University of Central Florida (UCF) PCB3023 Molecular Cell Biology Practice Exam 4

Cdc25 is a phosphatase that plays a critical role in the regulation of the cell cycle, specifically in the transition from G2 phase to M phase. In a healthy cell cycle, Cdc25 removes inhibitory phosphate groups from cyclin-dependent kinase (Cdk) complexes, allowing the activation of S-Cdk and M-Cdk, which are essential for the cell to progress into mitosis (M phase).

When DNA is damaged or not fully replicated during S phase, checkpoints in the cell cycle are activated to prevent the cell from progressing into M phase until the damage is resolved or DNA replication is completed. One such checkpoint response involves the inhibition of Cdc25. When Cdc25 is inhibited, it can no longer activate the M-Cdk by removing the inhibitory phosphates, thereby preventing the cell from entering mitosis.

The role of p53 is significant in DNA damage response, as it helps to activate the transcription of genes that cause cell cycle arrest or apoptosis, but it does not directly regulate the progression from G2 to M phase. ORC, or the origin recognition complex, is involved in the initiation of DNA replication, but its function does not directly block the transition to mitosis in response to DNA damage. S-C