THE COORDINATION OF CENTROSOME REPRODUCTION WITH NUCLEAR EVENTS OF THE CELL-CYCLE IN THE SEA-URCHIN ZYGOTE

Centrosomes repeatedly reproduce in sea urchin zygotes arrested in S p hase, whether cyclin-dependent kinase 1-cyclin B (Cdk1-B) activity rem ains at prefertilization levels or rises to mitotic values. In contras t, when zygotes are arrested in mitosis using cyclin B Delta-90, anaph ase occurs at the normal time, yet centrosomes do not reproduce. Toget her, these results reveal the cell cycle stage specificity for centros ome reproduction and demonstrate that neither the level nor the cyclin g of Cdk1-B activity coordinate centrosome reproduction with nuclear e vents. In addition, the proteolytic events of the metaphase-anaphase t ransition do not control when centrosomes duplicate. When we block pro tein synthesis at first prophase, the zygotes divide and arrest before second S phase. Both blastomeres contain just two complete centrosome s, which indicates that the cytoplasmic conditions between mitosis and S phase support centrosome reproduction. However, the fact that these daughter centrosomes do not reproduce again under such supportive con ditions suggests that they are lacking a component required for reprod uction. The repeated reproduction of centrosomes during S phase arrest points to the existence of a necessary ''licensing'' event that resto res this component to daughter centrosomes during S phase, preparing t hem to reproduce in the next cell cycle.