I love watching a good thriller movie.
We’ve all seen those alien invasion or zombie apocalypse movies; you know, where some kind
of malicious entity invades and destroys everything as we know it.
That would be one of our worst nightmares!
But what if I told you that foreign life forms are actually closer than you think; in fact,
they’re inside our very cells?
And what if I told you that instead of ruining society, they’re actually essential to our
survival?
Let me tell you the story of mitochondria, the tiny foreign entities inside our cells
that give us the energy we need to live, and the scientists who helped us understand where
they came from.
To start, let’s zoom into one of our cells.
You might know that mitochondria are the powerhouse of the cell; they're organelles that convert
glucose from the food we eat into energy!
What’s really interesting, though, is that mitochondria aren’t human…
they don’t have human DNA.
In that case, how did they get into our cells?
This mystery piqued the curiosity of our protagonists – a group of scientists from the UK, led
by S. Anderson.
Anderson’s team knew about a theory proposed by other scientists, called the endosymbiotic
theory, which hypothesises where mitochondria came from.
We can categorise living things into two types: prokaryotes and eukaryotes.
Prokaryotes are simple, single-celled organisms, whereas eukaryotes have more complex components
and can be multicellular.
Eukaryotes also contain mitochondria while prokaryotes don’t.
The endosymbiotic theory claims that this is because eukaryotes evolved from prokaryotes!
2.7 billion years ago, one prokaryote ingested another, and the two of them began living
in a symbiotic relationship.
While the outer prokaryote provided protection, they inner prokaryote was very efficient at
respiration and provided energy for them both.
They eventually evolved into the first eukaryotic cell, and over time the inner prokaryote became
modern mitochondria!
Though this was plausible, there was little to no genetic evidence to prove it.
That is, until our heroes intervened!
Anderson and his team knew that human mitochondria actually had a completely unique DNA sequence.
So, if they could sequence mitochondrial DNA and compare it with what they knew about human
DNA, they might be able to learn what was going on inside our little mitochondrial buddies.
Using a method called Sanger sequencing, the team found the genetic makeup of mitochondria.
Then, they tested for homology with the human genome.
This helped them understand what mitochondrial DNA coded for!
What they found was astounding.
Mitochondrial DNA encodes cytochrome, which allows them to produce energy.
In addition, mitochondria encode their own amino acids, which is something independent
organisms can do.
They also encode their own rRNA and tRNA, which are needed to create these proteins,
so they can basically complete protein synthesis independently!
Fascinatingly, the process of mitochondrial protein synthesis was also unique.
For one, mitochondria used unconventional start and stop codons.
They can use AUA as an alternative start codon.
Mitochondria use the universal stop codon UGA for tryptophan.
Instead, they use AGA and AGG as stop codons!
That’s not all; it turns out mitochondria also use their own, unique tRNA!
Anderson found that mitochondrial tRNA has fewer tertiary interactions than tRNA in the
rest of the cell.
This means their tRNA is much simpler but has more variability, allowing mitochondria
more freedom to evolve.
Overall, what Anderson and his team saw was basically an entire, independent organism
within the human cell.
This played a key role in solving the mitochondrial mystery.
Their findings showed that the endosymbiont hypothesis was probably true!
This was hugely significant for scientific progress.
We were one step closer to understanding how eukaryotes, and how humans like you and I,
came to be!
Because the endosymbiont theory is also applicable to chloroplasts in plants, Anderson’s discoveries
have also helped in understanding the origin of plant cells.
Overall, this knowledge now forms a core part of what all biologists learn in school today.
Here's what’s really interesting, though.
The team still thought mitochondria were neither prokaryotic nor eukaryotic.
Their tRNA was different from eukaryotes, but their rRNA had low homology with modern
prokaryotes, so mitochondria weren’t exactly either.
“Well then, what are they?”
I hear you ask.
Not to worry; our heroes tackled this mystery as well.
Remember how we said mitochondria have more freedom to evolve?
Well, Anderson believed that mitochondria and modern prokaryotes probably both evolved
from similar ancient prokaryotes.
However, since mitochondria lived inside of a cell, they experienced vastly different
selective pressures to other prokaryotes.
As a result, ancestor prokaryotes would have evolved to form the completely different structures
that we know today!
So Anderson’s team has helped us write a new chapter in our knowledge of the history
of life.
Here is where our story ends.
But what specific organism did mitochondria evolve from?
What about the outside prokaryote that evolved into the human cell?
There’s still so much we don’t know!
Until more scientists like our protagonists unearth more evidence proving this, that’s
a story for another day.
So, next time you watch an apocalypse movie, instead of freaking out, try to remember the
little foreign life forms hard at work inside you right now.
Thanks to them, we have the energy to keep watching the movies we love… and the energy
to run from zombies, if it ever comes to that!