(evolutionary biology) A theory suggesting that the organelles such as mitochondria and chloroplasts within the eukaryotic cell came about as a result of the early endosymbiosis between prokaryotic endosymbionts and eukaryotic host cell
Endosymbiotic theory is an evolutionary theory that assumes endosymbiosis between prokaryotic endosymbionts and eukaryotic host cell is the means by which organelles such as mitochondria and chloroplasts occur within eukaryotic cells.1 Proponents of this theory posited that about 1.5 billion years a larger cell took in smaller free-living prokaryotes (bacteria) and inside the cell the prokaryotes lived as endosymbionts. Research findings that seem to back up this theory implicate that the mitochondria arose from proteobacteria (such as SAR11 clade)2 whereas the chloroplasts arose from cyanobacteria (particularly the nitrogen-fixing cyanobacteria)3. Other proofs that seem to support this theory are as follows:
- Mitochondria and plastids are capable of reproducing their own through a process akin to prokaryotic binary fission
- Both mitochondria and plastids have single circular DNA similar to that of bacteria in terms of size and structure but different from that of the nucleus of the cell.
- Porins in the outer membranes of mitochondria and chloroplasts are similar to those in bacterial cell membrane. Cardiolipin, a membrane lipid, is found only in bacterial cell membrane and inner mitochondrial membrane.
1 endosymbiosis. (n.d.). Collins English Dictionary - Complete & Unabridged 10th Edition. Retrieved from Dictionary.com website .
2"Mitochondria Share an Ancestor With SAR11, a Globally Significant Marine Microbe". (2011). Retrieved from ScienceDaily [http://www.sciencedaily.com/releases/2011/07/110725190046.htm.
3 Deusch, O.; et al. (2008). "Genes of cyanobacterial origin in plant nuclear genomes point to a heterocyst-forming plastid ancestor". Mol. Biol. Evol 25: 748–761