Inside our cells, each of us has a second set of genes that are completely separate from the 23 pairs of chromosomes we inherit from our parents.
And that's not just the case with humans—it's true for every animal, plant, and fungus, and almost every multicellular organism on Earth.
This second genome belongs to our mitochondria, an organelle inside our cells. They're not entirely a part of us, but they're not separate either—so why are they so different from anything else in our bodies?
About 1.5 billion years ago, scientists think a single-celled organism took over the ancestor of mitochondria, giving rise to all multicellular organisms.
Mitochondria play an important role:
they convert the energy from our food and oxygen into a form of energy that our cells can use, a molecule called ATP. Without this energy, our cells begin to die.
Humans have more than 200 types of cells, and all but adult red blood cells contain mitochondria. This is because the red blood cell's job is to deliver oxygen, which the mitochondria will use up before it reaches its destination.
So all mitochondria use oxygen and metabolites to produce energy and have their own DNA, but mitochondrial DNA varies more among species than other DNA.
In mammals, mitochondria normally contain 37 genes.
Some plants, such as cucumbers, have up to 65 genes in their mitochondria, and some fungal mitochondria have only 1.
Some microbes that live in oxygen-deprived environments appear to be on the verge of losing their mitochondria entirely, and one group, the Oxymond monocercomonides. , already has.
This type exists because mitochondria are still evolving on their own timeline, together with the organisms that contain them.
To understand how this is possible, it helps to take a closer look at what the mitochondria inside us are doing, from the moment we are conceived. In almost all species, mitochondrial DNA is passed down from only one parent.
In humans and most animals, that parent is the mother.
The tail of sperm has about 50 to 75 mitochondria, to help them swim. They fuse with the tail after conception.
Meanwhile, an egg contains thousands of mitochondria, each containing multiple copies of mitochondrial DNA.
It translates the more than 150,000 copies of mitochondrial DNA that we inherit from our mothers, each of which is independent and may differ slightly from the others.
As a fertilized egg grows and divides, thousands of mitochondria are distributed throughout the developing embryo's cells.
By the time we differentiate into tissues and organs, variations in mitochondrial DNA are randomly scattered throughout our bodies. To complicate matters further, mitochondria have a separate replication process from our cells.
So as our cells divide and replicate, mitochondria are lost in new cells, and each time they are fusing and dividing themselves, on their own timeline.
As mitochondria fuse and separate,
they isolate damaged DNA or mitochondria that are not functioning properly for removal.
All of this means that the random selection of mitochondrial DNA from your mother that you inherit at birth can change throughout your life and throughout your body.
So mitochondria are dynamic and somewhat independent, but they are also shaped by their environment: us. We think that long ago, some of their genes were transferred into their host's genome.
So today, although mitochondria have their own genome and replicate separately from the cells that contain them, they cannot do so without the guidance of our DNA.
And although mitochondrial DNA is inherited from one parent, the genes involved in building and regulating mitochondria come from both.
Mitochondria continue to defy the neat hierarchy.
Their story is still unfolding within each of our cells, simultaneously separate and inseparable from our own.
Learning more about them can give us tools to protect human health in the future, and teach us more about our history.
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