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How p53 works and functions as a tumor suppressor protein?

p53 is a 53kd tumor suppressor protein that prevents cell cycle progression of cells with damaged DNA. P53 gene is mutated in nearly 50% of all tumors suggesting its importance in preventing cancer cell formation. Defective p53 allows abnormal DNA damaged cells to proliferate leading to cancer.
The two major functions of p53 are:
1. Cell cycle arrest and associated DNA repair and
2. Apoptosis or programmed cell death

Pathway I: Cell cycle arrest
If there is DNA damage, p53 level rise and prevents cell cycle progression to S-phase thereby preventing replication of damaged DNA. Further, it activates transcription of proteins involved in DNA repair. It induces the production of another protein called p21 which is a CDK inhibitor. Thus progression from G1 to S phase is prevented.

p 53 mediated cell cycle arrest and Apoptosis
Pathway II: Apoptosis
Apoptotic pathway is activated in response to irreparable DNA damage. In this pathway, p53 on activation induces expression of BAX gene. The BAX gene product, a proapoptotic protein induces cell to undergo apoptosis or programmed cell death.

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