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Endosymbiotic theory - This theory states that eukaryote organelles may have evolved when large eukaryotic organisms engulfed but did not digest smaller organisms and a symbiotic relationship arose.
The endosymbiotic theory explains how relatives of ancient bacteria ended up in modern-day cells.
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The endosymbiotic theory can be applied to chloroplasts too. It is supposed that these organelles were primitive photosynthetic prokaryotes because they have their own DNA, RNA and ribosomes and they can self-replicate too.
32. What are the main components of the cytoskeleton?
The endosymbiotic theory suggests that mitochondria and plastids were formerly small prokaryotes living within larger cells.
The term endosymbiont is used for a cell that lives within a host cell.
Indeed, the endosymbiotic theory for the origin of organelles sees a form of horizontal gene transfer as a critical step in the evolution of eukaryotes such as fungi, plants, and animals.
Evidence for Endosymbiotic Theory
Chloroplasts and mitochondria have DNA
does not code for all proteins
some genes in nucleus
proteins imported rom cytoplasm ...
According to the endosymbiotic theory, mitochondria might have been the remnants of early bacteria engulfed by ancient eukaryotic cells a billion years ago that might have evolved and become energy-yielding structures within eukaryotic cells at present.
In 1967 she proposed a contentious new hypothesis which became her most important scientific contribution as the endosymbiotic theory of the origin of mitochondria as separate organisms that long ago entered a symbiotic relationship with eukaryotic cells through endosymbiosis.
This endosymbiotic theory of eukaryotic organelles was championed by Lynn Margulis. Originally controversial, this theory is now accepted.
The mitochondria of eukaryotes probably evolved from endosymbiotic bacteria. Because of the similarities of their genomes, rickettsias may be the closest relatives to the ancestors of mitochondria.
Link to discussion of the endosymbiotic theory of the origin of mitochondria.
It has also been noticed that bacterial genes exist in eukaryotic nuclear genomes and that these genes generally encode mitochondrial and plastid proteins, giving support to the endosymbiotic theory of the origin of these organelles.
See also: What is the meaning of Theory, Bacteria, Organ, Cells, Trans?