Final answer:
Upon DNA damage, the p53 protein triggers G1 cell cycle arrest, facilitating DNA repair. If repair is unachievable, p53 prompts apoptosis to avoid replication of defective DNA. Additionally, p21 helps maintain the arrest by inhibiting Cdk/cyclin complexes.
Step-by-step explanation:
Pathway of DNA Damage Inducing G1 Arrest
When DNA damage occurs within a cell, the protein p53 plays a crucial role in arresting the cell cycle at the G1 phase, allowing time for DNA repair mechanisms to fix the damage. If the damage is beyond repair, p53 is instrumental in initiating apoptosis (programmed cell death) to prevent the replication of defective DNA. The G1 checkpoint is a critical control point where proteins such as Rb, p53, and p21 interact to monitor and respond to DNA integrity. Upon detecting DNA damage, p53 levels increase, leading to the production of p21. p21 then inhibits the activity of Cdk/cyclin complexes, enforcing the cell cycle arrest dictated by p53. If DNA repair is successful, the cell can proceed to the S phase; otherwise, continuous high levels of p53 and p21 due to unresolved stress or damage may signal the cell to undergo apoptosis.