Evidence for Endosymbiosis
Abundant evidence has been found for endosymbiosis: 1. Mitochondria and chloroplasts are similar in size and morphology to bacterial prokaryotic cells, though the mitochondria of some organisms are known to be morphologically variable.
2. Mitochondria and chloroplasts divide by binary fission, just as bacteria do, and not by mitosis as eukaryotes do. Both types of organelle have Fts proteins at their division plane.
3. Chemically distinct membrane systems:
The double membrane found in mitochondria and chloroplasts appears to be a relic of the absorption of the prokaryotic bacteria by the eukaryotic host cells. The inner membrane is of a different chemical composition – like that of eubacteria – than the outer membrane of the organelle. Some enzymes and inner membrane systems resemble prokaryotic inner membrane systems. The outer membrane is of similar composition to the plasma membrane of the eukaryote, as is the membrane of other cellular organelles such as the nuclear membrane, endoplasmic reticulum, and Golgi apparatus of eukaryotes (in support of the invagination hypothesis of their origin). Several primitive eukaryotic microbes, such as Giardia and Trichomonas possess a nuclear membrane yet have no mitochondria.
4. Mitochondria and chloroplasts have their own DNA and their own ribosomes:
The DNA of mitochondria and chloroplasts is different from that of the eukaryotic cell in which they are found. As Margulis predicted, both types of organelle include DNA that is like that of prokaryotes – circular, not linear. Further, the DNA of mitochondria and chloroplasts, like that of the eubacteria, usually has neither introns nor histones. The first amino acid of mitochondrial and plastic transcripts is equivalent to that of eubacteria, and different from that of eukaryotes.
Proteins encoded by mitochondrial DNA do not account for all of the mitochondrial proteins. The ingested prokaryotes are believed to have relinquished certain genes to the nuclei of their host cells, a process known as endosymbiotic gene transfer. For this reason, mitochondria and chloroplasts now depend on their hosts to synthesize most of their components.
The DNA of these organelles evolves independently – and at a different rate – from the nuclear DNA of the eukaryotic cell. (Mitochondrial DNA is employed to trace evolutionary lines of human maternally- derived cells because virtually all DNA mtDNA is contributed by the oocyte, unlike nuclear DNA which derives from both parents, and unlike the Y-chromosome contributed solely by the father.)
5. Mitochondria arise from preexisting mitochondria;
chloroplasts arise from preexisting chloroplasts (they are not manufactured through the direction of nuclear genes).
6. Organelle ribosomes are more similar in size to prokaryotic ribosomes:
Mitochondria and chloroplasts produced their own ribosomes, which have 30S and 50S subunits, and not the 40S and 60S subunits of the eukaryotic cells in which they occur.
7. Many antibiotics that kill or inhibit bacteria also inhibit protein synthesis of these organelles:
Antibiotics such as streptomycin block the synthesis of proteins in eubacteria, mitochondria, and chloroplasts, but not cytoplasmic protein synthesis in eukaryotes. Similarly, the antibiotic rifampicin infibits the RNA polymerase of eubacteria and mitochondria, but does not inhibit the RNA polymerase of the eukaryotic nucleus. Conversely inhibitors of eukaryotic protein synthesis, such as bacterially derived diphtheria toxin, do not affect protein synthesis within eubacteria, mitochondria, or chloroplasts.
8. Phylogenetic studies using comparative ribosomal RNA
sequencing demonstrates that both mitochondria and plastids are related to Bacteria.Phylogenetic analyses have clearly demonstrated that mitochondria and plastids derive from bacterial lines related to modern-day proteobacteria and cyanobacteria, respectively. Experimental observations confirm growth of bacterial endosymbionts in numerous organisms.
9. Microbiologist Kwang Jeon observed Legionella-like x-bacterial infection of strains of Amoeba proteus (xD) with which he was working. The infection killed many of the amoeba, but he raised the most hardy of the survivors. After many generations, the amoeba became dependent upon the bacterium, and endosymbiotic gene switching occurred. Free Full Text Article 2004 Detailed description xD amoeba experiments.
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