In a study published in Nature Aging, a research team led by Prof. LI Xin at the Shanghai Institute of Nutrition and Health of the Chinese Academy of Sciences discovered a "biphasic mitochondrial molecular clock" that tracks aging across human organs.

Mitochondria, cellular powerhouses with their own fast-mutating DNA, act as aging "timers." Researchers analyzed mitochondrial RNA from 47 tissues in 838 individuals, and identified two aging signatures: proliferative tissues and post-mitotic tissues.

Proliferative tissues (e.g., skin and gut) experience a "stochastic spread" of random DNA replication errors. This accelerated accumulation of sporadic mutations contributes to age-related dysfunction and tumor risk. 

Post-mitotic tissues (e.g., heart and brain) accumulate mutations at specific "deterministic damage hotspots." This reflects the cumulative burden of high energy demand and mitochondrial turnover, akin to "wear and tear."

The "mitochondrial clock" is like equipping each organ with its own "aging GPS." This biphasic model which unifies "replicative" and "metabolic" aging could empower age-related diagnostics and therapies by providing tissue-specific insights.

The work shows that mitochondria encode aging through two distinct modes, offering a new perspective on why organs age at different rates