Figure 7.

A model of how declining Cdc25 and Cdk1 activity results in S-phase 14 prolongation. (A) Cdk1 activity is low throughout S-phase 14, first due to low cyclin, then due to low Cdc25 (as a result of its developmental down-regulation), which allows late-replicating sequences to replicate late and S-phase 14 to be long. (B) Our injections of Cdc25 mRNA raise the level of Cdc25 during S-phase 14, causing higher Cdk1 activity, which causes the late-replicating sequences to replicate early, leading to a shortened S phase. (C) In the pre-MBT S phases, such as cycle 13 depicted here, all sequences basically replicate early, with only a couple of minutes difference in the time that individual sequences replicate. We propose that, while the late-replicating sequences are specified, Cdk1 activity during the pre-MBT S phases results in the early replication of late-replicating sequences during those cycles as well, leading to their short S phases. (D) Our injections of RNAi against the mitotic cyclins extend the length of S-phase 13. We propose that this is due to a reduction in Cdk1 activity, which is necessary to advance the late-firing sequences in the pre-MBT S phases.