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Figure 1
The many roles that Plk1 plays in the cell cycle. Plk1 has been implicated in the phosphorylation and activation (small arrows) or inactivation (blunt-ended lines) of the effectors of a variety of cell-cycle events. Plk1 may also be part of the negative feedback mechanism that prevents normal chromosome segregation when there is DNA damage.
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Figure 2
Model for the signalling pathways that act at different stages of Xenopus meiotic maturation in response to progesterone. RINGO–CDK1 complexes stimulate the transition between G2 and meiosis (M) I, whereas cyclin–CDK1 complexes are required for the transition between MI and MII under the influence of Cdc25 and Myt1. The MAP kinase pathway arrests cells in MII in response to activation by mos, possibly through stimulating Bub1. Fertilization is required to release the cells from this arrest.
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