Bursty mRNA production: gene ON/OFF switching + Poisson transcription + degradation
Gillespie SSA simulation. Gene switches ON→OFF (k_off) and OFF→ON (k_on). mRNA produced at k_r when ON, degraded at γ·n. Beta-Poisson mixture: Fano = 1 + k_r·k_off/((k_on+k_off)·(k_on+k_off+γ)).