<span> They contain their own DNA in the form of a circular plasmid which encodes some - but not all - of the molecules needed for chloroplast function.. It is theorized that this is the DNA from the free-living photosynthetic prokaryote that became an endosymbiont and then an organelle. This theory, The Endosymbiotic Theory, is widely accepted as a plausible explanation for the evolution of organelles. </span>
