The presence of ribosomes in both mitochondria and chloroplasts could be explained by the theory of endosymbiosis. Mitochondria and chloroplasts are conjectured to have once been free-living organisms that were engulfed by a larger prokaryote. Evidence to support this include the presence of DNA within these organelles that are distinct from that in the cell nucleus, a double membrane, and ribosomes which are the sites of protein assembly in cells. Furthermore, the DNA within mitochondria and chloroplasts have been found to be very closely related to that of Rickettsia and cyanobacteria, respectively, further supporting the endosymbiotic theory.