Figure 252

Schematic structure of centrioles. In nondividing cells, centrioles are arranged in pairs in which one centriole is aligned at a right angle to the other. One centriole is also more mature (generated at least two cell cycles ago) than the other centriole, which was generated in the previous cell cycle. Centrioles are located in close proximity to the nucleus. The basic building components of each centrosome are microtubule triplets that form the cylindrical structure surrounding an internal lumen. The proximal part of the lumen is lined by ytubulin, which provides the template for nucleation and arrangement of the microtubule triplets. The distal part of each lumen contains the protein centrin. In some species, two protein bridges, the proximal and distal connecting fibers, connect each centriole in a pair. In some species, but not in humans, the proximal end of each centriole is attached to the nuclear envelope by a contractile protein known as nucleus-basal body connector (NBBC).

connections between the centriole pair have been identified in human lymphocytes. In other organisms, two protein bridges, the proximal and distal connecting fibers, connect each centriole in a pair (see Fig. 2.52). In dividing cells, these connections participate in segregating the centrioles to each daughter cell. In some organisms, the proximal end of each centriole is attached to the nuclear envelope by contractile proteins called nucleus-basal body connectors (NBBCs). Their function is to link the centriole to the mitotic spindle poles during mitosis. In human cells, the cen-trosome-nucleus connection appears to be maintained by filamentous structures of cytoskeleton. A distinctive feature of mammalian centrioles is the difference between individual centrioles in the pair. One centriole (termed the mature centriole) contains stalk-like satellite processes and sheet-like appendages whose function is not known (see Fig. 2.52). The other centriole (termed the immature centriole) does not possess satellites or appendages.

Prior to cell division, a single new centriole is formed at a right angle and adjacent to each preexisting centriole

Prior to cell division, when DNA is being replicated during the S phase of the cell cycle (see page 68), centrioles also duplicate. A small mass of granular and fibrillar material, the procentriole, appears at the side of each centriole and gradually enlarges to form a right-angle appendage to the parent. Microtubules develop in the mass as it grows (usually during the S to late G2 phase of the cell cycle), appearing first as single tubules, then as doublets, and finally as triplets. Thus, a single new immature centriole is formed adjacent to each existing centriole, and its duplication occurs at a precise right angle to the preexisting centriole. Centrioles are the only organelles besides the cell nucleus that undergo such exact duplication. After duplication, the parent-daughter pairs separate and produce astral microtubules. In doing so, they define the poles between which the mitotic spindle develops.

However, during certain processes (e.g., development of ciliated epithelium), large clusters of centrioles may assemble de novo, independent of preexisting centrioles. The phenomenon of de novo centriole formation can be explained by simple maturation of invisible procentrioles already existing in the cytoplasm of the cell.

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