Endosymbiotic Origin of Eukaryotes

Initial observations

• In 1966, a scientist studying amoeba (single celled organism) found his population of amoeba infected with bacteria

• Only a few amoeba survived the epidemic• Survivors still had

bacteria inside but they were no longer sick.

• If antibiotics were used to kill bacteria, the amoeba died too = they needed each other

Two species living in close relationship

• Mutualism – both organisms benefit

• Commensalism – one organism benefits without helping or harming the other

• Parasitism – one organism benefits at the expense of the other

• It was found that the bacteria and amoeba divided on their own, got their own energy, used their own genes, and made their own proteins.

• Over time they could begin to work together

• The eukaryotic cell and its organelles likely evolved like this

From prokaryotes to eukaryotes• Archaea and Bacteria are

small, relatively simple cells surrounded by a membrane and a cell wall with a circular strand of DNA containing the genes (often called prokaryotes)

• Eukaryotes are more complex with linear DNA, a nucleus, and organelles

• Evidence supports that eukaryotic cells are descendents of separate prokaryotic cells that joined together in symbiosis.

• The mitochondria is the result of a free-living bacterium that was engulfed by another cell (perhaps as food) but ended up staying permanently.

• The host benefited from the chemical energy of the bacterium and the bacterium benefited from the protection, nutrient rich environment

Lynn Margulis (1970) provided the evidence1. The double membrane found in

mitochondria and chloroplast appear to be a relic of the prokaryotic bacteria by the eukaryotic host cells. The inner membrane is like that of prokaryotes and the outer membrane is similar to the eukaryote.

2. Mitochondria and chloroplast have their own circular DNA like a prokaryote

3. Mitochondria and chloroplast multiply by pinching in half – similar in process to prokaryote. If a cell’s mitochondria are removed, it can’t build new ones from scratch.

4. Mitochondria/Chloroplast have slightly different ribosomes than the rest of the cell, more similar to prokaryotes (subunits are different in size) .

The endosymbiotic event that generated mitochondria must have happened early in history of eukaryotes, because all eukaryotes have them. Then later, a similar event brought chloroplasts into some eukaryotic cells, creating the lineage that led to plants.

Watch Ted talks video on David Gallo 1998