AP BIOLOGY

Detailed explanation-1: -The sequence of events that define the extrinsic phase of apoptosis are best characterized with the FasL/FasR and TNF-/TNFR1 models. In these models, there is clustering of receptors and binding with the homologous trimeric ligand.

Detailed explanation-2: -Fas is a membrane protein belonging to the death receptor family. Cross-linking of Fas by its ligand, FasL, or agonistic anti-Fas antibodies, induces apoptosis of cells expressing Fas on the membrane by triggering a cascade of caspases.

Detailed explanation-3: -In both the intrinsic and extrinsic pathway of apoptosis, signaling results in the activation of a family of Cys (Cysteine) proteases, named caspases, that act in a proteolytic cascade to dismantle and remove the dying cell (1).

Detailed explanation-4: -The extrinsic pathway involves the binding of ligands to cell surface ‘death receptors’ (DR) which in turn initiates the caspase cascade. Death receptors are part of the tumour necrosis factor (TNF) gene superfamily and provide a rapid and efficient route to apoptosis.

Detailed explanation-5: -Apoptosis, one of the better-known forms of regulated cell death, is activated when cell-surface death receptors like Fas are engaged by their ligands (the extrinsic pathway) or when BCL-2-family pro-apoptotic proteins cause the permeabilization of the mitochondrial outer membrane (the intrinsic pathway).