Which hypothesis explains the origin of mitochondria and chloroplasts in eukaryotic cells via symbiosis with prokaryotes?

• A. Endosymbiotic theory
• B. Abiogenesis
• C. Biogenesis
• D. Panspermia

Answer: (A)

Endosymbiosis explains how mitochondria and chloroplasts originated in eukaryotic cells. The idea is that a host cell, likely one with a developed nucleus, engulfed free-living prokaryotes that then became permanent, work-taking partners. Over time these partner organisms evolved into the mitochondria and chloroplasts we see today. Strong evidence supports this: both organelles contain circular DNA that resembles bacterial genomes, and their genes and ribosomes are more like bacteria (70S-type) than typical eukaryotic organelles. They also reproduce by a process similar to bacterial binary fission inside the cell, and they have double membranes, consistent with an engulfment event. Phylogenetic analyses link mitochondria to proteobacteria and chloroplasts to cyanobacteria, aligning with the idea of two ancient endosymbiotic events. This explanation best accounts for these shared, bacterial-like features and the independent replication that fits inside a eukaryotic cell.

Abiogenesis refers to life arising from nonliving matter, which is about the origin of life itself rather than the origin of organelles within a living cell. Biogenesis is the general principle that life comes from existing life, not a mechanism for how specific organelles originated inside cells. Panspermia suggests life began elsewhere and was delivered to Earth, which doesn’t address why mitochondria and chloroplasts resemble bacteria or how they came to reside inside eukaryotic cells.