IGNOU MZO 004 2026 - Explain the theory of endosymbiosis and describe how primary endosymbi

Question

Explain the theory of endosymbiosis and describe how primary endosymbiosis led to the origin of mitochondria and chloroplasts, including gene transfer to the host nucleus and loss of independence of these organelles.

29 Jan 2026

Answer :

Word Count : 458

The theory of endosymbiosis, first popularized by Lynn Margulis in the 1960s, proposes that certain organelles within eukaryotic cells, particularly mitochondria and chloroplasts, originated from free-living prokaryotic organisms that were engulfed by ancestral eukaryotic cells. According to this theory, rather than being digested, these prokaryotes established a symbiotic relationship with the host cell, eventually evolving into organelles essential for cellular metabolism and energy production. This process is a cornerstone in understanding the evolution of eukaryotic cells and the diversification of life forms. Primary endosymbiosis refers to the initial event in which a eukaryotic ancestor engulfed a free-living prokaryote, specifically an alpha-proteobacterium in the case of mitochondria and a cyanobacterium in the case of chloroplasts. In the case of mitochondria, the engulfed alpha-proteobacterium possessed a complete respiratory machinery capable of oxidative phosphorylation. By entering a symbiotic relationship with the host, it provided an efficient source of ATP in exchange for protection and nutrients. Over evolutionary time, the engulfed bacterium transferred many

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