This is a fascinating question. Mitochondria also have their own ribosomes and do their own separate division.

So what's up? After all, every cell in your body (except mature red blood cell, which have no nuclei, and egg cells) has the same DNA in its nucleus. Why would mitochondria need

The best story we have now is that mitochondria are actually the descendants of bacteria that hitched a ride in larger, more complex cells. Our cells, and those of plants, amoeba-type things, and fungi, are "eukaryotic." Bacterial cells are called "prokaryotic." They don't have nuclei, they are much smaller than our cells, and don't have organelles like our cells. Perhaps the tiny prokaryotes invaded early eukaryotes a long time ago (like 1.4 billion years ago) in order to absorb their food, acting like parasites. Then the eukaryote were able to use the products of the bacteria, so it was actually a relationship that helped both the eukaryote and the prokaryote.

Sound far-fetched? A lot of people thought so too until the 1960's and 70's. A scientist name Lynn Margulis took this idea (which was proposed in the 1800's) and developed it in her Ph.D. dissertation. In the 1970's Dr. Kwang Jeon found that some of the amoebas in his lab had been invaded by bacteria. Some died, but others lived just fine with their bacterial hitchhikers. Years later, this line of amoebas actually required the bacteria in order to live. Now this idea of a mitochodria being the descendants of free-living bacteria is well accepted.

Mitochondrial DNA has a lot of uses because you only inherit it from your mother, so it doesn't mix every generation like your nuclear DNA. Can you find some examples of how mtDNA has been used to solve mysteries?

Plant cells also have another organelle with its own DNA. What is it?

Why do mitochondria need DNA? Well, to answer that question, we would have go back in biological evolutionary history some 2.1 to 2.7 billion years ago (a wee bit longer than when I was in 7th grade...) when bacteria (prokaryotes, or prokaryotic cells) shared the earth with newly evolved types of cells called eukaryotes (have you learned yet what the differences between these two types of cells are?)

Scientists believe that eukaryotes somehow ingested or were parasitized by the smaller prokaryotes, and that when this happened the prokaryotes somehow (no one knows how yet!) evaded being eaten by the eukaryotes, and escaped being killed by the eukaryotes' immune system (no one knows how yet!). In fact, these prokaryotes actually provided such a large benefit to the new host eukaryote, that they were slowly (over many many generations) incorporated into the necessary replication and metabolism of the host cell. When two organisms provide mutual benefit to one another, scientists call this relationship a symbiosis. When one of those organisms (the dependent cell) lives inside the other one (the "boss cell" or host cell), this symbiosis is more specifically called an endosymbiosis (meaning inner symbiosis!). For example, corals and the plant-like single cells living within them (called dinoflagellates) form an endosymbiosis.

However, unlike corals and their endosymbiotic dinoflagellates, the ancient original eukaryotic host cells and their prokaryotic symbionts became very very dependent on each other- to the point that now they now cannot function without each other! Could we live without mitochondria? Could mitochondria live without our cells as their hosts? The answer is definitely "no". In this situation, the host cell and symbiont have become essentially one organism over millions of years! However, the prokaryotes (now mitochondria) kept some of their DNA (left over from the time when they were free living cells), and thus have the ability to replicate certain types of proteins for use in the overall functions of the cell. But remember, they are not able to make all the types of proteins and enzymes they need by themselves- many are made by the nucleus.

So now, can you answer why do mitochondria need DNA, and why don't other organelles in a typical cell have DNA? (except of course the nucleus). It all has to do with their evolutionary history, doesn't it?

Now here is a further question for you: What organelles do photosynthesizing cells (plant-like) have that are also thought to also be ancient prokaryotic symbionts? (Hint: see Virginia Borden's class notes below). Also, I encourage you to look up information on a very famous woman scientist named Professor Lynn Margulis and her theory of endosymbiosis for more information! She and her collaborators overcame quite a bit of ridicule from the scientific community in the 1960-80s when she first proposed her theory.

Thanks for asking a wonderful question! Always keep your enthusiasm for asking questions about the way the world works, and being an observant scientist.

If I'm remembering correctly, chloroplasts have their own DNA, too. Scientists believe that mitochondria and chloroplasts were primitive cells without nuclei (prokaryotes) that got inside of primitive cells with nuclei (eukaryotes) and then stayed there. So they came in with their own DNA, and now they still have some of their own DNA that carries the code for some of their own proteins. They also need some proteins coded by genes in the nuclear DNA. A woman scientist first suggested all this, Lynn Margulis.