Figure 248

Structure of the microtubule-organizing center (MTOC). This diagram shows the location of the MTOC in relation to the nucleus and the Golgi apparatus. In some species, the MTOC is tethered to the nuclear envelope by a contractile protein, the nucleus-basal body connector (NBBC). The MTOC contains the centrioles and an amorphous protein matrix with an abundance of -y-tubulin rings. Each y-tubulin ring serves as the nucleation site for the growth of a single microtubule. Note that the minus (-) end of the microtubule remains attached to the MTOC and the plus (+) end represents the growing end directed toward the plasma membrane.

Centrioles provide basal bodies for cilia and flagella and align the mitotic spindle during cell division

• Mitotic spindle formation. During mitosis, centrioles are necessary for the formation of the MTOC and astral microtubules. Astral microtubules are formed around each individual centriole in a star-like fashion. They are crucial in establishing the axis of the developing mitotic spindle. In some animal cells, the mitotic spindle itself (mainly kinetochore microtubules) is formed by MTOC-independ-ent mechanisms and consists of microtubules that originate from the chromosomes. Recent experimental data indicate that in the absence of centrioles, astral microtubules fail to develop, causing errors in mitotic spindle orientation (Fig. 2.50). Thus, the primary role of centrioles in mitosis is to position the mitotic spindle properly by recruit-

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