Which of the following statements does NOT provide evidence in support of the endosymbiotic origin of eukaryotic cells?

Question

BlackTom AI · Accepted Answer

Mitochondria are not free-living organisms; they are intracellular organelles dependent on the host cell for many functions and for most of their proteins — therefore the statement that “Mitochondria are often found free-living outside of a eukaryotic cell” does NOT support endosymbiotic origin.

1) Relevant knowledge points and concepts
- Endosymbiotic theory: mitochondria and chloroplasts originated from free-living bacteria that were engulfed by ancestral eukaryotic cells and became stable intracellular symbionts.
- Key molecular and structural evidence: organelle genomes are circular (like bacteria), organelle ribosomes resemble prokaryotic ribosomes (structure and antibiotic sensitivity), organelle DNA sequences are phylogenetically close to particular bacterial groups, and organelles replicate by a fission-like process.

2) Step-by-step reasoning
- Examine each option against known evidence.
  a. “Mitochondria are often found free-living outside of a eukaryotic cell.” If true, this would show mitochondria behave as independent organisms; endosymbiosis predicts they are internalized and integrated, not commonly free-living. In reality mitochondria are never found as autonomous, free-living bacteria in nature.
  b. “The ribosomes found in mitochondria are similar to prokaryotic ribosomes.” This matches evidence: mitochondrial ribosomes resemble bacterial ribosomes in structure and sensitivity to some antibiotics.
  c. “The DNA of chloroplasts is circular like the DNA of phototrophic bacteria.” Circular genomes are characteristic of bacteria and are found in chloroplasts.
  d. “The DNA sequence of mitochondria is similar to the DNA sequence of respiring bacteria.” Sequence and phylogenetic similarity directly support origin from bacterial ancestors.

3) Calculations
- No numerical calculations required.

4) Why the correct answer is correct
- The stated choice is not evidence because it asserts an observation that contradicts the integrated, intracellular nature of mitochondria. Endosymbiotic origin is supported by shared molecular features, not by mitochondria being free-living today.

5) Why the other options are incorrect (i.e., why they do support endosymbiosis)
- Option b: Similar ribosomes indicate bacterial ancestry.
- Option c: Circular chloroplast DNA mirrors bacterial genome structure.
- Option d: Mitochondrial DNA sequence similarity to respiring bacteria provides direct phylogenetic evidence.

6) Conclusion
- The statement “Mitochondria are often found free-living outside of a eukaryotic cell.” does NOT provide evidence for endosymbiotic origin. This matches the provided correct answer.

Which of the following statements does NOT provide evidence in support of the endosymbiotic origin of eukaryotic cells? 单项选择题

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