Practice question

Aggregation centers broadcast pulsatile 3',5'-cyclic AMP, the universal chemoattractant for Dictyostelium. Starving cells relay cAMP outward as spiral or concentric waves detectable by dark-field optics. Neighbors sense increasing concentration via high-affinity G protein-coupled cAR1 receptors that activate adenylyl cyclase ACA, producing further cAMP. This excitable relay generates directional movement up gradient. Calcium ions, ATP, or ADP do not elicit chemotaxis in this species. Periodic cAMP secretion every six minutes arises from adaptation of receptor and degradation by extracellular phosphodiesterase PdsA, ensuring wave propagation over centimeter-scale territories.