Download Endosymbiotic Hypothesis (Endosymbiosis)

Endosymbiotic Hypothesis
(Endosymbiosis)
Or…”Why
Chloroplasts and
Mitochondria
seem to have
Bacterial Ancestry”
Old Ideas Resurrected
• 1883: Botanist by the name of Schimper notices
chloroplasts resemble cyanobacteria in that they divide
similarly (binary fission)
• 1920: Ivan Wallin notices same thing for
mitochondrion.
• (nobody notices)
• In 1967, Margulis offers evidence that these organelles
have separate DNA from the rest of the cell (their own
separate genome), and…..
A Hypothesis is Born
Endosymbiosis (n.):
 The theory (remember what theory means!)
that proposes that certain organelles,
including mitochondria and chloroplasts,
came from free-living prokaryotes.
 One organism lives within the body of another
and both benefit.
A Hypothesis is Born
Endosymbiosis (n.):
 A simpler prokaryotic cells was taken up by larger ones.
 Some of these smaller ones survived inside.
 The smaller ones inside evolved to become mitochondria or
chloroplasts.
A Hypothesis is Born
Endosymbiosis (n.):
 Advantages:
 If smaller one acted like a mitochondria, the larger
one got cellular energy from it.
 If it acted like a chloroplast, the larger one could
do photosynthesis.
 In exchange, the mitochondria and chloroplasts
found stable environment and nutrients.
What’s the Evidence?
1. Organelle replication method.
2. Membrane Similarity
3. Mitochondria and Chloroplasts have their
own DNA
4. Ribosome Similarity
5. Size Similarity
6. Protein Similarity
What’s the Evidence?
1. Mitochondria and chloroplasts replicate
differently than other organelles.
– While the rest of the cell divides via mitosis, these
(like bacteria), replicate by binary fission.
• Binary Fission: Cell splits in half to reproduce.
What’s the Evidence?
2. Membrane Similarity:
– Mitochondria and chloroplasts have double
membranes, and the inner membrane is made of
the protein “peptidoglycan.”
•
Except for these organelles, only bacteria have
membranes like this.
What’s the Evidence?
3. Mitochondria and chloroplasts have their
own set of DNA which is NOT in the nucleus
(like the rest).
– These sets of DNA come in circular structures
called “plasmids,” just like bacterial DNA.
– Statistical analysis shows it has a lot of the same
bases also.
What’s the Evidence?
4. Ribosomes: There are 2 kinds of ribosomes.
– Eukaryotes have larger “80S” ribosomes.
– Bacteria have smaller “70S” ribosomes.
•
Mitochondria and chloroplasts have 70S ribosomes
like bacteria.
What’s the Evidence?
5. There is a size similarity.
– Mitochondria and chloroplasts are about the
same size as their proposed prokaryotic
counterparts (namely, cyanobacteria).
What’s the Evidence?
6. Protein similarity. Eukaryotic proteins always
start with the same amino acid: methionine.
– Bacterial proteins starts with a different one: Nformylmethionine.
– Proteins in mitochondria and chloroplasts all
start with N-formylmethionine, too.
Think Hard…
• “Regular” (Nuclear) DNA comes from mommy
and daddy; each contributing 50%.
– Why is that? Think about how your DNA got to be
where it is.
Here’s Why!
• Nuclear DNA is (re)produced by the process of
“mitosis” (the process of cell division), starting
with a fertilized egg (which had DNA from
mommy’s egg and daddy’s sperm).
Mom’s Contribution Only
• Mitochondrial DNA is ONLY maternal.
– Daddy only gave nuclear DNA.
– Mitochondria came from mommy only (from the
cytoplasm of the unfertilized egg).
• So your mitochondrial DNA matches hers.
How could this happen?
In your notes, draw and label this diagram.
How could this happen?
Also, add the 6 pieces of evidence that support this.
1. Mitochondria and Chloroplast replicate (make copies
of themselves) like bacteria.
2. Membrane Similarity with bacteria.
3. Mitochondria and Chloroplasts have their own DNA
4. Ribosome size is similar to bacteria.
5. Size of cells is similar to bacteria.
6. Proteins are similar to bacteria.