Write a brief account on the origin of mitochondria and chloroplast

Mitochondria and chloroplasts are essential organelles in eukaryotic cells. Mitochondria are involved in aerobic respiration and ATP production, while chloroplasts are the site of photosynthesis in plant and algal cells. The widely accepted explanation for their origin is the Endosymbiotic Theory.

The earliest idea related to the symbiotic origin of plastids was proposed by Konstantin Mereschkowski in 1905, who suggested that chloroplasts evolved from autotrophic cyanobacteria like ancestors through a process of symbiogenesis. Later in the 1920s, Ivan Wallin, an American biologist, extended this idea to mitochondria, proposing that they originated from aerobic bacteria. However, these early ideas received little acceptance at that time.

A major revival and refinement of the endosymbiotic theory came with Lynn Margulis in 1967, who in her seminal paper "On the Origin of Mitosing Cells" provided detailed evidence that both mitochondria and chloroplasts originated from free-living prokaryotes. She proposed that an ancestral eukaryotic cell engulfed an aerobic bacterium (which became mitochondrion) and later a photosynthetic cyanobacterium (which became chloroplast) in two separate events. This model gained strong support later through molecular and genetic studies.

Endosymbiotic Theory of Origin

According to this theory, mitochondria and chloroplasts originated from free-living prokaryotic cells that were engulfed by ancestral eukaryotic cells. These prokaryotes entered into a symbiotic relationship with the host cell, eventually becoming permanent internal components of the eukaryotic cell.

Mitochondria are believed to have originated from aerobic alpha-proteobacteria, while chloroplasts are thought to have evolved from photosynthetic cyanobacteria. Over time, most of their genes were transferred to the host nuclear genome, but they retained a small, independent genome for essential functions.

Structural and Genetic Evidence Supporting Endosymbiosis

The following structural and genetic features support the endosymbiotic origin of these organelles:

• Double Membrane Structure: Both mitochondria and chloroplasts possess a double membrane, similar to that of gram-negative bacteria, which suggests engulfment by endocytosis.

• Circular DNA: These organelles contain their own circular DNA, not associated with histones, much like bacterial DNA.

• 70S Ribosomes: They contain 70S-type ribosomes (prokaryotic type), unlike the 80S ribosomes found in the eukaryotic cytoplasm.

• Binary Fission: Both organelles replicate independently through binary fission, which is a typical bacterial method of reproduction.

• Gene Sequence Similarities: Molecular studies have shown that mitochondrial genes are closely related to alpha-proteobacteria, and chloroplast genes resemble cyanobacterial genes.