It's not always the case that you can't have too much of a good thing. For example, although two centrosomes are essential for assembly of the bipolar spindle during mitosis, more than that can lead to genome instability. It's important, then, that centrosomes are duplicated only once per cell cycle. But how is this regulated?

In January's Nature Cell Biology, Steven Reed and co-workers propose a model for duplication of the spindle pole body (SPB; the yeast equivalent of the centrosome). Based on the premise that DNA replication — which also must occur only once per cell cycle — is coordinated with cell-cycle progression, the authors asked whether cyclin/CDK activities might activate duplication of the SPB and inhibit reduplication until completion of the cycle.

According to Reed and colleagues' model, the three G1 cyclins (Clns 1, 2 and 3) in budding yeast are involved in controlling SPB duplication. Subsequent maturation, an essential step that must be completed before SPBs can reduplicate, is directed either by the two S-phase B cyclins, Clb5 and Clb6, or by one of four mitotic B cyclins (Clbs 1, 2, 3 and 4). Finally, the four mitotic B cyclins can block SPB reduplication until mitosis has been completed or under checkpoint-arrest conditions. It seems that a fine balance between the positive and negative effects of cell-cycle proteins does, indeed, regulate SPB duplication.