Centriole duplication and temporal activation of PLK4. In G1, PLK4 is present at centrioles but active kinase cannot be detected indicating that PLK4 is inactive at this point in the cell cycle. At the G1/S boundary centriole duplication begins with the formation of procentrioles at the proximal end of each parental centriole. This coincides with activation of PLK4 at the mother centriole (active kinase drawn as a red star). PLK4 becomes active at the replicating daughter centriole later on in the cell cycle in G2. By mitosis both centrosomes possess a similar amount of active PLK4 and procentriole elongation has been completed.