the end of G2 phase start of the M phase (mitotic phase).

   

The second checkpoint is located at the end of G2 phase, triggering the start of the M phase (mitotic phase). In order for this checkpoint to be passed, the cell has to check a number of factors to ensure the cell is ready for mitosis. If this checkpoint is passed, the cell initiates the many molecular processes that signal the beginning of mitosis. The CDKs associated with this checkpoint are activated by phosphorylation of the CDK by the action of a "Maturation promoting factor" (Mitosis Promoting Factor, MPF).

The molecular nature of this checkpoint involves an activating phosphatase, known as Cdc25, which under favorable conditions removes the inhibitory phosphates present within the MPF (term for the cyclin B/CDK1 complex). However, DNA is frequently damaged prior to mitosis, and, to prevent transmission of this damage to daughter cells, the cell cycle is arrested via inactivation of the Cdc25 phosphatase. This is done by the ATM kinase protein which phosphorylates Cdc25 which leads to its ubiquitinylation and destruction.