All of the following are true regarding the regulation and execution of apoptosis EXCEPT:

Explanation

## Mechanism of Caspase-9 Activation **Key Point:** Caspase-9 activation requires **cytochrome c** (not its absence). Cytochrome c is released from mitochondria during MOMP and is essential for apoptosome formation. ## Apoptosome Formation — The Correct Mechanism 1. Mitochondrial outer membrane permeabilization (MOMP) releases cytochrome c into the cytoplasm 2. Cytochrome c binds to Apaf-1 (apoptotic protease-activating factor-1) 3. The cytochrome c–Apaf-1 complex recruits procaspase-9 and dATP/ATP 4. This forms the **apoptosome**, a wheel-like structure that activates caspase-9 5. Activated caspase-9 then cleaves and activates executioner caspases-3 and -7 [cite:Robbins 10e Ch 7] ## Correct Statements Explained ### PARP Cleavage by Caspase-3 - PARP is a DNA repair enzyme that detects DNA breaks and recruits repair machinery - Caspase-3 cleaves PARP into 89 kDa and 24 kDa fragments - This inactivation is a hallmark of apoptosis and prevents futile DNA repair in dying cells - PARP cleavage is used as a marker of apoptosis in flow cytometry and immunohistochemistry [cite:Robbins 10e Ch 7] ### BH3-Only Proteins as Stress Sensors - BH3-only proteins (Bid, Bim, Puma, Noxa) contain only the BH3 domain - They are activated by cellular stress (DNA damage, growth factor withdrawal, ER stress) - They antagonize anti-apoptotic Bcl-2 and Bcl-xL by direct binding, allowing pro-apoptotic Bax/Bak to oligomerize - This is the key regulatory step of the intrinsic pathway [cite:Robbins 10e Ch 7] ### IAPs and Smac/DIABLO - Inhibitor of apoptosis proteins (XIAP, cIAP1/2) bind to caspases-3, -7, and -9 and block their activity - Smac/DIABLO is released from mitochondria during MOMP - Smac/DIABLO binds to IAPs and relieves their inhibition of caspases ("second mitochondrial-derived activator of caspases") - This is a critical checkpoint in apoptosis regulation [cite:Robbins 10e Ch 7] ## Why Option 3 Is Wrong **High-Yield:** The stem says caspase-9 is activated "in the **absence** of cytochrome c," which is incorrect. Cytochrome c is **required** for apoptosome formation and caspase-9 activation. This is a classic exam trap — confusing the trigger (presence of cytochrome c) with the absence of a signal.