The cytoskeleton Flashcards
what is the cytoskeleton?
a complex system of filaments within the cell that aid in organization, structure, and behavior. It plays important roles in cell shape, locomotion, cell division, organelle organization, intracellular transport, mechanical strength, and muscle contraction
what are the three major cytoskeleton filaments structure?
microtubules: rigid hollow tube formed from tubule dimers
microfilaments (actin): flexible double helix of actin monomers
intermediate filaments: strong fiber composed of protein subunits, rope like structure
what do each of the three major cytoskeleton filaments do?
microtubules: determine position of organelles, direct transport, form mitotic spindles
Actin (microfilaments): determine cell shape and are essential for cell crawling and movement
Intermediate filaments: provide mechanical strength to the cell
where are each of the three major filaments found in a cell?
microtubules: heavily concentrated inside cell
Actin: in cell cortex (edges of cell)
intermediate filaments:
Major important features of actin (how common and what it does)
actin is ubiquitously expressed in all eukaryotic cells and essential for cells. actin has many functions like motility, protrusion, contractions, and connections
what are the different arrangements actin can make?
contractile bundles: antiparallel fibers in stress fibers
gel-like network: many cross-crossing fibers in cell cortex
tight parallel bundle: parallel fibers in filopodium
what is the structure of actin? F vs G actin
actin exists as monomers called G actin (globular). The monomers form dimers and trimers that build filaments. The polymer of actin is called F actin (filamentous)
what does it mean that actin is dynamic?
it is always being made and taken apart. monomers and small oligomers diffuse rapidly within cytosol and polymers assemble where and when they are needed
what does it mean that actin is polarized?
actin has a plus end and a minus end. The minus end binds ATP and additions are more likely to occur on plus end
how do actin monomers bind together?
each actin monomer makes 2 lateral and 2 longitudinal bonds with neighboring monomers. they are noncavelent bonds and are pretty weak. They form a double helical twist but are not two separate strands like DNA
what are barbed and pointed ends?
the actin filaments that have myosin motor domains resemble arrowheads. The barbed end is plus and the pointed end is minus
what are the stages of actin polymerization?
- nucleation
- elongation
- steady state
what are the requirements for nucleation? why does this step matter?
Nuclei greatly accelerate actin polymerization. Nuclei consist of small actin oligomers in the cytosol that cells maintain by binding accessory proteins to block further polymerization. cell signals remove the blocking to allow further polymerization.
what happens during the steady state period?
actin filament subunits are added and removed at equilibrium so the size stays the same.
why does the lag phase occur in actin polymerization?
actin monomers bind weakly to each other when there are few of them. When only two monomers are bound, they will be weak and dissociate. The binding of a third monomer makes the group more stable and then polymerization can take off
