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Fig. 3. The canonical centrosome duplication cycle. Cells in early G1 phase have a single centrosome, which comprises a pair of perpendicular centrioles (dark rectangle: mother centriole; grey rectangle: daughter centriole) and surrounding PCM (shaded disk). Usually at the G1-to-S transition, the two parental centrioles loose their arrangement and split slightly from each other (A). During S phase, a new daughter centriole (white rectangle) forms perpendicular to the proximal side of each parental centriole (B) and elongates (C). The two newly formed centriole pairs then disconnect fully (D), as does the PCM (E). Steps D and E involves the Nek2 kinase and its substrate C-Nap1. A working model posits that C-Nap1 connects centrioles within a pair during the bulk of the cell cycle and is phosphorylated by Nek2 in G2 phase, resulting in disconnection of parental centrioles and generation of two distinct centrosomes (Fry, 2002). Full acquisition of appendages on daughter centrioles is achieved by the end of the subsequent cell cycle (F, shown for daughter centriole from previous duplication round).
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