The theory essentially states that mitochondria and possibly chloroplast were once free living organisms that were incorporated into the cellular system through phagocytes, one cell engulfing another. The fact that both chloroplasts and mitochondria each have a complex internal membrane structure supports the theory. Both Mitochondria and chloroplasts also have DNA that is separate and distinct from the DNA found in the nucleolus of the cell. In both cases, this DNA encodes for proteins specific to the function of the organelle.