Endosymbiosis

Summary

In this podcast, Mr. Andersen explores endosymbiosis, a key concept in biology that explains the evolution from prokaryotic to eukaryotic cells. He discusses how mitochondria and chloroplasts, once free-living bacteria, became integral parts of eukaryotic cells through a symbiotic relationship. The podcast highlights Dr. Lynn Margulis' pioneering work on endosymbiosis, which was initially met with skepticism but is now widely accepted. Evidence from cellular structures, reproduction methods, and DNA similarities supports this theory, illustrating a fascinating example of cooperation in the natural world.

Takeaways

• 🌿 **Endosymbiosis**: The theory that eukaryotic cells evolved from prokaryotic cells through a process where one cell lived inside another.
• 🔬 **Cell Types**: There are two major cell types: prokaryotic cells (like bacteria) and eukaryotic cells (with a nucleus and organelles).
• 🕰️ **Timeline**: Prokaryotic cells existed for billions of years before eukaryotic cells appeared around 2 billion years ago.
• 🤔 **Evolutionary Puzzle**: The shift from prokaryotic to eukaryotic cells was a mystery until the endosymbiosis theory was proposed.
• 🌱 **Photosynthesis and Respiration**: Ancient aerobic bacteria and cyanobacterium were thought to be engulfed by a host cell, leading to the formation of mitochondria and chloroplasts.
• 🧬 **DNA Evidence**: Mitochondria have their own DNA, which is similar to that of certain bacteria, supporting the endosymbiosis theory.
• 👩‍🔬 **Lynn Margulis**: She was a key proponent of endosymbiosis in the 1960s, and her ideas are now widely accepted.
• 🔍 **Technological Advancements**: The ability to analyze DNA sequences provided evidence supporting the endosymbiosis theory.
• 🌐 **Symbiotic Relationships**: Examples like the relationship between coral and algae (Symbiodinium) illustrate modern symbiotic relationships.
• 👶 **Mitochondrial Inheritance**: Mitochondria are passed down from mother to offspring, as they were not contributed by the sperm during fertilization.

Q & A

What are the two major groups of cells mentioned in the podcast?

The two major groups of cells mentioned are prokaryotic cells and eukaryotic cells.

What is a characteristic feature of prokaryotic cells?

Prokaryotic cells are characterized by having a cell membrane, cell wall, and their DNA is organized in a nucleoid region without a nucleus.

When did eukaryotic cells first appear in the fossil record according to the podcast?

Eukaryotic cells first appeared in the fossil record around 2 billion years ago.

What does the term 'endosymbiosis' mean?

Endosymbiosis means living together within one another, where organisms live inside other organisms.

How did the idea of endosymbiosis help explain the evolution from prokaryotic to eukaryotic cells?

Endosymbiosis theory suggests that aerobic bacteria and cyanobacterium were engulfed by a host cell and eventually became mitochondria and chloroplasts, leading to the formation of eukaryotic cells.

Who was the first scientist to propose endosymbiotic evolution in eukaryotic cells?

Dr. Lynn Margulis was the first scientist to propose endosymbiotic evolution in eukaryotic cells.

What evidence supports the idea that mitochondria originated from bacteria?

The evidence includes similar membrane structures, reproduction methods, and DNA sequences between certain bacteria and mitochondria.

How do mitochondria reproduce within our cells?

Mitochondria reproduce through a process of asexual reproduction that resembles binary fission in bacteria.

Why do mitochondria have their own DNA?

Mitochondria have their own DNA because they were once independent prokaryotic cells that became incorporated into eukaryotic cells through endosymbiosis.

How are mitochondria passed from one generation to the next?

Mitochondria are passed from one generation to the next through the egg cell from the mother, as the sperm does not contribute mitochondria.

What is the relationship between mitochondria and chloroplasts in terms of their origin?

Both mitochondria and chloroplasts are believed to have originated from endosymbiotic events where prokaryotic cells were engulfed and became part of eukaryotic cells.

Outlines

00:00

🌿 The Theory of Endosymbiosis

Mr. Andersen introduces the concept of endosymbiosis, a key topic in biology, which explains the evolution from prokaryotic to eukaryotic cells. He discusses the differences between the two cell types, noting that eukaryotic cells contain organelles like mitochondria and chloroplasts, which were once free-living organisms. The theory suggests that these organelles were once bacteria that were engulfed by a host cell, eventually becoming integral parts of the cell. This idea was initially met with skepticism but is now widely accepted due to evidence such as similar DNA sequences and reproductive methods between mitochondria and bacteria. Dr. Lynn Margulis is credited with championing this theory in the 1960s, despite initial resistance from the scientific community.

05:00

🧬 Evidence for Endosymbiosis

The second paragraph delves into the evidence supporting endosymbiosis. It highlights the striking similarities between mitochondria and certain bacteria, such as their double membrane structure and mode of reproduction. The paragraph explains that mitochondria have their own DNA, which is more similar to bacterial DNA than to the nuclear DNA of the eukaryotic cell. This genetic similarity is a strong indicator that mitochondria were once independent bacteria. The discussion also touches on the role of symbiotic relationships in nature, using the example of coral and algae, to illustrate how such relationships may have led to the development of complex eukaryotic cells. The paragraph concludes with a question about the inheritance of mitochondria, explaining that they are maternally inherited, as sperm cells do not contribute mitochondria during fertilization.

Keywords

💡Endosymbiosis

Endosymbiosis is a biological theory that explains the origin of eukaryotic cells through the fusion of different types of prokaryotic cells. It suggests that larger cells engulfed smaller cells, which then became organelles like mitochondria and chloroplasts. In the video, endosymbiosis is the central theme, illustrating how these once-independent cells now coexist within eukaryotic cells, contributing to their complex functions.

💡Prokaryotic cells

Prokaryotic cells are simple, single-celled organisms without a nucleus or membrane-bound organelles, such as bacteria. The video discusses how life began with only prokaryotic cells, which were the dominant form of life for billions of years before eukaryotic cells appeared.

💡Eukaryotic cells

Eukaryotic cells are more complex than prokaryotic cells, containing a nucleus and other membrane-bound organelles. The video highlights the puzzle of how eukaryotic cells evolved, with endosymbiosis being the key theory to explain their origin and the presence of organelles like mitochondria.

💡Mitochondria

Mitochondria are organelles found in eukaryotic cells known as the 'powerhouses' of the cell because they generate energy through cellular respiration. The video explains how mitochondria are thought to have originated from aerobic bacteria that were engulfed by a host cell, eventually becoming an integral part of eukaryotic cells.

💡Chloroplasts

Chloroplasts are organelles in plant cells that carry out photosynthesis, converting light energy into chemical energy. The script suggests that, similar to mitochondria, chloroplasts may have originated from endosymbiotic events involving photosynthetic bacteria.

💡Cellular respiration

Cellular respiration is a process in which cells break down food in the presence of oxygen to produce energy. The video mentions aerobic bacteria that performed cellular respiration and were later engulfed by other cells, contributing to the development of mitochondria.

💡Photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy. The video draws a parallel between modern photosynthetic organisms and the ancient event where cyanobacteria were engulfed, leading to the formation of chloroplasts.

💡Symbiodinium

Symbiodinium is a type of dinoflagellate algae that lives in a symbiotic relationship with coral, performing photosynthesis within the coral's tissues. The video uses this example to illustrate the concept of endosymbiosis in modern organisms, mirroring the ancient events that led to the formation of eukaryotic cells.

💡Mitosis

Mitosis is the process of cell division in eukaryotic cells, where the nucleus divides into two identical copies. The video contrasts mitosis with binary fission, the method of reproduction in bacteria, to highlight the differences between prokaryotic and eukaryotic cells.

💡Binary fission

Binary fission is a simple form of asexual reproduction in prokaryotic cells, where the cell divides into two identical daughter cells. The video notes the similarity between binary fission in bacteria and the way mitochondria reproduce within eukaryotic cells, suggesting a common evolutionary origin.

💡DNA

DNA, or deoxyribonucleic acid, is the molecule that carries genetic instructions for the development and functioning of all living organisms. The video discusses how the DNA of mitochondria is similar to that of certain bacteria, providing evidence for the endosymbiotic theory.

Highlights

Endosymbiosis is a key concept in understanding the evolution from prokaryotic to eukaryotic cells.

Prokaryotic cells, like bacteria, are simple with a cell membrane and nucleoid region for DNA.

Eukaryotic cells contain a nucleus and organelles such as mitochondria and chloroplasts.

Fossil records show prokaryotic cells existed for billions of years before eukaryotic cells appeared.

Endosymbiosis theory suggests that aerobic bacteria and cyanobacterium were engulfed by a host cell, becoming mitochondria and chloroplasts.

Mitochondria are considered to have been hijackers inside our cells for billions of years.

Dr. Lynn Margulis was a pioneer in promoting endosymbiotic evolution in the 1960s.

Margulis' theory was initially met with skepticism due to a lack of evidence.

Symbiotic relationships, like that of anemone and clown fish, serve as modern examples of endosymbiosis.

Coral and algae relationship demonstrates a more intimate form of symbiosis, with algae living within coral tissues.

Mitochondria share similar properties with bacteria, including double membranes and reproduction methods.

Mitochondria have their own DNA, which is similar to that of specific bacteria, supporting the endosymbiosis theory.

Advancements in DNA sequencing technology provided evidence for the endosymbiotic origin of mitochondria.

Chloroplasts, like mitochondria, are believed to have originated from endosymbiotic events.

Mitochondria are inherited from mother to offspring through the egg cell, not the sperm.

Endosymbiosis theory has become widely accepted as scientific fact due to accumulating evidence.

Transcripts

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.