The genes you don't get from your parents (but can't live without) - Devin Shuman
Inside our cells, each of us has
a second set of genes
completely separate from the 23 pairs
of chromosomes
we inherit from our parents.
And this isn’t just the case for humans—
it’s true of every animal,
plant, and fungus,
and nearly every multicellular
organism on Earth.
This second genome belongs
to our mitochondria,
an organelle inside our cells.
They’re not fully a part of us,
but they’re not separate either—
so why are they so different
from anything else in our bodies?
Approximately 1.5 billion years ago,
scientists think a single-celled organism
engulfed the mitochondria’s ancestor,
creating the predecessor
of all multicellular organisms.
Mitochondria play an essential role:
they convert energy from the food we eat
and oxygen we breathe
into a form of energy our cells can use,
which is a molecule called ATP.
Without this energy,
our cells start to die.
Humans have over 200 types of cells,
and all except mature red blood cells
have mitochondria.
That’s because a red blood cell’s job
is to transport oxygen,
which mitochondria would use up before
it could reach its destination.
So all mitochondria use oxygen
and metabolites to create energy
and have their own DNA,
but mitochondrial DNA varies more
across species than other DNA.
In mammals, mitochondria usually
have 37 genes.
In some plants, like cucumbers,
mitochondria have up to 65 genes,
and some fungal mitochondria have only 1.
A few microbes that live
in oxygen-poor environments
seem to be on the way to losing
their mitochondria entirely,
and one group, oxymonad monocercomonoides,
already has.
This variety exists because mitochondria
are still evolving,
both in tandem with the organisms
that contain them,
and separately, on their own timeline.
To understand how that’s possible,
it helps to take a closer look at what
the mitochondria inside us are doing,
starting from the moment we’re conceived.
In almost all species, mitochondrial DNA
is passed down from only one parent.
In humans and most animals,
that parent is the mother.
Sperm contain approximately
50 to 75 mitochondria in the tail,
to help them swim.
These dissolve with the tail
after conception.
Meanwhile, an egg contains thousands
of mitochondria,
each containing multiple copies
of the mitochondrial DNA.
This translates to over 150,000 copies
of mitochondrial DNA
that we inherit from our mothers,
each of which is independent
and could vary slightly from the others.
As a fertilized egg grows and divides,
those thousands of mitochondria are
divvied up into the cells
of the developing embryo.
By the time we have
differentiated tissues and organs,
variations in the mitochondrial DNA are
scattered at random throughout our bodies.
To make matters even more complex,
mitochondria have a separate replication
process from our cells.
So as our cells replicate by dividing,
mitochondria end up in new cells,
and all the while they’re fusing
and dividing themselves,
on their own timeline.
As mitochondria combine and separate,
they sequester faulty DNA or mitochondria
that aren’t working properly for removal.
All this means that the random selection
of your mother’s mitochondrial DNA
you inherit at birth
can change throughout your life
and throughout your body.
So mitochondria are dynamic and,
to a degree, independent,
but they’re also shaped
by their environments: us.
We think that long ago,
some of their genes were transferred
to their host’s genomes.
So today, although mitochondria
have their own genome
and replicate separately from the cells
that contain them,
they can't do this without
instruction from our DNA.
And though mitochondrial DNA
is inherited from one parent,
the genes involved in building
and regulating the mitochondria
come from both.
Mitochondria continue to defy
tidy classification.
Their story is still unfolding
inside of each of our cells,
simultaneously separate
and inseparable from our own.
Learning more about them can
both give us tools
to protect human health in the future,
and teach us more about our history.
