Schematic illustration of centrioles, centriole duplication, PLK4 symmetry breaking, and Turing reaction–diffusion systems.

(A) Most higher-eukaryotic cells are born with a single pair of centrioles comprising an older mother and a younger daughter formed around a 9-fold symmetric central cartwheel (dark green) surrounded by 9-sets of microtubule doublets or triplets (light green), depending on the species. The centrioles depicted here are from Drosophila, which have doublet MTs and which lack the distal and sub-distal appendages found on human mother centrioles. (B) The centriole pair (i) duplicates once each cell cycle when the mother (M) and daughter (D) separate (ii) (for simplicity only the original mother is shown here); the daughter matures into a mother (not shown) and both mother centrioles recruit PLK4 (red) symmetrically around themselves (iii). PLK4 symmetry is broken (iv) when PLK4 becomes concentrated in a single spot that defines the site of new daughter centriole assembly (v). PLK4 usually dissociates from the centriole by the time daughter centriole assembly is complete [74]. (C) Diagrams illustrate the logic of the 2 chemical reaction schemes that can form Turing reaction–diffusion systems (see text for details).