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1 Apr 2018

Polymerization of tubulin subunits into microtubules occurs with an increase in the orderliness of the subunits. Yet tubulin polymerization occurs with an increase in entropy (decrease in order). How can that be? (please dumb it down)

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Related questions

Activity: Dynamic Instability of Microtubules

Part A

What causes catastrophe of the microtubule in vitro?

the lack of tubulin heterodimers
non-motor Microtubule Associated Proteins (MAPs)
GTP hydrolysis
mutation of the β-tubulin

Part B

What is the role of GTP in microtubule polymerization?

GTP binds the alpha and beta tubulin subunits together to form the tubulin monomer.
GTP hydrolysis provides the energy for the polymerization of the microtubule.
GTP is a second messenger that signals the need for polymerization/de-polymerization.
GTP stabilizes the tip of the microtubule, allowing more monomers to be added.

Part C

What would happen in the treadmilling experiment if a non-hydrolyzable analogue of GTP were used?

The monomers would be unable to add to the plus end, and the microtubules would shrink until they disappeared.
The microtubule would treadmill until the new tubulin, with non-hydrolyzable GTP, reached the minus end, and then it would only extend at the plus end.
The microtubule would add monomers at both the plus and minus ends, growing in both directions.
The non-hydrolyzable GTP would stabilize both ends, causing treadmilling to stop.

Part D

What is the difference between the plus and minus ends of the microtubule in in vitro experiments?

The beta subunit of the tubulin is exposed on the minus end.
Polymerization occurs at the plus end.
The plus end has a lower critical concentration for tubulin heterodimers.
Catastrophe occurs at the minus end.

Part E

How would the drug taxol affect the in vitro dynamic instability and treadmilling experiments?

Taxol would block catastrophe at the plus end in the dynamic instability experiment but not depolymerization at the minus end in the treadmilling experiment.
Taxol would block the addition of tubulin in both experiments, leading to a destruction of the microtubules.
Taxol would stabilize the microtubules in both experiments, leading to polymerization without catastrophe.
Taxol would block depolymerization at the minus end in the treadmilling experiment but not catastrophe at the plus end in the dynamic instability experiment.

To divide, cells undergo a process called mitosis (discussed in Ch. 18) in which polymers of a protein called tubulin assemble to form microtubules, which are rope-like structures that help separate a cell into two cells during division. The graphic below depicts the polymerization of tubulin into a microtubule. If a cell in mitosis is cooled to 0°, the microtubules depolymerize into tubulin monomers. The same is true for microtubules made from pure tubulin in a test tube; they assemble readily at 37°C, but disassemble at low temperature. This behavior is governed by the basic thermodynamic equation: ∆G = ∆H − TΔS A. Decide what signs (positive or negative) of ΔH and ΔS must be to account for the temperature-dependent polymerization described above, and show how your choices account for polymerization of tubulin at high temperature and its depolymerization at low temperature. (Assume that the ΔH and ΔS values do not change with temperature).

You have discovered a single cellular eukaryotic organism that has very little vesicle trafficking. You suspect that this may be due to the relatively few formed microtubules that are observed in this species. Upon close examination, you find the microtubules that are present are unusually stable. When looking at an amino acid comparison with human alpha and beta tubulin, you notice that the alpha tubulin is identical, but the beta tubulin is different in the region responsible for GTP hydrolysis. You isolated the heterodimer subunits from this new species and put them in a cell free system in a GTP bound form of the heterodimer along with centrosome nucleation sites. How would the dynamics be different from normal heterodimer subunits in the same situation?

a) The new species’ microtubules would treadmill at a maximum concentration of heterodimers

b)The new species’ microtubules would be less susceptible to catastrophe

c)The new species’ microtubules would loose more from the (-) end than (+) end

d)The new species’ microtubules would grow to a longer length than normal cellular microtubules

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