Mitochondria have DNA because they need to be controlled individually
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Mitochondria have DNA because they need to be controlled individually by 
Ciro Santilli 34 Updated 2024-11-19 Created 1970-01-01
Ciro Santilli 34 Updated 2024-11-19 Created 1970-01-01Argued at Power, Sex, Suicide by Nick Lane (2006) page 212.
Basically, energy supply has to be modulated rather quickly, because we spend a lot sometimes, and very little other times.
Even not turning it off quickly enough is a problem, as it starts to generate free radicals which fuck you up.
If control came from the nucleus, it has no way to address different mitochondria. But it might be that only one of the mitochondria needs the change. If the nucleus tells all mitochondria to stop producing when only one is full, the others are going to say: "nope, I'm not full, continue producing!" and the one that need to stop will have its signal overriden by the others.