THE ENDOSYMBIOTIC THEORY
The overall purpose of this Learning Object is to learn how certain organelles in eukaryotic cells arose from prokaryotic bacteria.
It is thought that life arose four billion years ago to form simple microbes similar to today's prokaryotic cells. The earliest fossil evidence of eukaryotic cells is about one billion years ago. The endosymbiotic theory states that some of the organelles in today's eukaryotic cells were once prokaryotic microbes.
In this theory, the first eukaryotic cell was probably an amoeba-like cell that got nutrients by phagocytosis and contained a nucleus that formed when a piece of the cytoplasmic membrane pinched off around the chromosomes. Some of these amoeba-like organisms ingested prokaryotic cells that then survived within the organism and developed a symbiotic relationship. Mitochondria formed when bacteria capable of aerobic respiration were ingested; chloroplasts formed when photosynthetic bacteria were ingested. They eventually lost their cell wall and much of their DNA because they were not of benefit within the host cell. Mitochondria and chloroplasts cannot grow outside their host cell.
Evidence for this is based on the following:
1. Mitochondria and chloroplasts are the same size as prokaryotic cells, divide by binary fission, and, like bacteris, have Fts proteins at their division plane.
2. Mitochondria and chloroplasts have their own DNA that is circular, not linear.
3. Mitochondria and chloroplasts have their own ribosomes that have 30S and 50S subunits, not 40S and 60S.
4. Several more primitive eukaryotic microbes, such as Giardia and Trichomonas have a nuclear membrane but no mitochondria.
Although evidence is less convincing, it is also possible that flagella and cilia may have come from spirochetes.
Copyright © Gary E. Kaiser
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Updated: Jan. 7, 2002