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Why do mitochondria have their own DNA?

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Fluorescent confocal image of HeLa cells with green microtubules outlining cell shapes, red mitochondria, and blue nuclei. — mitochondrion Mitochondria (red) are found throughout the cytoplasm of almost all eukaryotic cells (cell nucleus is shown in blue; cytoskeleton is shown in yellow).

Mitochondria possess their own DNA because they are the evolutionary descendants of free-living prokaryotes that were incorporated into primitive eukaryotic cells billions of years ago, a process known as endosymbiosis.21

According to the endosymbiont hypothesis, these ancient prokaryotes entered into a stable, mutually beneficial relationship with a host cell. Over millions of years, the internalized organisms underwent genome reduction and lost the ability to survive independently, eventually becoming the mitochondria found in modern eukaryotic cells.21

The presence of their own circular DNA—which is similar to that of bacteria—along with their ability to reproduce by binary fission and their unique double-membrane structure, serves as evidence of this shared evolutionary past. While most mitochondrial proteins are now encoded by the cell's nucleus, the mitochondrial genome retains specific genes, such as those required for components of the electron transport chain, which are essential for the organelle's role in energy production.31