Example 1
An organelle has its own circular DNA and bacterial-like ribosomes. Which origin model fits?
These features support bacterial endosymbiotic ancestry, especially when combined with sequence evidence.
Year 12 · SACE Stage 2
Early life and endosymbiosis
Classify evidence about cell origins and shared ancestry.
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Method and worked examples
Evidence including ancient microfossils and chemical traces supports life on Earth around 3.5 billion years ago. Early prokaryotic cells preceded the eukaryotic fossil record. All known cellular life uses membranes and nucleic acids, consistent with shared evolutionary history. Models of the earliest cells include possible roles for RNA, which can carry information; some RNAs called ribozymes also catalyse reactions.
Endosymbiotic theory explains the ancestry of mitochondria and chloroplasts from bacteria living within ancestral host cells. Evidence includes their own DNA, bacterial-like ribosomes, division patterns and membrane organisation, together with sequence comparisons. Modern organelles depend heavily on host-cell genes and are not simply independent bacteria. Distinguish evidence consistent with a theory from a feature found in all organelles: the existence of a membrane alone is less specific than several converging observations.
Example 1
Example 2
Convert 3.5 billion years into millions of years.
One billion is 1000 million, so 3.5 billion years =3500 million years.
Watch out: A ribozyme is an RNA molecule with catalytic activity.