from dm vasudevan a textbook of Biochemistry for Medical Students, 11th edition. Write a 3-mark short answer on p53 is called as the guardian of the genome.
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I now have all the key content needed. The DM Vasudevan book is not in the library, but the content on p53 as "guardian of the genome" is well-covered across the available biochemistry textbooks, and the concepts are standard across all medical biochemistry texts including Vasudevan. Here is the 3-mark short answer:
p53 is Called the "Guardian of the Genome"
p53 is a tumor suppressor protein encoded by the TP53 gene and functions as a transcription factor that regulates the cell cycle and apoptosis. It is called the "guardian of the genome" because it protects the integrity of the genome by preventing the replication and propagation of cells with damaged DNA.
Mechanism of Action
1. Sensing DNA damage:
When a cell suffers DNA damage (due to mutagens, ionizing radiation, UV light, or other genotoxic stress), p53 levels rise rapidly. Normally, p53 is short-lived and kept low by its association with MDM2, a protein that targets p53 for degradation. Upon DNA damage, protein kinases (e.g., ATM) phosphorylate p53, releasing it from MDM2 and stabilizing it.
2. Cell cycle arrest (G1 arrest):
Stabilized p53 acts as a transcription factor and stimulates the transcription of p21 (a CDK inhibitor). p21 inhibits cyclin-CDK complexes, preventing phosphorylation of Rb and blocking the cell from entering S phase. This halt gives the cell time to repair damaged DNA.
3. DNA repair:
p53 also induces transcription of GADD45 (Growth Arrest and DNA Damage), a DNA repair enzyme that facilitates repair of the damaged DNA.
4. Apoptosis (if damage is irreparable):
If DNA damage cannot be repaired, p53 activates pro-apoptotic genes (e.g., BAX), driving the cell into programmed cell death. This prevents a cell with a permanently damaged genome from proliferating into a tumor.
Clinical Significance
- Mutations in p53 are found in more than 50% of human tumors, making TP53 the most commonly mutated gene in cancer.
- Loss of p53 means damaged DNA is replicated unchecked, dramatically increasing the frequency of further mutations.
- Inherited mutation of one p53 allele causes Li-Fraumeni syndrome, characterized by multiple malignancies at a young age.
In summary, p53 acts as a checkpoint guardian - it arrests the cell cycle after DNA damage, facilitates repair, and eliminates irreparably damaged cells by apoptosis, thereby maintaining genomic stability and preventing carcinogenesis.
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