Which characteristic is commonly cited as evidence for the endosymbiotic origin of mitochondria and chloroplasts?

Which characteristic is commonly cited as evidence for the endosymbiotic origin of mitochondria and chloroplasts?

• A. They contain circular DNA similar to bacteria.
• B. They lack ribosomes.
• C. They are inherited exclusively via paternal line.
• D. They are found only in the nucleus.

Answer: (A)

Endosymbiotic origin is supported by the presence of circular DNA in mitochondria and chloroplasts. This circular genetic material is arranged like bacterial genomes, separate from the linear, histone-wrapped DNA of the host cell’s nucleus, and it shows that these organelles likely started as independent organisms. The circular DNA explains why mitochondria and chloroplasts can replicate their genomes and function somewhat independently within the cell, a hallmark of their prokaryotic ancestry. Other features—such as ribosomes similar to bacterial ones and double-membrane envelopes—also align with this origin, but the circular DNA is the most direct, observable clue linking these organelles to bacteria.

Endosymbiotic origin is supported by the presence of circular DNA in mitochondria and chloroplasts. This circular genetic material is arranged like bacterial genomes, separate from the linear, histone-wrapped DNA of the host cell’s nucleus, and it shows that these organelles likely started as independent organisms. The circular DNA explains why mitochondria and chloroplasts can replicate their genomes and function somewhat independently within the cell, a hallmark of their prokaryotic ancestry. Other features—such as ribosomes similar to bacterial ones and double-membrane envelopes—also align with this origin, but the circular DNA is the most direct, observable clue linking these organelles to bacteria.