Figure 2.
The ability of Cyclins E and A to induce DNA replication is consistent with models in which their respective requirements for S phase change during development. (a) In endocycles, Cyclin A is not expressed and Cyclin E is solely responsible for S phase. S phase occurs apparently only when Cyclin E is present and ends upon the disappearance of Cyclin E, even though the genome is not completely replicated by that time. (b) In mitotic cycles that lack a G1 phase – such as the first 16 cycles of Drosophila embryogenesis – Cyclin E is present at constantly high levels and entry into S phase is limited only by the restoration of replication competence that accompanies mitosis. As a result of perpetually high levels of Cyclin E, Cyclin A may not be required for S phase in these cells. (c) In mitotic cycles with a G1 phase, Cyclin E is downregulated prior to the end of S phase in preparation for the following cell cycle. In these cells, the absence of Cyclin E late in S phase may create a requirement for Cyclin A for the completion of replication.