The Origin Of The Eukaryotic Cell

As known eukaryotic cells are much advance and complex then prokaryotic cells. It raise a question that how more complex eukaryotic cells are arose. Some scientist says that in ancient time different prokaryotic cells come together and start living together. There growth, nutrition and product they form are being share among them. This is how a mutual relationship in prokaryotic cells form a modern eukaryotic cells.

To promote their view scientists put example of mitochondria, chloroplast like organelles of eukaryotic cells. These organelles have their own genetic material and are capable to reproduce independently as per the need of cell. Some biologist believe that original “protoeucaryote” was a large, aerobic archaeon or bacterium that formed mitochondria, chloroplasts, and nuclei when its plasma membrane invaginated and enclosed genetic material in a double membrane. The organelles could then evolve independently. It is also  possible that a large cyanobacteria lost its cell wall and became phagocytic. 

Subsequently, primitive chloroplasts, mitochondria, and nuclei would be formed by the fusion of thylakoids and endoplasmic reticulum cisternae to enclose specific areas of cytoplasm.

By far the most popular theory for the origin of eukaryotic cells is the endosymbiosis theory. In brief, it is supposed that the ancestral prokaryotic cell, which may have been an archaeon, lost its cell wall and gained the ability to obtain nutrients by phagocytosing other prokaryotes.

If an aerobic, amoeboid, phagocytic prokaryotes – possibly already possessing a developed nucleus – engulfed an aerobic bacterial cell and established a permanent symbiotic relationship with it, the host would be better adapted to its increasingly oxidase environment. The endosymbiotic aerobic bacterium eventually would develop into the mitochondrion. Similarly, symbiotic associations with cyanobacteria would lead to the formation of chloroplasts and photosynthetic eukaryotes.

There are evidence to support the endosymbiotic theory. Both mitochondria and chloroplasts resemble bacteria in size and appearance, contain DNA in the form of a closed circular like that of bacteria, and reproduce semiautonomously.

Mitochondrial and chloroplast ribosomes resemble prokaryotic ribosomes more closely than those in the eucaryotic cytoplasmic matrix.

The sequence over the mitochondrial and chloroplast DNA is much similar to bacterial gene sequence, then the sequence of eukaryotic cell.

Finally, there are symbiotic associations that appear to be bacterial endosymbioses in which distinctive prokaryotes characteristics are being lost.

Reference:Prescott Microbiology