Figure 2.
The oscillatory levels of key factors in endocycle maintenance. (A) The canonical mitotic cell cycle is composed of four sequential phases: G1, S, G2, and M. This cell cycle gives rise to two identical daughter cells. (B) The endocycle is composed of two alternating phases: G and S. This leads to increased DNA ploidy within a single cell. (C) The endocycle is driven by oscillations in key cell cycle factors. Levels of the E2F1 transcription factor rise at the end of G phase to turn on transcription of cycE and is degraded during S phase. Cyclin E/CDK2 activity rises at the start of S phase to initiate DNA replication and falls at the end of S phase after the completion of replication. The activity of the E3 ubiquitin ligase CRL4-Cdt2 peaks in S phase, when it marks E2F1 for degradation. In addition, the activity of APC/CFzr/Cdh1 peaks when Cyclin E/CDK2 activity levels are low and targets Geminin for degradation during G phase.