Single-nucleoid imaging in whole cells defines the dynamics of the mtDNA life cycle
The mitochondrial genome (mtDNA) is essential for oxidative phosphorylation, and mammalian cells typically contain hundreds or thousands of copies. Although specific aspects of mtDNA replication and degradation have been explored, a complete account of the mtDNA life cycle has remained elusive, particularly for nondividing cells. Using super-resolution and 4D lattice light-sheet imaging, we quantified the full mtDNA life cycle in quiescent primary human cells. We show that cells maintain steady state by replicating and degrading 1.5 {+/-} 0.2% of total mtDNA content each hour, a remarkably rapid flux. Younger mtDNA molecules are closer to the nucleus, spared from degradation, and serve as foci for further replication events. Non-proliferative cells regulate nucleoid density within the mitochondrial network, and the mitochondrial membrane potential sustains mtDNA copy number. These findings provide a foundational understanding of the dynamics underlying mtDNA homeostasis and a mechanism explaining how mutations accumulate in aging and disease.