Endosymbiotic theory of mitochondrial origin

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The endosymbiotic theory of mitochondrial origin is an endosymbiotic theory that has been proposed to explain the evolutionary origin of the mitochondrion, an organelle found in most eukaryotic cells. The theory briefly states the following: mitochondria are evolutionary descended from endosymbionts in the eukaryotic cells that were independent living organisms (bacteria, which are prokaryotes). The symbiotic relationship was as follows: the endosymbionts performed aerobic respiration for the cell, and in turn, the cell gave them food and sustenance.

The theory was proposed by the biologist Lynn Margulis and is also termed the Lynn Margulis hypothesis.

Evidence used to support the theory

Evidence type How it supports the endosymbiotic theory
Size The cell of mitochondria () is in the size range for the prokaryotic cells from which they are probably descended. The overall prokaryotic cell size range is , but the older ones from which mitochondria are believed to have descended have sizes at the lower end of the range.
Existence of own DNA Mitochondria have their own DNA that differs from the nuclear DNA. This DNA is very similar to the DNA of bacteria. The coding rules for mitochondrial DNA also match those of bacterial DNA rather than that of the nuclear DNA of eukaryotes.
Mode of propagation Mitochondria replicate using binary fission with their own DNA, similar to prokaryotes. This is unlike other eukaryotic organelles, where new instances of the organelles are assembled from scratch in the cell
Existence of double membrane Each mitochondrion has a double membrane: an inner mitochondrial membrane and an outer mitochondrial membrane separated by an intermembrane space. This is unlike other organelles in eukaryotic cells, which have single biological membranes (note that the single membrane comprises a lipid bilayer).
A double membrane can be explained by the endosymbiotic theory: the inner membrane was the cell membrane of the endosymbiont bacterium, and the outer membrane was a membrane created by the host cell to regulate flow of materials between it and the endosymbiont. The mode of regulation of the entry and exit of materials is also similar to the mode of regulation for endosymbionts.