(A) Centriole assembly begins during late G1 or early S phase with the assembly of the cartwheel, which depends on HsSAS-6 for the central hub and Cep135 (Bld10p ortholog) for the spokes and/or the pinheads. (B) CPAP (SAS-4 ortholog) triggers γ-tubulin-dependent nucleation of the A-tubules and their attachment to the pinheads of the cartwheel, possibly by participating in the recruitment of the γ-tubulin ring complex (γ-TuRC) at the proximal end of the cartwheel. Each A-tubule is nucleated by a γ-TuRC and grows unidirectionally from proximal (P) to distal (D). The A-tubule remains capped by the γ-TuRC throughout the assembly process but is lost from daughter and mother centrioles. A cap structure containing CP110 and Cep97 forms at the distal end of the procentriole. The cap is required to control procentriole microtubule growth and probably also to stabilize the nascent procentriole. (C) The B- and C-tubules form by a γ-TuRC-independent mechanism and grow bidirectionally until they reach the length of the A-tubule. The microtubule triplets are stabilized by ε- and δ-tubulin and centriole elongation begins. (D) During S phase, procentrioles elongate up to ~70% of their final length. This step is dependent on hPOC5, and possibly involves hPOC1 as well. (E) Procentriole elongation continues after the transition into G2. Two mechanisms control centriole length at this stage: 1) a balance between the activities of CPAP, which promotes α/β-tubulin incorporation at the distal end, and the cap structure containing CP110 and Cep97; 2) an Ofd1-dependent mechanism. (F) After mitosis, the procentrioles become daughter centrioles. Tilted discs surrounded by electron-dense material are observed from this stage onwards in the distal part of the centriole. Markers like centrin and hPOC5 accumulate within the distal lumen as cell cycle progresses, which could reflect the progressive maturation of the centriole. HsSAS-6 is no longer associated with the proximal part of the daughter centrioles, possibly correlating with the disassembly of the central hub of the cartwheel. In contrast, the spokes could be conserved to some extent as Cep135 remains associated with mother and daughter centrioles. (G) After the second mitosis, centriole maturation is completed when the distal and sub-distal appendages assemble. The assembly of both types of appendage depends on ODF2, a maturation-specific marker recruited at the distal end of the daughter centriole during the previous G2 phase. Assembly of the distal appendages is also dependent on Ofd1, and possibly as well on the distal appendage component Cep164. Schematic representations of the centrosome during the successive steps of centriole assembly and maturation are shown in the lower parts of panels A–G, in which the assembling centrioles are highlighted (brown when only the cartwheel is present, yellow for later stages).