LECTURE 3 NEW

Question 1

Microtubule structure

Answer 1

Microtubules are composed of α- and β-tubulin heterodimers assembled into a hollow, tube like cylinder.

Dimers align end-to-end to form protofilaments, which then aggregate side by side (staggered packing) to create the microtubule’s wall.

Question 2

Microtubule polarity

Answer 2

Microtubules have a (+) end (where β-tubulin is exposed) and a (-) end (associated with microtubule-organizing centers, MTOCs).

Question 3

Provide several examples of the roles of microtubules within the cell.

Answer 3

Providing structure and support for the cell.

Creating highways for intracellular transport.

Separating chromosomes during cell division.

Allowing for cellular movement through cilia and flagella.

Question 4

Role of MTOCs (Microtubule Organizing Centers)

Answer 4

MTOCs are structures that nucleate (start) and organize microtubules.

Centrosomes: The main MTOCs in animal cells, consisting of centrioles surrounded by pericentriolar material.

γ-tubulin Ring Complex (γ-TuRC): Essential for microtubule nucleation, binding αβ-tubulin dimers and acting as a template

Question 5

Microtubule Dynamics

Answer 5

Dynamic Instability: Microtubules rapidly grow and shrink; their lifetime varies significantly across different cell types.

Critical Concentration (Cc): The minimum concentration of tubulin required for assembly; varies for (+) and (-) ends.

Question 6

GTP cap model

Answer 6

GTP-bound β-tubulin stabilizes growing microtubules; hydrolysis to GDP exposes destabilizing regions that can lead to disassembly.

Rescue events can occur due to GTP-β-tubulin islands along the length of a microtubule, potentially pausing disassembly and allowing growth to resume.

Question 7

Microtubule-Associated Proteins (MAPs)

Answer 7

Regulate stability and dynamics:
Stabilizing MAPs: MAP2, MAP4, Tau
Destabilizing MAPs: Kinesin-13, Op18/Stathmin
Growth Modulators: XMAP215 promotes growth, CLASP suppresses catastrophes.
Severing Proteins: Such as katanin, disassemble microtubules using ATP hydrolysis.

Question 8

Medical Importance of Microtubule-Binding Drugs

Answer 8

Colchicine: Inhibits microtubule polymerization, used in treating gout by reducing inflammation.

Paclitaxel (Taxol): Stabilizes microtubules, preventing disassembly; used in cancer treatment (e.g., breast and ovarian cancer).