Question

1) Explain how a symbiosis b/t a single-celled primitive eukaryote and bacteria let do the evolution of the mitochondria and chloroplast. In your explanation, include the benefit that each symbiont conferred to the other. Provide evidence that the two aforementioned organelles have bacterial origins.

Answer 1

According to the endo-symbiotic theory on the evolution of eukaryotic cells, mitochondria and chloroplast were once free-living prokaryotes later engulfed by a large prokaryote cell and finally resulted in a symbiotic relationship between two.

According to the theory, Mitochondria is the result of endocytosis of an aerobic bacterium and chloroplast is the result of endocytosis of a photosynthetic bacterium, both by a sizeable anaerobic bacterium. This process finally resulted in a mutually beneficial relationship (Symbiotic relationship) between the aerobic bacteria and the anaerobic (in the case of mitochondria) or photosynthetic (in the case of chloroplast) bacteria. The aerobic bacteria would have handled the toxic oxygen for the anaerobic bacteria, and the photosynthetic bacteria would have helped in photosynthesis. The larger anaerobic bacteria would have given food and shelter for the symbionts.

Evidence

DNA – Both mitochondria and chloroplast have their own circular DNA, and the size is similar to bacterial chromosome.

Size – Size of both mitochondria and chloroplast (1 -10 microns) is almost same as free-living prokaryotic bacteria.

Double membrane – Both mitochondria and chloroplast have a double membrane. The inner lipid bilayer would have been the bacterial cell's plasma membrane, and the outer lipid bilayer came from the cell that engulfed it.

Cell division – Both mitochondria and chloroplast divide independently from the cell through binary fission (one cell splits into two) which is similar to bacterial cells.

Ribosomes – The ribosomes present in mitochondria and chloroplast is 70s like those found in bacteria, rather than 80s found in eukaryotes.

Gamma subunit of the eukaryotic DNA polymerase III - DNA polymerase III, used in mitochondrial DNA synthesis, is very similar to the bacterial Replication Factor C.