What is Endosymbiotic Theory???

• Palaeontological evidence suggests the primary appearance of prokaryotic cells around 4 billion years ago while eukaryotic cells appeared just 1.8 billion years ago. This led to a quest for whether the ancestor of all eukaryotic cells was a prokaryote?
• Endosymbiotic theory states that one prokaryotic cell(host) engulfed another prokaryotic cell through the process of infolding where both cells lived in a symbiotic relationship and thrived together. Newly formed cells were capable of reproduction and were called eukaryotes for having their membranes bound to organelles.

Evidence supporting this theory are: 

• Membranes: Mitochondria and chloroplast are membrane-bound- the same as prokaryotic cells.
• DNA: Mitochondria and chloroplast have their own circular DNA where chloroplast DNA resembles photosynthetic blue-green bacteria and mitochondrial DNA resembles aerobic bacteria.
• Reproduction: Mitochondria and chloroplast reproduce by binary fission as the bacteria do.
• Size and morphology: Mitochondria and chloroplast exhibit similar size and morphology to bacteria.
• Nature and size of ribosomes: Ribosomes of bacteria, chloroplasts, and mitochondria are similar in size and are the 70S (50S and 30S subunits) type.
• Sensitivity to antibiotics: Antibiotics suited to inhibit protein synthesis in bacteria also inhibit protein synthesis in chloroplast and mitochondria.