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Line 26: − − == Evidence == − − A great body of evidence exists in the fields of [[molecular biology]], [[biochemistry]], and [[genetics]] that supports the theory that mitochondria and plastids arose from bacteria<ref>Kimball, J. 2010. Kimball's Biology Pages. Accessed October 13, 2010. An online open source biology text by Harvard professor, and author of a general biology text, John W. Kimball.</ref><ref>Reece, J., Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson, 2010. Campbell Biology. 9th Edition Benjamin Cummings; 9th Ed. (October 7, 2010)</ref><ref>Raven, P., George Johnson, Kenneth Mason, Jonathan Losos, Susan Singer, 2010. Biology. McGraw-Hill 9th Ed. (January 14, 2010)</ref>. − * New mitochondria and plastids are formed only through a process similar to binary fission. In some [[algae]], such as Euglena, the plastids can be destroyed 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 membranes, and the innermost of these shows differences in composition from the other membranes of the cell. They are composed of a peptidoglycan [[cell wall]] characteristic of a bacterial cell. − * Both mitochondria and plastids contain DNA that is different from that of the cell nucleus and that is similar to that of bacteria (in being circular in shape and in its size). − * DNA sequence analysis and [[phylogeny|phylogenetic]] estimates suggest that nuclear DNA contains genes that probably came from plastids. − * These organelles' [[ribosomes]] 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 thylakoids and particular [[chlorophyll|chlorophylls]], is very similar to that of cyanobacteria. Phylogenetic estimates constructed with bacteria, plastids, and eukaryotic genomes also suggest that plastids are most closely related to cyanobacteria. − * Mitochondria have several [[enzyme|enzymes]] and transport systems similar to those of bacteria. − * Some proteins encoded in the nucleus are transported to the 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 mitochondrial and plastid function are located in the nucleus. Many originate from the bacterial endosymbiont. − * Plastids are present in very different groups of protists, some of which are closely related to forms lacking plastids. This suggests that if chloroplasts originated de novo, they did so multiple times, in 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 eukaryotes that acquired their plastids directly from bacteria (known as Primoplantae), the glaucophyte algae have chloroplasts that strongly resemble cyanobacteria. In particular, they have a peptidoglycan cell wall between the two membranes. − * Mitochondria and plastids are similar in size to bacteria.
→Evidence
The possibility that peroxisomes may have an endosymbiotic origin has also been considered, although they lack DNA. Christian de Duve proposed that they may have been the first endosymbionts, allowing cells to withstand growing amounts of free molecular [[oxygen]] in the [[Earth|Earth's]] atmosphere. However, it now appears that they may be formed ''de novo'', contradicting the idea that they have a symbiotic origin<ref>Gabaldón T, Snel B, van Zimmeren F, Hemrika W, Tabak H, Huynen MA (2006). "Origin and evolution of the peroxisomal proteome". Biol. Direct 1 (1): 8. doi:10.1186/1745-6150-1-8. PMC 1472686. PMID 16556314. (Provides evidence that contradicts an endosymbiotic origin of peroxisomes. Instead it is suggested that they evolutionarily originate from the Endoplasmic Reticulum)</ref>. It is believed that over millennia these endosymbionts transferred some of their own DNA to the host cell's nucleus during the evolutionary transition from a symbiotic community to an instituted eukaryotic cell (called "serial endosymbiosis"). This hypothesis is thought to be possible because it is known today from scientific observation that transfer of DNA occurs between [[bacteria]] species, even if they are not closely related. Bacteria can take up DNA from their surroundings and have a limited ability to incorporate it into their own [[genome]].
The possibility that peroxisomes may have an endosymbiotic origin has also been considered, although they lack DNA. Christian de Duve proposed that they may have been the first endosymbionts, allowing cells to withstand growing amounts of free molecular [[oxygen]] in the [[Earth|Earth's]] atmosphere. However, it now appears that they may be formed ''de novo'', contradicting the idea that they have a symbiotic origin<ref>Gabaldón T, Snel B, van Zimmeren F, Hemrika W, Tabak H, Huynen MA (2006). "Origin and evolution of the peroxisomal proteome". Biol. Direct 1 (1): 8. doi:10.1186/1745-6150-1-8. PMC 1472686. PMID 16556314. (Provides evidence that contradicts an endosymbiotic origin of peroxisomes. Instead it is suggested that they evolutionarily originate from the Endoplasmic Reticulum)</ref>. It is believed that over millennia these endosymbionts transferred some of their own DNA to the host cell's nucleus during the evolutionary transition from a symbiotic community to an instituted eukaryotic cell (called "serial endosymbiosis"). This hypothesis is thought to be possible because it is known today from scientific observation that transfer of DNA occurs between [[bacteria]] species, even if they are not closely related. Bacteria can take up DNA from their surroundings and have a limited ability to incorporate it into their own [[genome]].
== Problems ==
== Problems ==