Which of the following statements about microtubules is true?
(a) Motor proteins move in a directional fashion along microtubules by using the inherent structural polarity of a protofilament.
(b) The centromere nucleates the microtubules of the mitotic spindle.
(c) Because microtubules are subject to dynamic instability, they are used only for transient structures in a cell.
(d) ATP hydrolysis by a tubulin heterodimer is important for controlling the growth of a microtubule.
Choice (a) is true. Microtubules are nucleated by the centrosome [not the centromere, choice (b)]. Although microtubules are subject to dynamic instability, their interaction with microtubule-binding proteins can stabilize them so that they can be used to form stable structures such as cilia and flagella [choice (c)]. GTP (not ATP) hydrolysis is important for controlling the growth of a microtubule [choice (d)].
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