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Chapter 20 - Microtubules

20.1: Microtubule Organization and Dynamics

  • GTPases have a branch of tubulin that belong to the same family, and those tubulins polymerize into microtubules which are small hollow cylindrical structures which a diameter of 25 nm

  • Microtubules exhibit structural and functional polarity like actin microfilaments

  • Protofilaments are formed by dimeric aB-tubulin which interact with end to end, and protofilaments are laterally associated with microtubules

  • The (+) of microtubules lose and gain subunits, and microtubules have structural polarity

  • The critical concentration of aB- tubulin affects the assembly and disassembly of microtubules:

    • If the Cc is above, the microtubules assemble

    • If the Cc is below, the microtubules disassemble

  • There are two situations that are shown regarding tubulin concentrations:

    • Tread milling, subunits increasing and decreasing

    • Dynamic instability, lengthening, and shortening of the oscillation

  • GTP bound to B-tubulin on the (+) side or if the GTP has been hydrolyzed to GDP affects the balance in the growth and shrinkage of microtubules

  • MAPs (microtubule-associated proteins) affect the stability of and organize microtubules:

    • MAPs can control cytosolic microtubule depolymerization

    • They can also organize and crosslink microtubules into bundles and membranes for the mediation of filaments in the membrane

  • Drugs like colchicine and taxol can help with cancer as they disrupt microtubules and provide an antimitotic effect

  • Microtubule organizing centers (MTOCs) determine cell polarity

20.2: Kinesin and Dynein-Powered Movements

  • Kinesin and dynein, two different families of motor proteins, help with the transport of proteins, organelles, and vesicles along microtubules

  • Kinesins move cargo to the (+) side and dyneins move cargo to the (-) side

  • Kinesins work as dimers with head domains that bind microtubules and ATP and tail domains that bind other cargo or vesicles

  • Dynactin links dyneins to their cargoes which consist of vesicles and chromosomes

  • Dynactin is a multiprotein complex that also works to bind microtubules

  • A myosin motor probably powers the transport of vesicles along microfilaments in areas that don’t have a large amount of microtubule in the cell

  • The cell is moved forward by flagellar beating and materials are moved through the cell by ciliary beating

20.3: Microtubule Dynamics and Motor Proteins in Mitosis

  • Mitosis is the process of replicated chromosomes being separated and partitioned into two daughter cells

  • Mitosis consists of four sub-stages:

    • Prophase

    • Metaphase

    • Anaphase

    • Telophase

  • Mitotic apparatus of animal cells:

    • Astral microtubules (asters)

    • Polar and kinetochore microtubules (football-shaped spindle)

    • Spindle pores

    • Chromosomes attached to the microtubules of the kinetochore

  • Microproteins Bimc, CENP-E, and cytosolic dynein move into and stay in all spindles

  • Motor protein-mediated interactions help establish the orientation of spindles

  • The chromosomes are centered by opposing forces created by motor proteins

  • Both microtubule dynamics and microtubule motors are needed for chromosomal transport to the opposite poles at anaphase

  • Daughter cells are created during cytokinesis through cleavage

20.1: Microtubule Organization and Dynamics

  • GTPases have a branch of tubulin that belong to the same family, and those tubulins polymerize into microtubules which are small hollow cylindrical structures which a diameter of 25 nm

  • Microtubules exhibit structural and functional polarity like actin microfilaments

  • Protofilaments are formed by dimeric aB-tubulin which interact with end to end, and protofilaments are laterally associated with microtubules

  • The (+) of microtubules lose and gain subunits, and microtubules have structural polarity

  • The critical concentration of aB- tubulin affects the assembly and disassembly of microtubules:

    • If the Cc is above, the microtubules assemble

    • If the Cc is below, the microtubules disassemble

  • There are two situations that are shown regarding tubulin concentrations:

    • Tread milling, subunits increasing and decreasing

    • Dynamic instability, lengthening, and shortening of the oscillation

  • GTP bound to B-tubulin on the (+) side or if the GTP has been hydrolyzed to GDP affects the balance in the growth and shrinkage of microtubules

  • MAPs (microtubule-associated proteins) affect the stability of and organize microtubules:

    • MAPs can control cytosolic microtubule depolymerization

    • They can also organize and crosslink microtubules into bundles and membranes for the mediation of filaments in the membrane

  • Drugs like colchicine and taxol can help with cancer as they disrupt microtubules and provide an antimitotic effect

  • Microtubule organizing centers (MTOCs) determine cell polarity

20.2: Kinesin and Dynein-Powered Movements

  • Kinesin and dynein, two different families of motor proteins, help with the transport of proteins, organelles, and vesicles along microtubules

  • Kinesins move cargo to the (+) side and dyneins move cargo to the (-) side

  • Kinesins work as dimers with head domains that bind microtubules and ATP and tail domains that bind other cargo or vesicles

  • Dynactin links dyneins to their cargoes which consist of vesicles and chromosomes

  • Dynactin is a multiprotein complex that also works to bind microtubules

  • A myosin motor probably powers the transport of vesicles along microfilaments in areas that don’t have a large amount of microtubule in the cell

  • The cell is moved forward by flagellar beating and materials are moved through the cell by ciliary beating

20.3: Microtubule Dynamics and Motor Proteins in Mitosis

  • Mitosis is the process of replicated chromosomes being separated and partitioned into two daughter cells

  • Mitosis consists of four sub-stages:

    • Prophase

    • Metaphase

    • Anaphase

    • Telophase

  • Mitotic apparatus of animal cells:

    • Astral microtubules (asters)

    • Polar and kinetochore microtubules (football-shaped spindle)

    • Spindle pores

    • Chromosomes attached to the microtubules of the kinetochore

  • Microproteins Bimc, CENP-E, and cytosolic dynein move into and stay in all spindles

  • Motor protein-mediated interactions help establish the orientation of spindles

  • The chromosomes are centered by opposing forces created by motor proteins

  • Both microtubule dynamics and microtubule motors are needed for chromosomal transport to the opposite poles at anaphase

  • Daughter cells are created during cytokinesis through cleavage