- A Eukaryotic cells are sometimes proposed to have evolved directly from ancestral virus particles, despite viruses lacking the cellular machinery, ribosomes, and membranes that characterize mitochondria
- B Mitochondria and chloroplasts originated as free-living bacteria engulfed by a host cell and became permanent endosymbionts; evidence: double membrane, circular DNA, 70S ribosomes, binary fission
- C Mitochondria are sometimes proposed to have evolved directly from the cell nucleus rather than from any independent organism, despite possessing their own circular DNA and double membrane distinct from the nucleus
- D The endosymbiont theory is sometimes thought to remain a purely speculative idea lacking supporting evidence, when in fact double membranes and bacterial-like DNA provide strong corroborating evidence
Correct answer: B. Mitochondria and chloroplasts originated as free-living bacteria engulfed by a host cell and became permanent endosymbionts; evidence: double membrane, circular DNA, 70S ribosomes, binary fission
Explanation: Endosymbiont theory (Lynn Margulis, 1967): mitochondria arose from alpha-proteobacteria (Rickettsia-like); chloroplasts from cyanobacteria. Evidence: (1) Double membrane (outer = host membrane, inner = original bacterial membrane); (2) Circular DNA (like bacteria, not histoned); (3) 70S ribosomes (bacterial size, sensitive to same antibiotics); (4) Binary fission; (5) Phylogenetic analysis of mitochondrial/chloroplast DNA sequences places them within bacterial lineages.
Concept context
Origin of life, Darwin's theory, natural selection, speciation, and evidence for evolution.