3 Types of Cells
(Quelle: NY Times)
In 1977, shortly after the first DNA sequences of genes became widely available, Dr. Carl R. Woese of the University of Illinois showed that all life originated from three basic types of cell,
- eukaryotes (the type of cell in all multicellular organisms, like plants and animals),
- prokaryotes like bacteria and
- archaea, the last a kind of bacteria found in boiling geysers and around volcanic fissures in the ocean floor.
Simulations indicate that eukaryotes were formed from a merger of an ancient photosynthetic bacterium with an archaea cell. The findings fit with the fossil evidence, because the earliest bacterial-type cells are 3.8 billion years old and the earliest known eukaryotic cells occur much later, in rocks that are just 1.4 billion years old.
Just as life seems to have evolved on earth just once, the creation of the eukaryotic cell with its mitochondria seems also to have been a one-time event. Many experts theorize that all mitochondria in the far-flung kingdom of eukaryotic creatures trace from a single ancestor.
Mitochondrien
Quelle und Details auf Kimball's Biology Pages.
Why do mitochondria have their own genome?
Many of the features of the mitochondrial genetic system resemble those found in prokaryotes like bacteria. This has strengthened the theory that mitochondria are the evolutionary descendants of a prokaryote that established an endosymbiotic relationship with the ancestors of eukaryotic cells early in the history of life on earth.
However, many of the genes needed for mitochondrial function have since moved to the nuclear genome. The recent sequencing of the complete genome of Rickettsia prowazekii has revealed a number of genes closely related to those found in mitochondria. Perhaps rickettsias are the closest living descendants of the endosymbionts that became the mitochondria of eukaryotes (Zellen von Tieren, Pflanzen, Pilzen).
Mitochondria
Mitochondria are membrane-enclosed organelles distributed through the cytosol of most eukaryotic cells. Their main function is the conversion of the potential energy of food molecules into ATP. Mitochondria have:
- an outer membrane that encloses the entire structure
- an inner membrane that encloses a fluid-filled matrix between the two is the intermembrane space
- the inner membrane is elaborately folded with shelflike cristae projecting into the matrix.
- a small number (some 5–10) circular molecules of DNA
Mitochondrial DNA
Quelle und Details bei ActionBioscience.org.
DNA is present inside the nucleus of every cell of our body but it is the DNA of the cell's mitochondria that has been most commonly used to construct evolutionary trees. Mitochondria have their own genome of about 16,500 bp (basenpaaren) that exists outside of the cell nucleus. Each contains 13 protein coding genes, 22 tRNAs and 2 rRNAs.
They are inherited only from the mother, which allows tracing of a direct genetic line. They don't recombine. The process of recombination in nuclear DNA (except the Y chromosome) mixes sections of DNA from the mother and the father creating a garbled genetic history.
Chloroplasten
Chloroplasten, die Teile der Pflanzenzelle die mit Hilfe des Sonnenlichts den Sauerstoff zum Atmen erzeugen, haben übrigens auch ihre eigene DNA. Man vermutet, dass durch einen sehr ähnlichen Prozess, nämlich die symbiotische Vereinigung von Zellen mit Cyanobakterien, die Pflanzenzellen entstanden sind. Auch heute noch wird 60% des Sauerstoffs nicht von den Pflanzen, sondern von diesen Bakterien erzeugt, die auch erdgeschichtlich viel älter als die Pflanzen sind (3,5 Milliarden Jahre).
Philipp Schaumann, http://philipps-welt.info/
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