G1 cyclin driven DNA replication.

The mitotic cell cycle is driven by Cyclin-Dependent Kinases (CDK). CDK activation requires the binding of activatory subunits termed cyclins. Different waves of cyclins are expressed during the cell cycle, enabling CDKs to trigger phase specific events. For instance, S phase cyclins promote the initiation of DNA replication but not chromosome segregation. There are at least 2 explanations for how such regulation is achieved. According to one of the visions, cyclins confer intrinsic substrate specificity to the CDK catalytic subunit. Alternatively a quantitative model has been proposed, according to which ever-increasing CDK activity is required to trigger cell cycle events from G1 to M. If a quantitative control prevails, then an early cyclin should trigger later cycle events if accumulated at high enough levels at the right time and place. We show here that a G1 phase cyclin bears the potential to trigger DNA replication and promote S and G2 phase specific transcription.