Centrosomes nucleate microtubules and duplicate once per cell cycle. This d
uplication and subsequent segregation in mitosis results in maintenance of
the one centrosome/cell ratio. Centrosome duplication occurs during the G(1
)/S transition in somatic cells and must be coupled to the events of the nu
clear cell cycle; failure to coordinate duplication and mitosis results in
abnormal numbers of centrosomes and aberrant mitoses. Using both in vivo an
d in vitro assays, we show that centrosome duplication in Xenopus laevis em
bryos requires cyclin/cdk2 kinase activity. Injection of the cdk (cyclin-de
pendent kinase) inhibitor p21 into one blastomere of a dividing embryo bloc
ks centrosome duplication in that blastomere; the related cdk inhibitor p27
has a similar effect. An in vitro system using Xenopus extracts carries ou
t separation of the paired centrioles within the centrosome. This centriole
separation activity is dependent on cyclin/cdk2 activity; depletion of eit
her cdk2 or of the two activating cyclins, cyclin A and cyclin E, eliminate
s centriole separation activity. In addition, centriole separation is inhib
ited by the mitotic state, suggesting a mechanism of linking the cell cycle
to periodic duplication of the centrosome.