The function of microtubules depends on their specific spatial organization with

Question

The function of microtubules depends on their specific spatial organization within the cell. How arc specific arrangements created, and what determines the formation and disappearance of individual microtubules? To address these questions, investigators have studied the in vivo assembly of alpha beta -tubulin dimers into microtubules. Below 15 mu M alpha beta-tubulin, no microtubules are formed: above 15 mu alpha beta -tubulin, microtubule readily formed (Fig. A). If centrosomes are added to the solution of tubulin, microtubules begin to form at less than 5 mu M (Fig. B). Why do you think that the concentration at which MT begin to form (the critical concentration) is different in the two experiments? Why do you think that the plot in figure A increases linearly with increasing tubulin concentration above 15 mu M, where the plot in figure B reaches a plateau at about 25 mu M. The concentration of alpha beta -tubulin dimers (the subunits for assembly) In a typical cell is 1 mg/ml and the MW of a tubulin tubulin dimer is 110,000. What is the molar concentration of tubulin dimmers in cells? How does the cellular concentration compare with the critical concentrations in the two experiments shown below? What are the implications for the assembly of microtubules In cells? Analysis of MT assembly. (A) Mass of MTs assembled in the absence of centrosomes as a function of tubulin concentration. (B) Average number of MTs per centrosome as a function of tubulin concentration. Concentrations refer to ab-tubulin dimers, which are the subunits of assembly.

Explanation / Answer

A. They are different because centrosomes probably act as a catalyst in the formation of tubulin dimers into microtubules. So, since centrosomes make it easier to form microtubules, a lower concentration of tubulin will still lead to microtubule assembly.

B. It may be that figure A and B both have the same shape and that figure A will plateu as well. This could be the case because when you compare figure A and B looking at microtubule mass between 0 and 30 both are linear in that region. As you increase the concentration of ab-tubulin in figure A, it will likely plateau as well just like figure B. It just takes a higher ab-tubulin concentration without centrosomes to get there.

C. Molecular weight (MW) is how many grams/Liter is needed of a substance to be 1 M. It is given that 110,000 g/L(or mg/mL) is needed to get 1 Molarity. So since we have 1 mg/mL, we do 1/110,000 to get how many M it is in the cell. That gives us .0000091 Molar, which is 9.1 micromolar. At this concentration in the experiments, without centrosomes present there would be no microtubules formed(they start forming at concentration ~15 micromolar), and with centrosomes we would have about 27 microtubule mass.

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