1、Endosymbiotic theoryThe endosymbiotic theory concerns the origins of and (e.g.), which are of cells. According to this theory, these organelles originated as separate organisms that were taken inside the cell as. Mitochondria developed from (in particular, or close relatives) and chloroplasts from.H
2、istoryThe endosymbiotic theory was first articulated by the Russian botanist in 1905. Mereschkowski was familiar with work by botanist, who had observed in 1883 that the division of in green plants closely resembled that of free-living, and who had himself tentatively proposed (in a footnote) that g
3、reen plants had arisen from a symbiotic union of two organisms. extended the idea of an endosymbiotic origin to in the 1920s.These theories were initially dismissed or ignored. More detailed electron microscopic comparisons between cyanobacteria and chloroplasts (for example studies by), combined wi
4、th the discovery that plastids and mitochondria contain their own DNA (which by that stage was recognized to be the hereditary material of organisms) led to a resurrection of the idea in the 1960s.The endosymbiotic theory was advanced and substantiated with microbiological evidence by in a 1967 pape
5、r, The Origin of Mitosing Eukaryotic Cells. In her 1981 work Symbiosis in Cell Evolution she argued that eukaryotic cells originated as communities of interacting entities, including endosymbiotic that developed into eukaryotic and. This last idea has not received much acceptance, because flagella l
6、ack DNA and do not show ultrastructural similarities to prokaryotes. See also.According to Margulis and, “Life did not take over the globe by combat, but by networking“ (i.e., by cooperation).The possibility that may have an endosymbiotic origin has also been considered, although they lack DNA. prop
7、osed that they may have been the first endosymbionts, allowing cells to withstand growing amounts of free molecular oxygen in the Earths atmosphere. However, it now appears that they may be formed de novo, contradicting the idea that they have a symbiotic origin.It is also believed that these endosy
8、mbionts transferred some of their own DNA to the host cells nucleus during the evolutionary transition from a symbiotic community to an instituted eukaryotic cell. This hypothesis is thought to be possible because it is known today from scientific observation that transfer of DNA occurs between prok
9、aryotic species, even if they are not closely related. Prokaryotes can take up DNA from their surroundings and have a limited ability to incorporate it into their own genome.Evidence New mitochondria and plastids are formed only through a process similar to. In some, such as, the plastids can be des
10、troyed by certain chemicals or prolonged absence of light without otherwise affecting the cell. In such a case, the plastids will not regenerate. They are surrounded by two or more, and the innermost of these shows differences in composition from the other membranes of the cell. The composition is l
11、ike that of a prokaryotic cell membrane. Both mitochondria and plastids contain that is different from that of the cell nucleus and that is similar to that of (in being circular in shape and in its size). DNA sequence analysis and estimates suggests that nuclear DNA contains genes that probably came
12、 from plastids. These organelles are like those found in bacteria (70s). Proteins of organelle origin, like those of bacteria, use N-formylmethionine as the initiating amino acid. Much of the internal structure and biochemistry of plastids, for instance the presence of and particular, is very simila
13、r to that of. estimates constructed with bacteria, plastids, and eukaryotic genomes also suggest that plastids are most closely related to cyanobacteria. Mitochondria have several enzymes and transport systems similar to those of prokaryotes. Some proteins encoded in the nucleus are transported to t
14、he organelle, and both mitochondria and plastids have small genomes compared to bacteria. This is consistent with an increased dependence on the eukaryotic host after forming an endosymbiosis. Most genes on the organellar genomes have been lost or moved to the nucleus. Most genes needed for mitochon
15、drial and plastid function are located in the nucleus. Many originate from the bacterial endosymbiont. Plastids are present in very different groups of, some of which are closely related to forms lacking plastids. This suggests that if chloroplasts originated de novo, they did so multiple times, in
16、which case their close similarity to each other is difficult to explain. Many of these protists contain “primary“ plastids that have not yet been acquired from other plastid-containing eukaryotes. Among the eukaryotes that acquired their plastids directly from bacteria (known as), the algae have chl
17、oroplasts that strongly resemble cyanobacteria. In particular, they have a cell wall between their two membranes. Mitochondria and plastids are just about the same size as bacteria. Secondary endosymbiosisPrimary endosymbiosis involves the engulfment of a bacterium by another free living organism. S
18、econdary endosymbiosis occurs when the product of primary endosymbiosis is itself engulfed and retained by another free living eukaryote. Secondary endosymbiosis has occurred several times and has given rise to extremely diverse groups of algae and other eukaryotes. Some organisms can take opportuni
19、stic advantage of a similar process, where they engulf an alga and use the products of its photosynthesis, but once the prey item dies (or is lost) the host returns to a free living state. Obligate secondary endosymbionts become dependent on their organelles and are unable to survive in their absenc
20、eOne possible secondary endosymbiosis in process has been observed by Okamoto where the remnant nucleus of the red algal symbiont (the) is present between the two inner and two outer plastid membranes.Despite the diversity of organisms containing plastids, the morphology, biochemistry, genomic organ
21、isation, and molecular phylogeny of plastid RNAs and proteins suggest a single origin of all extant plastids although this theory is still debated. Problems Neither mitochondria nor plastids can survive in oxygen or outside the cell, having lost many essential genes required for survival. The standa
22、rd counterargument points to the large timespan that the mitochondria/plastids have co-existed with their hosts. In this view, genes and systems that were no longer necessary were simply deleted, or in many cases, transferred into the host genome instead. (In fact these transfers constitute an impor
23、tant way for the host cell to regulate plastid or mitochondrial activity.) The transfer of genes from mitochondria and plastids to the “host genome” or cell nucleus raises a further problem: why were all genes not transferred? In other words, why do any genes at all remain in mitochondria and plasti
24、ds? This problem is addressed by the, which proposes that genes and respiratory chain proteins are Co-located for Redox Regulation. A large cell, especially one equipped for phagocytosis, has vast energetic requirements, which cannot be achieved without the internalisation of energy production (due
25、to the decrease in the surface area to volume ratio as size increases). This implies that, for the cell to gain mitochondria, it could not have been a primitive eukaryote, but instead a prokaryotic cell. This in turn implies that the emergence of the eukaryotes and the formation of mitochondria were
26、 achieved simultaneously. This may be explained by possibly a very close symbiotic relationship between two types of prokaryotes which eventually led to gene exchange and engulfing of the mitochondria precursors through partial fusion or engulfing by the host bacteria. Genetic analysis of small euka
27、ryotes that lack mitochondria shows that they all still retain genes for mitochondrial proteins. This implies that all these eukaryotes once had mitochondria. This objection can be answered if, as suggested above, the origin of the eukaryotes coincided with the formation of mitochondria. These last two problems are accounted for in the.
