Evidence that
mitochondria and chloroplasts arose via an ancient endosymbiosis of a bacteria is as follows:
* Both
mitochondria and chloroplasts contain DNA which is fairly different from that of the cell nucleus, and in a quantity similar to that of bacteria.
* Mitochondria utilize a different genetic code than the eukaryotic host cell, this code is very similar to bacteria and Archaea.
* They are surrounded by two or more membranes, and the innermost of these shows differences in composition compared to the other membranes in the cell. The composition is like that of a prokaryotic cell membrane.
* New
mitochondria and chloroplasts are formed only through a process similar to binary fission. In some algae, such as Euglena, the chloroplasts can be destroyed by certain chemicals or prolonged absence of light without otherwise affecting the cell. In such a case, the chloroplasts will not regenerate.
* Much of the internal structure and biochemistry of chloroplasts, for instance the presence of thylakoids and particular chlorophylls, is very similar to that of cyanobacteria. Phylogenies built with bacteria, chloroplasts, and eukaryotic genomes also suggest that chloroplasts are most closely related to cyanobacteria.
* DNA sequence analysis and phylogeny suggests that nuclear DNA contains genes that probably came from the chloroplast.
* Some genes encoded in the nucleus are transported to the organelle, and both
mitochondria and chloroplasts have unusually small genomes compared to other organisms. This is consistent with an increased dependence on the eukaryotic host after forming an endosymbiosis.
* Chloroplasts appear in very different groups of protists, which are in general more closely related to forms lacking them than to each other. This suggests that if chloroplasts originated as part of the cell, they did so multiple times, in which case their close similarity to each other is difficult to explain.
