Question

5. Place the following aspects of microtubules in order of size, from the smallest to the largest: protofilament microtubule α-tubulin tubulin dimer mitotic spindle

6. You are curious about the dynamic instability of microtubules and decide to join a lab that works on microtubule polymerization. The people in the lab help you grow some microtubules in culture using conditions that allow you to watch individual microtubules under a microscope. You can see the microtubules growing and shrinking, as you expect. The professor who runs the lab gets in a new piece of equipment, a laser beam that is very fine and can be used to sever microtubules. She is very excited and wants to sever growing microtubules at their middle, using the laser beam. A. Do you predict that the newly exposed microtubule plus ends will grow or shrink? Explain your answer. B. What do you expect would happen to the newly exposed plus ends if you were to grow the microtubules in the presence of an analog of GTP that cannot be hydrolyzed and then severed the microtubules in the middle using a laser beam?

Answer 1

5. The arrangement from smallest to largest: α-tubulin, tubulin dimer, protofilament, microtubule, mitotic spindle.

6.a) The plus ends of the microtubule exposed by the laser beam are most likely to shrink. The growth of the microtubules occurs by the addition of the GTP-carrying subunits at the end, which hydrolyzes over time. This leaves only a cap of the GTP-carrying subunits along with the remaining tubulin protofilament that contains the GDP-carrying subunits. Thus, by severing the microtubule in the middle, a plus-end containing the GDP-containing subunits will be created which are less tightly bound and will depolymerize causing shrinking of the microtubule.

b) If the microtubules were grown in teh presence of the non-hydrozylable GTP analogue, then the newly exposed plus end will have FTP cap and hence, they will continue growing.