Organelle origin — Which observation is NOT considered valid evidence supporting the endosymbiotic origin of mitochondria and chloroplasts?

Introduction / Context:The endosymbiotic theory proposes that mitochondria and chloroplasts evolved from free-living bacteria that became resident within early eukaryotic cells. A set of structural and genetic observations strongly supports this view. However, it is equally important to recognize what the theory does not claim about current organelles.

Given Data / Assumptions:

• Modern mitochondria and chloroplasts possess circular DNA and prokaryote-like ribosomes.
• They divide by binary fission-like processes and have double membranes.
• They depend on nuclear-encoded proteins for many essential functions.

Concept / Approach:Evaluate each statement against known evidence. The presence of their own DNA and ribosomes, bacterial-like division, and size similarity are classic lines of evidence supporting endosymbiosis. However, the idea that these organelles can now live independently is incorrect; they are integrated with the host cell and cannot survive as autonomous organisms.

Step-by-Step Solution:

Verification / Alternative check:Proteomic analyses show that a majority of mitochondrial and chloroplast proteins are encoded by nuclear genes and imported, explaining their present-day dependence on the host cell.

Why Other Options Are Wrong:

Common Pitfalls:Confusing ancestral autonomy with current autonomy; endosymbiosis asserts historical origin, not present independence.

Final Answer:Both organelles can live and function independently of the host cell.