Endosymbiotic theory
SIMPLY PUT, the theory of endosymbiosis is a concept that mitochondria and chloroplasts are the result of years of evolution initiated by endocytosis of bacteria and blue-green algae which,
instead of becoming digested, became symbiotic.
Factors in favour of mitochondrial and chloroplast endosymbiosis: outer membrane is similar to the plasma membrane; mitochondria looks a lot like bacteria; chloroplasts look a lot like blue-green algae; these organelles are similar to prokaryotes in that: both have circular, naked (little protein) DNA.
Most other quirks that distinguish prokaryotic DNA and eukaryotic DNA (except introns), suggest mitochondrial and chloroplast DNA is prokaryotic. Both have prokaryotic type and size ribosomes. Inner membrane lipids of mitochondria and chloroplasts are similar to their prokaryotic counterparts. Mitochondria and chloroplasts seem to divide independently of the rest of the eukaryotic cell/eukaryotes are good at endocytosis.
Factors against the theory: mitochondria and chloroplasts only code for a few proteins. Most of the proteins found in the organelles are actually coded for by the nuclear DNA. Because of this, it is not surprising that mitochondria nor chloroplasts have been shown to be able to live on their own when isolated from the eukaryotic cell. If the theory of endosymbiosis is true, then one must ask what was the original eukaryotic cell (without mitochondria or chloroplasts) and how did it survive (glycolysis?). Why have not any primitive eukaryotic cells ever be found that are devoid of these organelles?
V.S. Arun Kumar Sama,
Centre for Bio-technology, Acharya Nagarjuna University, Guntur
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