Microtubule astral form

Three types of microtubule can readily be defined in the mitotic spindle. Polar microtubules overlap (and probably interact) between the poles and are involved in pushing the poles apart in anaphase. Astral microtubules radiate in all directions and also help separate the poles. Kinetochore microtubules attach themselves to specialized protein structures (kinetochores) located on each side of the centromere of each chromosome. These microtubules are involved in moving the chromosomes to the metaphase plate and in separating sister chromatids at anaphase. The microtubules in the spindle are very dynamic and have a half-life of only a few seconds. This appears to be especially important in the capture of chromosomes by the kinetochore microtubules. Microtubules that miss the target kinetochores are quickly lost because their dynamic instability soon leads to depolymerization. The new microtubules that form may hit the target and be partially stabilized through plus-end capping. 

Fig. 12.13 (a) We distinguish the centrosomes and the kinetochores. There are three classes of microtubules in a mitotic spindle. Attached to the kinetochores are the kinetochore-microtubules and attached to the centrosomes are the astral and the polar microtubules, which become the spindle poles, (b) The centrosomes undergo characteristic changes during the cell cycle. In the S phase, daughter centrioles begin to form. Finally, the centrosome divides to form the mitotic spindle poles. 

In each half of the spindle, a single centrosome at the pole organizes three distinct sets of microtubules whose (—) ends all point toward the centrosome (Figure 20-3lb). One set, the astral microtubules, forms the aster they radiate outward from the centrosome toward the cortex of the cell, where they help position the mitotic apparatus and later help to determine the cleavage plane in cytokinesis. The other two sets of microtubules compose the spindle. The kinetochore microtubules attach to chromosomes at specialized attachment sites on the chromosomes called kinetochores. Polar microtubules do not interact with chromosomes but instead overlap with polar microtubules from the opposite pole. Two types of interactions hold the spindle halves together to form... 

When the duplicated centrosomes have become aligned, formation of the spindle proceeds, driven by simultaneous events at centrosomes and chromosomes. As just discussed, the centrosome facilitates spindle formation by nucleating the assembly of the spindle microtubules. In addition, the (—) ends of microtubules are gathered and stabilized at the pole by dynein-dynactin working with the nuclear/mitotic apparatus protein. The role of dynein in spindle pole formation has been demonstrated by reconstitution studies in which bipolar spindles form in Xenopus egg extracts in the presence of centrosomes, microtubules, and sperm nuclei. The addition of antibodies against cytosolic dynein to this in vitro system releases and splays the spindle microtubules but leaves the cen-trosomal astral microtubules in position (Figure 20-35). 

The mitotic apparatus of animal cells comprises the astral microtubules forming the asters, the polar and kine-tochore microtubules forming the football-shaped spindle, the spindle poles derived from the duplicated centrosomes, and chromosomes attached to the kinetochore microtubules (see Figure 20-31). 

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