Figure 1. Centriole architecture and the centrosome duplication-segregation cycle.

(A) (a) Schematic showing fully mature parent centriole (upright) and tightly associated procentriole. Prominent markers representative for the different structures are indicated to the right. (b) Micrograph shows lattice of in vitro reconstituted cartwheel hub and spoke structures visualized by cryo-electron microscopy. Adapted with permission from²³. (c) Image derived from cryotomogram sections of Chlamydomonas procentriole emphasizes cartwheel and triplet microtubules. Adapted with permission from¹⁹. (d) Transmission electron microscopy shows longitudinal section (top) and cross sections at proximal (lower left) and distal parts (lower right) of Paramecium basal body (Anne-Marie Tassin, unpublished). (B) Shared pathways ensure coordination of centrosome duplication-segregation and chromosome replication-segregation cycles. At the G1/S transition both centriole duplication and DNA replication depend on CDK2 as well as phosphorylation of the retinoblastoma protein pRb and liberation of E2F transcription factors²⁰³. Similarly, overlapping sets of enzymes, including the kinases CDK1 and PLK1 and the protease Separase govern entry into mitosis, chromosome segregation, and licensing of DNA and centrioles for a new round of duplication. Lastly, several proteins with well-established functions in DNA transactions have been proposed to play additional roles in the centrosome cycle, but indirect effects on centrosomes remain difficult to exclude²⁰⁴. Centrioles are depicted in different shades of grey to indicate different states of maturity. A procentriole (light grey) is a newly created centriole that is not yet duplication competent. A procentriole converts into an immature parent centriole (middle grey) following disengagement in mitosis. An immature parent centriole becomes a mature parent centriole (dark grey) following the acquisition of appendages. Appendage structures undergo a transient modification/disassembly during mitosis. Cartwheels are shown in red; loose tethers connecting parent centrioles in dashed green lines; tight linkers connecting procentrioles to their parents in dark blue; subdistal and distal appendages are shown in light and dark blue respectively.