USA Biology Olympiad (USABO) 2025 – 400 Free Practice Questions to Pass the Exam

The strongest evidence for the endosymbiotic origin of eukaryotic organelles, particularly mitochondria, is drawn from the similarities found between prokaryotes and mitochondria. This is primarily based on several key observations.

Mitochondria possess their own circular DNA, which is similar to bacterial DNA, rather than the linear DNA found in the nucleus of eukaryotic cells. This resemblance suggests that mitochondria may have evolved from free-living prokaryotic organisms that entered into a symbiotic relationship with an ancestral eukaryotic cell. Furthermore, mitochondria replicate through a process similar to binary fission, which is characteristic of prokaryotes rather than the mitotic process seen in eukaryotic cell division.

Additionally, the double membrane structure of mitochondria aligns with the idea that these organelles were once free-living bacteria. The inner membrane shows similarities with bacterial membranes in terms of composition and function, while the outer membrane resembles the membrane of eukaryotic cells.

These strands of evidence—circular DNA, replication methods, and membrane structures—collectively support the endosymbiotic theory, which posits that certain organelles within eukaryotic cells originated from engulfed, free