The nucleus serves as the pacemaker for the cell cycle

Mitosis is a dramatic cellular process that affects all parts of the cell. In Xenopus embryos and extracts it is driven by the activation of a bistable trigger circuit, whose various components are localized in the nucleus, centrosome, and cytoplasm. In principle, whichever cellular location has the fastest intrinsic rhythm should act as a pacemaker for the process. Here we followed tubulin polymerization and depolymerization in Xenopus egg extracts supplemented with demembranated sperm, and thereby identified locations where mitosis first occurred. We found that mitosis was commonly first initiated at sperm-derived nuclei and their accompanying centrosomes, and then spread outward in circular trigger waves. The cell cycle was ∼20% more rapid at the nucleus/centrosome-associated trigger wave sources than in the regions of the extract that appeared not to be entrained by trigger waves. Nuclei produced from phage DNA, which did not possess centrosomes, also acted as trigger wave sources, but purified centrosomes in the absence of nuclei did not. We conclude that the nucleus accelerates mitotic entry and propose that it acts as a pacemaker for cell cycle. One Sentence Summary Studies in cycling Xenopus egg extracts show that mitosis first occurs in the nucleus and then spreads outward through the cytoplasm in circular trigger waves.