent on time and thus the rate constant is Koff (seconds).
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What is the critical concentration?
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Kon=Koff - therefore there is no change in the length or size of actin fibres.
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At what end is polymerisation and depolymerisation faster?
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The positive end due to conformational changes
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How is polymerisation negatively regulated?
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By thymosin - binds to G-actin preventing it from adding to the F-actin. Effectively reduces the concentration of G-actin.
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How is polymerisation positively regulated?
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By profilin - keeps thymosin in check. The profilin actin complex can be added to the positive end which effectively increases the [G-actin] at the positive end.
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What is nucleation in actin?
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A larger filament assembly is required to start the growth of the actin fibre. Nucleation is a rate-limiting step in F-actin formation.
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How can the lag phase in the nucleation of actin be abolished?
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By supplying a pre-made nucleus - Arp can act as a nucleation site as actin can bind to the positive end of Arp.
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How can Arp affect branching?
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Arp complexes can bind to actin fibres at 70 degrees providing another nucleation site at this point from which another filament can be synthesised.
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What is myosin?
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The motor protein associated with actin.
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How does myosin work in muscle cells?
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Myosin II forms thick filaments in striated muscle cells. It interacts with actin to cause contractions - it changes the length of the sarcomeres in the muscle tissues by sliding against actin filaments.
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What is the process of myosin causing contractions in muscle cells?
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1. ATP binding to the myosin head causes it to detach from the actin filament
2. In the detached myosin head, ATP hydrolysis (ATP --> ADP) causes weak binding to the actin filament. 3. The release of PO4 increases actin binding. 4. The release of ADP causes the power stroke. |
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What are the three major steps in cell locomotion?
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1. Protrusion of the filopodia
2. Attachment 3. Traction |
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What happens in the protrusion of the filopodia?
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This is achieved through actin polymerisation. It involves treadmilling which occurs when actin is added to the + end more quickly than the negative end. The + end engages with the membrane pushing it forward.
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What happens in the attachment step in cell locomotion?
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Actin filaments connected by transmembrane proteins attach the cell to the substratum.
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What happens in the traction step in cell locomotion?
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Contraction by myosin-actin interactions move the rear of the cell forward. Focal contacts with the substratum are broken.
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How is cell motility regulated?
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1. External molecules bind to cell surface receptors.
2. Rac and Rho are activated. |
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What does Rac do?
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Rac activates actin polymerisation. It is only activated locally so where ever the receptors are is roughly the area where rac is activated. Thus actin polymerisation and protrusion only occurs near where the signal came from, giving the cell migration directionality.
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What does Rho do?
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Causes action myosin contraction in the back part of the cell (Rho is free to diffuse).
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What are the subunits in microtubulin?
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The subunits are tubulin. A tubulin heterodimer is formed with a beta-tubulin (+ end) and an alpha-tubulin (- end).
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How are the protofilaments in microtubulin formed?
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Tubulin heterodimers assemble head to head to form a protofilament (which has polarity).
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How does a microtubule form?
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13 protofilaments bind laterally to form a microtubule which has a lumen in the middle.
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How are microtubules assembled?
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Microtubules self-assemble from tubulin subunits and grow faster at the + (beta) end than the - (alpha) end. They alternate between phases of rapid assembly and disassembly which leads to dynamic instability.
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What is the importance of a GTP cap in microtubules?
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The GTP cap is present at the + end and allows microtubules to grow. Without it, the microtubule is destabilised and will undergo rapid depolymerisation (catastrophe).
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How can the GTP cap lost?
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If the rate of polymerisation is too long, the GTP in the beta subunit is hydrolysed to GDP, which makes the microtubule unstable.
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How is nucleation achieved in a microtubule?
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The Gamma-tubulin ring complex provides a nucleation site to which alpha-tubulin can bind, allowing for rapid polymerisation at the beta (+) end.
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what does the MAP and catastrophe factor (kinesin 13) do when it binds to the ends of the microtubules?
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MAP stabilises (suppresses frequency of catastrophes/enhances growth rate)
Catastrophe factor (kinesin 13) destabilised (increases frequency of catastrophes) |
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What does kinesin and dynein do?
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They are motor proteins hat use ATP hydrolysis. Kinesin moves towards positive end and dynein moves towards negative end.
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