Eukaryotic cells are cells which have a nucleus. Within that nucleus are various discrete structures with very specific functions that are, as a group, called 'organelles'.
In the 1970s a book by professor Lynn Margulis (titled Origin of Eukaryotic Cells) revived long-standing ideas (c.1910) that the organelles (e.g. mitochondria and plastids) had somehow evolved from free-living bacteria that had been symbiotically incorporated within eukaryotic host cells.
Technological developments in DNA cloning and sequencing in the 1970s and 1980s confirmed that the DNA of organelles such as is found in mitochondria had many similarities to that of some free living bacteria (e.g. Alphaproteobacteria). (See : Cold Spring Harb Perspect Biolv.4(9); )
There are various theories regarding how the bacteria might have been successfully incorporated into 'cells' in the distant evolutionary past. But the question of whether mitochondria and other organelles were formed at the same time or after the nucleus, remains controversial. As is the question of how the symbiotic relationship took hold (e.g. mitochondria currently synthesize ATP for the cell by metabolizing carbon-based macro-molecules)
The evolutionary processes underlying the differentness of prokaryotic and eukaryotic cells and the origin of the latter's organelles are still poorly understood. For about 100 years, the principle of endosymbiosis has figured into thoughts as to how these processes might have occurred.
Source : Biol Chem .382(11):1521-39
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