Question

The endosymbiont theory offers an explanation for how eukaryotic cells could have obtained mitochondria and chloroplasts through a symbiotic relationship with prokaryotic cells that changed over time. What structural feature of prokaryotic cells might you expect to have changed before any endosymbionts could be acquired by eukaryotic cells?

Answer 1

• Endosymbiotic theory: Eukaryotes evolved from prokaryotes.
• As per the endosymbiotic theory, engulfment of phototrophs formed chloroplast, while engulfment of heterotrohic bacteria formed mitochondria.

Evidences:

• Mitochondria were aerobic bacteria, which were endocytosed.
• Chloroplasts were endocytosed photosynthetic bacteria.
• Eukaryotic ribosomes resemble prokaryotic ribosome.
• Thus, according to the endosymbiotic theory, chloroplasts originated by the process of endocytosis of photosynthetic bacteria, like cyanobacteria.
• It has been found that certain introns have been acquired by the chloroplast genome from the ancestral bacteria.
• These introns are group II introns, which are specifically ribozyme.
• Some researchers also disregard the acquisition of these introns by horizontal gene transfer from other organisms and considers the presence of introns in chloroplast as a result of endosymbiotic theory form the cyanobacteria.

• Researchers believe that evolution of mitochondria may be from Rickettsia, which are obligate aerobes.
• rRNA analysis and sequence matching (like of chaperonin 60) indicate similarity of eukaryotic mitochondria with Alpha-proteobacteria.

• Transfer of genes from mitochondrial genome as well as deletion mutation, resulted in mitochondrial genome reduction and presence of some bacterial genome in nuclear genome.
• There could have been a subsequent disintegration of mitochondria. However, the nuclear genome retained some proteobacterial genome that were previously existed as endosymbiont.