Mitochondria, the energy powerhouses of cells, are distinct from other organelles because they have their own DNA. The genetic material is different from nuclear DNA and has led to the theory that mitochondria were once free-living and then ingested by bacteria.
What are Mitochondria?
Mitochondria are rod-shaped membrane-enclosed organelles that are found in the cytoplasm of most eukaryotic cells. Their size ranges from about 1 to 10 micrometers and they provide the chemical energy that cells need in order to be able to carry out their functions.
Where did Mitochondria Come From?
The origin of mitochondria has been the subject of much debate and discussion over the years. Amongst organelles they are unique because they contain their own DNA and this has led many to believe that they were once free-living organisms that were taken up and absorbed by bacteria at some point in the planet's history. This is known as the endosymbiosis theory (endosymbiosis is a symbiotic relationship between two organisms where one lives inside the other).
This theory about the origin of some organelles first came to the attention of scientists in 1905. Its proponent was a Russian botanist, Konstantin Mereschkowsky who was working with lichens. The idea was extended to mitochondria by biologist Ivan Wallin in the 1920s. But it received more widespread publicity in 1981 following the publication of Symbiosis in Cell Evolution by Lynn Margulis. She argued that some eukaryotic cell structures, including mitochondria, were once free-living and interacted as communities before being absorbed by another free living-organism.
There are several strong lines of evidence which suggest that the origin of mitochondria lies with bacteria - that these organelles were actually prokaryotic cells at some point in time.
Circular-shaped organisation of DNA which resembles bacterial plasmids
- Mitochondria possess double membranes, as do many different types of bacteria
- That they contain their own DNA even though they exist inside cells suggest that they were once free-living, independent organisms
- According to Margulis mitochondrial reproduction is similar to bacterial reproduction "Genetic recombination in (mitochondria) is far more reminiscent of phage and bacterial sexuality than it is of eukaryotic nuclear sexual behavior."
- Some mitochondrial enzymes are similar to bacterial enzymes
Mitochondria produce mitochondria - they cannot be created in a cell that does not already possess mitochondria. So they cannot be created by nuclear DNA.
Has the case been Proven?
The endosymbiotic origin of mitochondria has not yet been proven, and of course we can't turn the clock back to see what happened, but it is a widely accepted theory to explain the origin of mitochondria.
Lynn Margulis. Symbiosis in Cell Evolution. W.H.Freeman & Co Ltd (July 1981)