Great question. Why should an organelle in a cell need its own DNA when the code for everything else is in the DNA in the nucleus? The best explanation is really interesting, but a bit complicated.

Let’s start with how species interact. When individuals of two different species interact in a way that benefits both of them, we call it mutualism. For example, a bee gets nectar from visiting a flower. The plant’s pollen gets spread to another flower, spreading the genes of the first plant. Both benefit. Sometimes mutualists live in or on the other mutualist. We call that symbiosis.

Now let’s bring in something that seems totally different, how our cells convert energy. All cells need energy. The food we eat contains energy. Our cells take the energy in food like sugar and turn it into a chemical called ATP. Cells use ATP to do all sorts of work, from contracting muscles to making hormones.

If we didn’t have mitochondria, we could get about 2 ATP molecules by breaking down a molecule of sugar. With mitochondria, we get about 36 molecules of ATP from the energy in one molecule of sugar. Yay mitochondria! This is true for animal cells, plant cells, and all other cells that we call eukaryotic.

The best explanation we have for mitochondrial DNA is that it is a symbiosis between cells. A mitochondrion may be the descendant of a bacteria that could do some of that awesome processing that gets so much ATP out of a food molecule. An ancestor of today’s eukaryotic cell may have taken in the bacteria to eat it, but didn’t or couldn’t for some reason and kept on eating other food. The bacteria would then be surrounded by food that the big cell captured and broke down to molecules. That would be like moving into a house that provided all your food. The bacteria would have lots of energy and nutrients, and could multiply. The bacteria would make lots of ATP, some that would be available to the big cell. That would be like having someone move into your house that could take your food and multiply it many times. It would have been a win-win or a mutualism. The lucky big cell would have lots of energy to grow and divide. It could out-compete all the big cells that didn’t have the helpful bacteria. Each time the big cell divided, some of the symbiotic bacteria would end up in each of the new cells.

So the best explanation is that mitochondria are descendants of a totally different species that hitched a ride in our distant ancestor’s cell. These days, the egg cell provides mitochondria for new embryos, so we get them all from our mother.

You may want to study genetics or cell biology.

Mitochondria were once free-living bacteria, and had their own entire genomes in order to live as organisms. They still retain some of the cellular machinery including the DNA necessary to survive as the free-living organisms they once were. Chloroplasts are likewise.