Arrange the steps in apoptosis in sequence of occurrence: 1. Release of cyt c in cytoplasm2. Activation of APAF-13. Removal of BCL-2 4. Caspase activation
**Core Concept**
Apoptosis, or programmed cell death, is a crucial process in development and maintenance of tissue homeostasis. It involves a series of molecular events leading to cell death, including the activation of **caspases** and the release of **cytochrome c (cyt c)**. The **BCL-2 family** of proteins plays a significant role in regulating apoptosis.
**Why the Correct Answer is Right**
The correct sequence begins with the **removal of BCL-2**, which normally inhibits apoptosis by preventing the release of **cyt c** from the mitochondria. Once BCL-2 is removed or inactivated, **cyt c** is released into the cytoplasm, where it binds to **APAF-1**, leading to the **activation of APAF-1**. This complex then activates **caspases**, the key executioners of apoptosis.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the removal of BCL-2 must occur before the release of cyt c.
**Option B:** Incorrect as APAF-1 activation follows cyt c release.
**Option D:** Incorrect because caspase activation is a downstream event.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that the **intrinsic pathway** of apoptosis involves the mitochondria and the BCL-2 family, while the **extrinsic pathway** involves death receptors and **caspase 8**. Understanding these pathways is crucial for grasping the regulation of apoptosis.
**Correct Answer:** C. 3, 1, 2, 4