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
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.
Thanks for asking,