Explain how evidence from the replication of mitochondria supports the endosymbiotic theory for the origin of eukaryotic cells.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Mitochondria replicate by binary fission, the same way prokaryotes divide, showing they are autonomous.
Mitochondria cannot be synthesised de‑novo; new organelles arise only from pre‑existing mitochondria, just as a bacterial endosymbiont would have multiplied inside a host.
The replication is independent of the host nucleus, mirroring the early endosymbiotic event where the bacterial ancestor replicated on its own.
Mitochondria contain circular DNA and 70S ribosomes, allowing them to replicate their genome and synthesize proteins semi‑autonomously, a hallmark of prokaryotes.
Mitochondria cannot be synthesised de‑novo; new organelles arise only from pre‑existing mitochondria, just as a bacterial endosymbiont would have multiplied inside a host.
The replication is independent of the host nucleus, mirroring the early endosymbiotic event where the bacterial ancestor replicated on its own.
Mitochondria contain circular DNA and 70S ribosomes, allowing them to replicate their genome and synthesize proteins semi‑autonomously, a hallmark of prokaryotes.
Examiner tips
- Use the word ‘replicate’ and ‘binary fission’ to show understanding of the process. Mention the lack of de‑novo synthesis to link to endosymbiosis. Highlight the independent replication and the presence of 70S ribosomes and circular DNA. Keep each point brief but complete to hit all marks.
Common mistakes
- Confusing mitochondrial division with mitosis of the host cell. Forgetting to state that mitochondria cannot form de‑novo. Over‑emphasising the host’s role and ignoring the autonomous features of mitochondria.
Mark scheme (4 marks)
- Mitochondria divide by binary fission, the same process used by prokaryotes / bacteria.
- Mitochondria are never synthesised de novo / cannot arise from non-mitochondrial material; new mitochondria only form from pre-existing mitochondria.
- This mirrors the situation in endosymbiosis, in which the ancestral prokaryotic endosymbiont would have replicated independently within the host cell.
- Mitochondria possess their own circular DNA and 70S ribosomes, meaning they can carry out semi-autonomous replication / protein synthesis, consistent with a prokaryotic ancestor.
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Origins of cells questions
- Explain why the hypothesis that the first cells arose from pre-existing cells is…
- Explain how the RNA world hypothesis accounts for two major challenges in the or…
- Explain how the endosymbiotic theory accounts for the origin of mitochondria in …
- Explain how the properties of phospholipids contribute to the formation of the f…
- Explain how hydrothermal vents are considered a possible environment for the ori…
- Explain how the process of natural selection could have acted on protocells to g…