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

Although centrioles were discovered over a century ago, their precise functions, replication, and assembly remain largely unclear. The functions of centrioles can be organized into two categories:

• Basal body formation. One of the important functions of the centriole is to provide basal bodies, which are necessary for the assembly of cilia and flagella (Fig. 2.49). Basal bodies are formed by replication of centrioles that give rise to multiple procentrioles. Each procentriole migrates to the appropriate site on the surface of the cell, where it becomes a basal body. The basal body acts as the organizing center for a cilium. Microtubules grow upward from the basal body, pushing the cell membrane outward, and elongate to form the mature cilium. The process of ciliary formation is described on page 95.

astral microtubule centriole

ing the MTOC from which astral microtubules can grow and establish the axis for the developing spindle.

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