Why does mitochondria have DNA

Compared to other organelles inside the cell...

Thanks!

Because the organelle in the distant past was an independent purple non-sulfur Rickettsia that underwent the process of endosymbiosis to become an integrated organelle in a much larger complex eukaryote.

Mitochondria retain genetic material specific to the organelle's protein expression by 70s ribosomes and replication by binary fission. They do not have genes for all their proteins.

The transfer of genetic material to the nucleus tied the fission of the organelle to the nuclear regulation of the host cell replication. The cell had internal communication with its endosymbiont and cell cycle control, as a result the cell was more efficient.

Endosymbiosis in an observed selective setting:
Microbiologist Kwang Jeon, using artificial selection, created a new endosymbiosis between the eukaryotic Amoeba proteus and its infective bacteria. After many selective generations the amoeba became dependent upon the bacterium, and endosymbiotic gene switching occurred, moving some genes to the host as predicted by the gene transfer theory. The bacterium went from an independent, self reproductive organism to being an organelle with regulation of its reproduction tied to the amoeba's reproductive cycle. The organelle retained its circular DNA, limited protein expression and binary fission.
http://www.gate.net/~rwms/EvoEndosymbiot…
Gene transfer from the mitochondrion into the nucleus
http://jcs.biologists.org/cgi/content/fu…
http://mbe.oxfordjournals.org/cgi/conten…

The current most popular theory of the origin of mitochondria is that they originated as bacteria that invaded the ancestral eukaryote and started living in its cytoplasm. (It's actually pretty common for bacteria to live inside eukaryotic cells.) Instead of eventually killing off its host, though, its descendants developped a symbiotic relationship with their hosts. Over millions of years, the bacteria lost the ability to function on their own, and transfered some of their genetic material to the host chromosomes and lost some other genes. However, they still retain a few essential genes on a little loop of mitochondrial DNA.

Mitochondria, along with chloroplasts, were believed to once be separate organisms living alone in the environment. It was through evolution that the mitochondria were eventually integrated into larger, more complex organisms. Even though they are now part of the structure and intra-workings of a larger organism, they still retain their DNA from when they were once their own organism.

I assume because it was once a separate creature. The DNA is vestigial. I assume.