Endosymbiont

An organism that lives within another organism. For instance, some nitrification? bacteria and cellulose-digesting protozoa.

A few organelles of eukaryote cells probably originated as reduced endosymbiotic prokaryotes, namely the mitochondrion and chloroplast. Both of these have double membranes - the inner one corresponding to the bacterium, the outer to the cell - and contain some DNA, but this does not code for parts outside the organelle is the DNA in the nucleus does.

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