Temporal regulation of organelle biogenesis.

Organelle abundance in cells is tightly regulated in response to external stimuli, but the underlying mechanisms remain poorly understood. Time-lapse imaging of fluorescently labelled organelles enables single-cell measurements of organelle copy numbers, revealing the time evolution of their distribution across a cell population. Building on a recently proposed kinetic model of organelle biogenesis, which incorporates de novo synthesis, fission, fusion, and degradation, we explore the time-dependent dynamics of organelle abundance. While previous studies focused on steady-state properties, here we calculate the first two moments of: 1) organelle copy numbers over time, and 2) first passage times to reach a specified organelle count. We show that these two moments provide a powerful means to discriminate between different mechanisms of organelle biogenesis. Notably, the time-dependent behaviour of organelle biogenesis reveals richer dynamics compared to the steady-state scenario. Our findings shed light on how cells attain steady-state organelle abundance after cell division and environmental perturbation.