Figure 1.

Centrosomes, cilia, and the cell cycle. (A) The centrosome is composed of mother and daughter centrioles, and a protein matrix called pericentriolar material. The mother centriole has distal and subdistal appendages. Interconnecting fibers connect the two centrioles. (B) New centrioles (procentrioles) assemble in S phase and continue to elongate in G2. The two paired centrioles separate, and the original (oldest) daughter centriole acquires appendage proteins in late G2/early M, although these appendages are not visible at this stage (appendages are depicted as dotted lines). After mitosis, the paired centrioles become disengaged. In G0, the mother centriole migrates near the plasma membrane to become a basal body, and the primary cilium is formed. (C) The basal body localizes near the plasma membrane and nucleates a primary cilium. The mother centriole converts to the basal body, and structures that include the transition fibers/distal appendage, basal foot/subdistal appendage, and striated rootlet are observed. The transition fibers tether the basal body to the plasma membrane in the transition zone, in which triplet microtubules of the basal body transition to doublet microtubules in the axoneme. The axoneme is surrounded by a ciliary membrane. Ciliary pockets are observed near basal bodies.