Your cells contain tiny passengers with their own DNA. Mitochondria (and the chloroplasts of plants and algae) look and act surprisingly like bacteria. The explanation is one of the best-supported ideas about the history of eukaryotic cells.
Key idea
Mitochondria and chloroplasts descend from bacteria taken in by ancient cells, which kept them instead of digesting them. Mitochondria came from bacteria that used oxygen; chloroplasts came later from cyanobacteria, which photosynthesize.
- Their own DNA: a small circular chromosome, like a bacterium's, and unlike the linear chromosomes in the nucleus.
- Their own ribosomes, the smaller bacterial kind.
- Two membranes: the inner one like a bacterial membrane, the outer one from the host cell that wrapped around them.
- They divide by splitting in two, the way bacteria do, and a cell cannot build new ones from scratch.
- Their DNA sequences match bacteria most closely: mitochondria group with a group of bacteria called alphaproteobacteria, chloroplasts with cyanobacteria.
Why it works
The idea goes back to 1905, when Konstantin Mereschkowski proposed that chloroplasts were captured cyanobacteria. Lynn Margulis built it into a full theory in 1967. It was doubted for years, until DNA sequencing gave the decisive test: the genes inside mitochondria and chloroplasts are bacterial genes. The order of events also follows from the evidence. Nearly every eukaryote has mitochondria (or traces of them), but only plants and algae have chloroplasts, so the mitochondrial partnership came first, and a cell that already had mitochondria later took in a cyanobacterium.
Worked example: Weigh the evidence
Which observation supports the idea that chloroplasts were once free-living bacteria: (a) chloroplasts are green; (b) chloroplasts have circular DNA with genes most like cyanobacteria; (c) chloroplasts are found in leaves?
- (a) Color tells us about pigments, not ancestry.
- (c) Where they are found says nothing about where they came from.
- (b) Circular DNA with cyanobacterial genes is exactly what descent from cyanobacteria predicts.
Answer: (b)
Common mistake
Thinking the endosymbiosis theory is a guess with no evidence, or that chloroplasts came from mitochondria. Several independent lines of evidence (DNA, ribosomes, membranes, division, gene sequences) all agree, which is what makes it a theory in the scientific sense. And the DNA shows two separate events from two different kinds of bacteria: mitochondria from alphaproteobacteria, chloroplasts from cyanobacteria.