Investigation in Microtubule Dynamic Instability

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  • Topic: Microtubule, Nucleation, Mitosis
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  • Published : April 18, 2013
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Title: Investigation in microtubule dynamic instability
Introduction
Microtubules are important for maintaining cell structure, intracellular transport, formation of mitotic spindle, as well as other cellular processes. Investigation of dynamics of microtubule assembly and disassembly allow us to understand the malfunction of mitotic spindle formation or other cellular processes. This experiment is divided into two parts; we are going to find out the critical parameters for achieving greatest average length of microtubules in part one and achieving the greatest number of microtubules in part two.

Principle
In this experiment, we used a simulation programme to explore how various factors change the way microtubules grow out from centrosome, and the shrink back. Growth rate, shrink rate, catastrophe rate, rescue rate, release rate, minus end end depolymerization rate, nucleation rate and nucleation site are the factors we can adjust to see how them affects the average length and number of microtubules. The simulation time acceleration is set to 5x real time. Each time a parameter is varied and others are controlled factors. The record is taken when the simulation has reached steady state and graphs are plotted.

Results
Part1 – How to achieving greatest average length of microtubules Fixed
parameter| Shrink rate| Catastrope| Rescue
Release| MED| Nuc rate| Nuc
sites|
Variable Growth rate| 0.263| 0.042| 0.064
0.024| 0.8| 0.02| 180|
Result| 1| 2| 3| 4| 5| Mean|
0.14| 32.9| 21.12| 23.93| 23.95| 27.54| 25.888|
0.16| 33.19| 36.82| 32.5| 28.83| 30.15| 32.298|
0.18| 29.79| 39.11| 41.19| 40.8| 31.54| 36.486|
0.2| 40.77| 41.19| 45.94| 38.28| 47.66| 42.768|
0.22| 38.66| 47.49| 48.53| 48.55| 47.96| 46.238|
0.24| 42.25| 45.31| 45.25| 46.81| 40.95| 44.114|
Table1

Figure1
Fixed
parameter| Growth
rate| Shrink rate| Catastrop/
Release| MED| Nuc rate| Nuc
cites|
Variable Rescue| 0.12| 0.263| 0.042
0.024| 0.8| 0.02| 180|
Result| 1| 2| 3| 4| 5| mean|
0.084| 23.76| 22.77| 26.56| 30.78| 25.12| 25.798|
0.104| 18.88| 19.07| 17.82| 20.08| 17.55| 18.68|
0.124| 19.96| 16.69| 17.37| 19.37| 22.38| 19.154|
0.144| 21.34| 19.53| 20.54| 21.44| 21.95| 20.96|
0.164| 20.65| 18.76| 21.76| 16.33| 19.73| 19.446|
Table2

Figure 2
Discussion
Each free tubulin dimer contains one tightly bound GTP molecule that is hydrolyzed to GDP after the subunit is added to a growing microtubules. When polymerization is proceeding rapidly, tubulin molecules add to the end of the microtubule faster that the GTP they carry is hydrolyzed, and the microtubule growth. [1] Varied the growth rate and kept other factors constant, the average length of microtubules should always increase. However, the average length of microtubules rises as growth rate increase from 0.14 to 0.22µm/sec and stop increasing at 0.22µm/sec. It tends to level off rather than increase at 0.22µm/sec. It means the growth rate is no longer the limiting factor. Some factors other than growth rate, may be the rescue rate, limited the increase of the average length. Rescue rate is the rate at which a shrinking microtubule switches to growing state. We assume the greatest rescue rate, the more the microtubules undergo polymerization. So that the proportion of growing microtubules would increase and the average length rise. Instead of increase, the average length of microtubules drops from 0.084 to 0.104µm/sec. Increase the rescue rate may trigger the mechanism that lowers the average length of microtubules. It remains at around 20µm from 0.104 to 0.164µm/sec means that that there is no correlation between rescue rate and the average length beyond a point among 0.084 and 0.104µm/sec.

Part2 – How to achieve the greatest number of microtubules Fixed
parameter| Growth
rate| Catastrop| Rescue
Release| MED| Nuc rate| Shrink rate|...
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