An organelle's ancestor isn't the host cell's ancestor
- The idea
- Predict
- See it
- Why
- Check
The idea
“If a gene in a eukaryote matches a bacterium, the eukaryote must descend from that bacterium.”
Mitochondrial genes sit inside your cells, so their history seems like yours. Your cells are really two old partners with separate pasts.
The short version
The mistake: If a gene in a eukaryote matches a bacterium, the eukaryote must descend from that bacterium.
What's actually true: An organelle's genes trace to the bacterium that was taken in. The host cell's own lineage traces to an archaeal-type cell.
- A host cell took in a bacterium that became the mitochondrion. A cyanobacterium taken in later became the chloroplast.
- So one eukaryotic cell holds genes with different ancestries. Its core nuclear genes trace to the archaeal-type host.
- Many endosymbiont genes moved to the nucleus but still look bacterial. A gene's closest match shows which partner it came from.
On the exam: Ask whose gene it is. Organelle genes trace to the endosymbiont, core host genes to archaea.