Which of the following is the primary sensor of DNA damage in the intrinsic apoptotic pathway?

Explanation

## The Intrinsic Apoptotic Pathway **Key Point:** The intrinsic (mitochondrial) pathway is triggered by intracellular stress signals, primarily DNA damage, which activates p53 as the central "guardian of the genome." ### p53's Role in Apoptosis 1. **Sensor of DNA damage**: p53 is activated by ATM/ATR kinases in response to double-strand breaks or other genomic insults. 2. **Transcriptional activation**: p53 induces expression of pro-apoptotic genes (BAX, PUMA, NOXA) and suppresses anti-apoptotic genes (BCL-2, BCL-XL). 3. **Mitochondrial commitment**: Increased BAX/BAK oligomerization leads to outer mitochondrial membrane permeabilization (OMMP). 4. **Cytochrome c release**: OMMP allows cytochrome c to escape into the cytosol, forming the apoptosome with Apaf-1 and caspase-9. **High-Yield:** p53 mutations are found in >50% of human cancers because loss of p53-mediated apoptosis allows damaged cells to survive and proliferate. ### Comparison: Intrinsic vs. Extrinsic Pathways | Feature | Intrinsic | Extrinsic | |---------|-----------|----------| | **Trigger** | Intracellular stress (DNA damage, hypoxia, ROS) | Death receptor ligation (FasL, TNF-α) | | **Primary sensor** | p53 | Death receptor (Fas, TNF-R1) | | **Key molecules** | Bcl-2 family, mitochondria, caspase-9 | Death-inducing signaling complex (DISC), caspase-8 | | **Mitochondrial involvement** | Direct (OMMP) | Indirect (via caspase-8 → Bid cleavage) | **Clinical Pearl:** Chemotherapy and radiation induce apoptosis primarily through the intrinsic pathway by causing DNA damage and activating p53.