Midi-chlorians in The Phantom Menace are part of a recurring theme throughout the movie, that of symbiotic relationships.[21] They were loosely based on mitochondria, organelles that provide energy for cells; like midi-chlorians, mitochondria are believed to have once been separate organisms that inhabited living cells and have since become part of them; even now, mitochondria act in some ways as independent lifeforms, with DNA all their own. Lucas established this relation with mitochondria, in particular the necessity of midi-chlorians for life to exist, as a metaphor for society; namely, he says all parts of society must get along in much the way the midi-chlorians and their symbiont do.[26]