|MadSci Network: Evolution|
This question has been clarified over the last few years by 2 things; the identification of the Archaea and the acceptance of the endosymbiont theory. About 20 years ago through the study of a type of RNA present in all cells, the 5 kingdom system 4 of which were eukaryotic was shown not to be a valid model (Animals, plants, protists and fungi, the 5th being prokaryotes). Nowadays no scientist uses the 5 kingdom system and instead we use the 3 kingdom (domain) system (Eukaryotes, Bacteria and Archaea). These techniques were then applied to consider the mitochondria and chloroplast, the major difference between plant and animal cells and the endosymbiont theory was born. According to this theory, a larger prokaryote (bacteria or Archaea) (or perhaps early eukaryote (animals, plants fungi and algae)) engulfed or surrounded a smaller prokaryote some 1.5 billion years ago. Instead of digesting the smaller organism the large one and the smaller one entered into a co-operative relationship (symbiosis), where both organisms benefited and neither was harmed. The larger organism gained excess ATP (energy) provided by the "protomitochondrion" and excess sugar provided by the "protochloroplast", while providing a stable environment and the raw materials the endosymbionts required. Over millions of years the two organisms became so the same that they are now one. They are so dependant today that eukaryotic cells cannot survive without mitochondria (likewise photosynthetic eukaryotes (plants) cannot survive without chloroplasts), and the endosymbionts can not survive outside their hosts. Nearly all eukaryotes have mitochondria. What other evidence survives to support the theory. Well the mitochondrion and chloroplast have their own DNA (this is very similar to that in bacteria and cyanobacteria), own ribosomes, thay react to toxins like bacteria, and finally divide just like bacteria. If you would to know more and see some explanatory diagrams go to my course webpage: http://188.8.131.52/curricular/m2o1999/microlect3.html
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