The function of microtubules depends on their specific spatial organization with
ID: 7538 • Letter: T
Question
The function of microtubules depends on their specific spatial organization within the cell. How are 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 -tubulin dimers into microtubules. Below
15 M -tubulin, no microtubules are formed; above 15 M -tubulin, microtubule readily formed (Fig. A).
If centrosomes are added to the solution of tubulin, microtubules begin to form at less than 5 M (Fig. B).
A. Why do you think that the concentration at which MT begin to form (the critical concentration) is different in the two experiments?
B. Why do you think that the plot in figure A increases linearly with increasing tubulin concentration above 15 M, where the plot in figure B reaches a plateau at about 25 M.
C. The concentration of -tubulin dimers (the subunits for assembly) in a typical cell is 1 mg/ml and the MW of a tubulin dimer is 110,000. What is the molar concentration of tubulin dimers 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 the cell?
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 microtubule is a hollow cylinder about 24 nm in diameter. Along the microtubule axis, tubulin heterodimers are joined end-to-end to form protofilaments, with alternating a & b subunits. Staggered assembly of 13 protofilaments yields a helical arrangement of tubulin heterodimers in the cylinder wall.
Centrosome is the microtubule organizing center in the cell, where microtubules are produced. With this centrosome, centrioles are present which are made of three fused microtubules.
A). In the two experiments, the formation of microtubule differed because, with the addition of centrosomes to second experiment the microtubules are formed at a early concentration. This is due to the presence of microtubule forming particles in the centrosome(contains three microtubules already) adds in formation at an lower concentration.
B). In figure A, the formation of MT started from 15um and attained an constant growth. It is due to the absence of centrosome, which initiating the MT synthesis.
Where as in figure B, the centrosome addition assisted in MT formation, and grows exponentially from the origin.
C). Molar concentration is calculated by using the formula
M = moles / volume
moles = weight/molecular weight, where weight - 1mg = 0.000001
= 0.000001/ 110000= 9.09
M = 9.09 / 0.001 = 9090
Therefore the molar concentration is 9,090.
An a,b-tubulin heterodimer is the basic structural unit of microtubules.Related Questions
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