Which eukaryotic organelles were proposed to have arisen as endosymbionts and what kinds of evidence were presented to support each organelle as an endosymbiont? Give 3 examples here.
The correct answer and explanation is:
The theory of endosymbiosis proposes that certain eukaryotic organelles originated from free-living prokaryotes that were engulfed by ancestral eukaryotic cells. Over time, these engulfed prokaryotes formed a symbiotic relationship with the host cell, eventually evolving into essential eukaryotic organelles. Three major organelles that are considered to have arisen through endosymbiosis are the mitochondria, chloroplasts, and the plastids in certain protists.
- Mitochondria: Mitochondria are believed to have evolved from an ancient aerobic bacterium, possibly related to modern-day alpha-proteobacteria. Evidence for this includes the presence of their own circular DNA, which resembles the DNA of prokaryotes rather than eukaryotic linear chromosomes. Additionally, mitochondria replicate through binary fission, a method used by bacteria, not by typical eukaryotic organelles. Mitochondria also have double membranes, with the inner membrane being similar to the plasma membrane of bacteria. The genetic sequence of mitochondrial ribosomal RNA (rRNA) shares similarities with that of certain bacterial species, further supporting the theory of their prokaryotic origin.
- Chloroplasts: Chloroplasts, which are involved in photosynthesis, are thought to have originated from cyanobacteria. These bacteria, like chloroplasts, perform photosynthesis and have a similar structure, including a double membrane. Chloroplasts also contain their own DNA, which is circular and similar to the DNA of cyanobacteria. Furthermore, chloroplasts share similarities in their ribosomal RNA sequences with cyanobacteria, suggesting a common ancestry. The presence of pigments such as chlorophyll, which are also found in cyanobacteria, provides additional evidence supporting this endosymbiotic origin.
- Plastids in Protists: Certain protists, such as the red and green algae, contain plastids that are thought to have originated from a similar endosymbiotic event with a cyanobacterium. Evidence supporting this includes the fact that plastids also have their own DNA and a double membrane structure. Additionally, the genes and proteins in these plastids are closely related to those of cyanobacteria, further suggesting a shared evolutionary history. Some protists even retain photosynthetic capabilities similar to cyanobacteria, reinforcing the connection.
These organelles exhibit several key features that support their endosymbiotic origins, including the presence of their own DNA, bacterial-like ribosomes, and the ability to replicate independently of the host cell.