What is the consequence of having a version of Cdc6 that cannot be phosphorylated?

Having a version of Cdc6 that cannot be phosphorylated leads to the scenario where the cell's regulatory mechanisms governing the cell cycle are disrupted. Cdc6 is a critical component for the initiation of DNA replication, as it functions in preparing the DNA replication origins for the S phase of the cell cycle.

In a normal cell cycle, Cdc6 is phosphorylated, which is a necessary signal that helps to transition the cell from the G1 phase into the S phase, ensuring that DNA replication occurs only once per cycle. If Cdc6 cannot be phosphorylated, the signaling pathways that prevent premature entry into the next S phase are compromised. This results in a lack of proper control over the timing of DNA replication. Consequently, the cell might initiate another round of DNA synthesis without completing the previous cycle, leading to potential problems such as re-replication of the genome and genomic instability.

This mechanism highlights the importance of post-translational modifications, such as phosphorylation, in the regulation of the cell cycle. In summary, the unphosphorylatable form of Cdc6 disrupts the necessary checkpoints for DNA replication, causing the cell to enter another S phase prematurely.