To study the mechanism of centrosome duplication in cycling cells, we 
established a novel system of multiple centrosome formation in two types of 
cells: CHO cells treated with RO3306, a Cyclin-dependent kinase 1 (Cdk1) 
inhibitor and DT40 cells, in which Cdks were knocked out by chemical genetics. 
Cdk1-inactivated cells initiated DNA replication and centrosome duplication at 
the onset of S phase. They became arrested at the end of G2, but the centrosome 
cycle continued to produce supernumerary centrioles/centrosomes without DNA 
endoreplication in those cells. Centrosomes were amplified in a highly 
synchronous and reproducible manner: all of them were located next to the 
nucleus and spread widely apart from each other with several μm in distance. 
Double knockout of Cdk1 and Cdk2 caused cell cycle arrest at G1/S and 
centrosomes were no longer duplicated. However, cells continued to grow and 
increased their volume over 10-fold during 48 hr of culture. Centrosome 
components, including γ-tubulin and Cep135, were synthesized and accumulated 
during the arrest, allowing rapid centrosome multiplication upon recovery from 
the cell cycle arrest or expression of exogenous Plk4 in G1/S cells. Thus 
centrosome amplification results from the discoordination of the centrosome 
cycle from the progression of other cell cycle events, which is controlled by 
different levels of Cdk activities.