Centrosome duplication proceeds during mimosine-lnduced G₁ cell cycle arrest

Centrosome duplication must remain coordinated with cell cycle progression to ensure the formation of a strictly bipolar mitotic spindle, but the mechanisms that regulate this coordination are poorly understood. Previous work has shown that prolonged S-phase is permissive for centrosome duplication, but prolonging either G₂ or M-phase cannot support duplication. To examine whether G₁ is permissive for centrosome duplication, we release serum-starved G₀ cells into mimosine, which delays the cell cycle in G₁. We find that in mimosine, centrosome duplication does occur, albeit slowly compared with cells that progress into S-phase; centrosome duplication in mimosine-treated cells also proceeds in the absence of a rise in Cdk2 kinase activity normally associated with the G₁/S transition. CHO cells arrested with mimosine can also assemble more than four centrioles (termed "centrosome amplification"), but the extent of centrosome amplification during prolonged G₁ is decreased compared to cells that enter S-phase and activate the Cdk2-cyclin complex. Together, our results suggest a model, which predicts that entry into S-phase and the rise in Cdk2 activity associated with this transition are not absolutely required to initiate centrosome duplication, but rather, serve to entrain the centrosome reproduction cycle with cell cycle progression.