Final answer:
A G protein is activated when GDP is exchanged for GTP after a signaling molecule binds to a G-protein-coupled receptor, leading to cellular responses.
Step-by-step explanation:
A G-protein is activated when it is induced to change from being bonded with guanosine diphosphate (GDP) to being bonded with guanosine triphosphate (GTP). In cell signaling, this process involves a G-protein-coupled receptor. When a signaling molecule binds to this receptor, the G-protein changes shape and releases GDP. It then picks up GTP, causing the G-protein to activate. The activated G-protein can then trigger a cascade of cellular responses, which often involve the activation of other proteins or production of second messengers like cyclic AMP (cAMP). Eventually, the GTP is hydrolyzed back to GDP, deactivating the G-protein and allowing the subunits to reassociate, thereby resetting the signaling apparatus for another cycle.