Endosymbiosis

Hi. It's Mr. Andersen and in this podcast I'm going to talk about on of
my favorite topics in all of biology and that's endosymbiosis. If you didn't know this, there
are two major groups of cells. We have prokaryotic cells and then we have eukaryotic cells. An
example of a prokaryotic cell is like a bacteria. They simply have a cell membrane, cell wall.
All of their DNA is organized in a nucleoid region. And they're fairly simple and fairly
small. And a eukaryotic cell we're going to have a nucleus. We're going to have organelles
like endoplasmic reticulum, golgi apparatus, mitochondria. But what we find is when we
look in the fossil record, life started about 3.6 billion years ago. And we just see prokaryotic
cells for the longest of times. In other words, we don't see eukaryotic cells show up until
around 2 billion years ago. And it puzzled scientists how this shift was made because
there are clearly two different evolutionary pathways. The pathway of the small prokaryotic
cells and then the larger eukaryotic cells. And they eventually settled on an idea called
endosymbiosis. And what does that mean? Well let's break it down, endo means within. Symbiosis
means together. And bio just means living. And so basically we have organisms that are
living together within one another. So that's weird. What does that mean? Well basically
when I read about this the first time it puzzled me. What we think is way back in the day we
had these aerobic bacterium, ones that were doing cellular respiration so they were breaking
down food in the presence of oxygen. And we also had these cyanobacterium that were doing
photosynthesis and they were essentially engulfed by another host cell and they became the mitochondria
and the chloroplasts that we have today. And so that's pretty cool. In other words these
cells became part of a cell and eventually became that cell. That means that the mitochondria
that are found in all of your cells are kind of like hijackers that have been inside our
cell for billions of years. Now you can see why scientists would have a hard time kind
of believing that this is true. And the first scientist to really be a proponent of endosymbiotic
evolution in eukaryotic cells was Dr. Lynn Margulis. And in the 1960s, I think in 1967,
she wrote an article, a journal article, talking about this. This idea that maybe this is how
mitochondria and chloroplasts came to be. She shopped it around and no scientific journals
would pick it up. After going to about 14 different journals, one journal on theoretical
ideas eventually published it. And it was kind of not laughed down, but it was put aside
for a long period of time. And that's because there wasn't a lot of evidence that showed
that this was true. But Dr. Margulis kept working and working and working and pretty
much today we accept this as scientific fact, or as close to fact as it could be. And so
I wanted to start, before I get to the evidence of why this is probably true to talk about
how symbiosis works on our planet. And so you're maybe familiar with symbiotic relationships,
maybe like the anemone and the clown fish. But it becomes way more intimate than that.
And so this is a type of coral. And this coral can do photosynthesis. But coral is an animal
so how does it do photosynthesis? Well basically they have an algae called Symbiodinium, it's
a type of dinoflagellates and this is eaten by the coral. In other words the coral is
taking in this algae, just like it would be taking in food, but it doesn't break it down.
It doesn't destroy it. The algae lives within the coral. And you can see in this electron
microscopy, you can see these little individual algae cells that are found within the tissues
of the coral. And so what is it doing, it's producing food through photosynthesis. And
that food is then taken in by the coral and in return the coral is giving it a place to
live. And so we think something like this happened, you know billions of years ago,
and that created these first eukaryotic cells. Well, what evidence do we have that this is
true. Well let's just take a look at two. So basically we have a type of bacteria that
looks a lot like a mitochondria. They have a lot of similar properties. And so what evidence
do we have that mitochondria came from bacteria. Well the membranes are going to be very similar
in both of these. In a mitochondria we have this double membrane and we're going to see
the same thing in these bacteria. The way they reproduce is very similar. Now eukaryotic
cells, how do they reproduce? Basically they copy their chromosomes. The chromosomes line
up in the middle and then it divides in half. And we call that mitosis. Now that's not what
happens in bacteria. Bacteria are going to copy all of their DNA and then they just pinch
in half and we call that binary fission. What we find is that even in your cells the mitochondria
are making copies of themselves through a process of asexual reproduction that looks
a lot like asexual reproduction in bacteria. And so this is another piece of evidence.
But why really this idea was set aside for a long period of time is that the technology
to answer this question wasn't quite there. And once we got DNA and the ability to look
at the actual nucleotide sequences within the DNA, were we able to compare the DNA in
these prokaryotic cells and in the mitochondria and we find that it's very similar. What does
that mean? Mitochondria have their own DNA. So they're like a cell within our cell. And
so this is coding for proteins that are used by the mitochondria. And this DNA looks a
lot like a specific type of bacteria. And so again all of this evidence is piled up
and we now believe that this is one of the ways that cells became eukaryotic. We think
they also may have enfolded. In other words the membrane may have folded in on the side
to create some of the complexity, but we're pretty sure chloroplasts and mitochondria
came through this idea of endosymbiosis. And so I remember reading about that and then
thinking up this question and I think it's a pretty good one and a lot of my smart students
will come up with this. And the idea is, okay, if mitochondria are within our cells but they
weren't technically part of our cells then how are they copied from generation to generation.
In other words, where do I get my mitochondria from? Well you can thank your mom for that.
And so basically what happens is in an egg cell, in your mom, we've got a nucleus, but
we also have all these other parts of a cell and so in that egg cell you're going to have
a bunch of mitochondria. Mitochondria that have been passed from mother to daughter to
mother to daughter all through time. And so basically what happens is when that egg is
fertilized by a sperm the sperm doesn't bring mitochondria with it. It just gives genetic
information because the mitochondria are already there. And so when that cell splits in half
we've got mitochondria in each of those individual cells. And so mitochondria used to be cells
of their own. They're now obligate symbionts inside us. That means they can't live on their
own but we have this wonderful relationship where we let them make energy for us and in
plants they have chloroplasts and mitochondria that came from the same origins. And so that's
endosymbiosis and I hope that's helpful.