Endosymbiotic Theory - Evidence

Evidence

Evidence that mitochondria and plastids arose from bacteria is as follows:

  • 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. This shows that the plastid regeneration relies on an extracellular source, such as from cell division or endosymbiosis.
  • They are surrounded by two or more membranes, and the innermost of these shows differences in composition from the other membranes of the cell.
  • Both mitochondria and plastids contain DNA that is different from that of the cell nucleus and that is similar to that of bacteria (both in their size and their having a circular form).
  • DNA sequence analysis and 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 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 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 Archaeplastida), the glaucophyte algae have chloroplasts that strongly resemble cyanobacteria. In particular, they have a peptidoglycan cell wall between the two membranes.

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