Mitochondria were acquired by host cells through a process known as endosymbiosis. Here are some key points about this process:
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Endosymbiotic Theory: Mitochondria are believed to have originated from free-living prokaryotic organisms, likely an ancestral alpha-proteobacterium, that were engulfed by an ancestral eukaryotic cell. This endosymbiotic relationship allowed the host cell to benefit from the energy-producing capabilities of the engulfed bacteria.
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Double Membrane: Mitochondria have a double membrane structure that is consistent with the idea that they evolved from a prokaryotic organism. The inner membrane is highly folded into structures called cristae, which increase the surface area for energy production.
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DNA and Ribosomes: Mitochondria contain their own circular DNA (mtDNA) and ribosomes, similar to those found in bacteria. This supports the idea that mitochondria were once independent organisms capable of reproducing themselves.
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Gene Transfer: Over time, most of the genes from the engulfed prokaryote were transferred to the host cell's nuclear genome. As a result, modern mitochondria are no longer capable of living independently and rely on the host cell for most of their functions.
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Mutual Benefit: This relationship is mutually beneficial; the mitochondria provide ATP (energy) to the host cell through oxidative phosphorylation, while the host cell provides protection and nutrients to the mitochondria.
These points reflect the widely accepted understanding of how mitochondria were acquired by host cells in the evolutionary history of eukaryotes.