Electron micrographs of centrioles from P lk 1-inhibited cells. Synchronous population of HeLa cells was treated with Plk1 inhibitors BI and GSK in G1. Cells were allowed to progress through the cell cycle and were fixed at the peak of mitotic entry (21 h after shake-off). (A, B, D, and E) Low magnification images are illustrating phases of the cell cycle. In addition, two or three 80-nm-thick serial sections (S1–3) through mother centrioles associated with a procentriole are shown. (A) Control late G2 cell with duplicated mother centriole surrounded with expanding pericentriolar material and nucleated microtubules (insert). (B) Examples of shorter-than average procentrioles in Plk1-inhibited prophase cell. Green arrows point to the regions with uneven length of procentriole microtubules. (C) Example of shorter-than-average procentrioles in Plk1-inhibited prometaphase cell. (D) Example of a cross-sectioned short procentriole lacking the third set of microtubules. (E) Example of shorter-than-average procentrioles in prometaphase cell. (F) Examples of procentrioles (white arrows) from late S and G2 cells. One section is shown for each centriole. Procentrioles appear narrower and shorter than the typical control procentrioles in late S and G2. Procentriole in cross-section lacks the full set of triplet microtubules. Some serial sections in B and C are shown in Fig. 5 D of the main text. Scale bars, 4 µm for low magnification, 0.4 µm nm for serial sections and insert in A.