Endosymbiosis is best described as:

• A. A symbiotic relationship where one organism lives inside the other
• B. A mutualistic relationship where both organisms gain nothing
• C. A symbiotic relationship where two organisms exchange DNA
• D. A relationship where one organism dies for the other

Answer: (A)

Endosymbiosis is a relationship where one organism lives inside another. The defining feature is internal residency, not just a close association. A classic example is how mitochondria and chloroplasts in eukaryotic cells resemble free-living bacteria, with their own circular DNA and ribosomes, and their ability to reproduce separately. This supports the idea that such organelles originated from formerly independent organisms that became integrated within a host cell.

So, the description that fits best is one where the key idea is one organism living inside the other. The alternative descriptions don't capture that internal living aspect: mutualism implies benefits to both, not necessarily involving one organism inside the other; DNA exchange can occur but is not the defining criterion; and a relationship where one dies for the other describes parasitism or predation, not endosymbiosis.