Mitochondria were engulfed. The nuclear envelope folded in.
- The idea
- Predict
- See it
- Why
- Check
The idea
“Mitochondria came from the cell membrane folding inward, and the nucleus came from an engulfed cell.”
Both stories end with a compartment wrapped in membrane. So it's easy to attach each story to the wrong organelle.
The short version
The mistake: Mitochondria came from the cell membrane folding inward, and the nucleus came from an engulfed cell.
What's actually true: Mitochondria and chloroplasts came from engulfed bacteria. The nuclear envelope, ER and Golgi folded in from the cell's membrane.
- The cell's own membrane folded in to make the endomembrane system, including the nuclear envelope, ER and Golgi. None has its own genome.
- In endosymbiosis, a host cell engulfed a prokaryote and kept it alive. Mitochondria came from an aerobic bacterium, and chloroplasts from a cyanobacterium.
- Mitochondria, chloroplasts and the nucleus all have two membranes. So look for what a bacterium has, like circular DNA, bacteria-like ribosomes and splitting in two.
On the exam: Its own DNA ring and bacteria-like ribosomes mean it was engulfed. Without them, it folded in.