Why do mitochondria in cells have their own DNA?
According to the endosymbiont theory, an anaerobic cell probably ingested an aerobic bacterium but failed to digest it. The aerobic bacterium flourished within the cell because the cell’s cytoplasm was abundant in half-digested food molecules. The bacterium digested these molecules with oxygen and gained great amounts of energy. Because the bacterium had so much energy, it probably leaked some of it as ATP into the cell’s cytoplasm. This benefited the anaerobic cell because it enabled it to digest food aerobically. Eventually, the aerobic bacterium could no longer live independently from the cell, and it therefore became a mitochondrion. The origin of the chloroplast is very similar… Source: http://en.wikipedia.org/wiki/Endosymbiosis In other words, they have separate DNA because they originally evolved as distinct organisms. The scientist who pioneered this theory was Lynn Margulis.
