Explain the MAPK signaling cascade and its biological significance.

The MAPK (Mitogen-Activated Protein Kinase) cascade is a three-tier kinase pathway: MAPKKK activates MAPKK, which activates MAPK. The classical ERK pathway begins with receptor tyrosine kinase activation, recruiting Ras GTPase, which activates Raf (MAPKKK), MEK (MAPKK), then ERK (MAPK). Activated ERK phosphorylates transcription factors and other targets regulating cell proliferation, differentiation, and survival. Parallel pathways (JNK, p38) respond to stress. Signal amplification occurs at each step. MAPK pathway mutations, particularly in Ras and Raf, drive many cancers, making them important therapeutic targets.