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Endosymbiotic theory

Page history last edited by Charles Forstbauer 14 years ago

Totaled 3/29 Mr F

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Biography: Lynn Margulis

The Endosymbiotic Theory was developed by Lynn Margulis. American microbiologist who is currently distinguished professor in the Department of Geosciences at the University of Massachusetts, Amherst. She originated the endosymbiotic theory and was one of the pioneers, along with James Lovelock, of the Gaia Hypothesis. From 1977 to 1980, she chaired the Space Science Board Committee on Planetary Biology and Chemical Evolution. Margulis obtained a B.A. from the University of Chicago(1957), an M.S. from the University of Wisconsin (1960), and Ph.D. from the University of California, Berkeley. Her former husband was Carl Sagan.

 

The endosymbiotic theory concerns the origins of mitochondria and plastids (e.g. chloroplasts), which are organelles of eukaryotic cells. According to this theory, these organelles originated as separate prokaryotic organisms that were taken inside the cell as endosymbionts. Mitochondria developed from proteobacteria (in particular, Rickettsiales or close relatives) and chloroplasts from cyanobacteria.

 

Endosymbiotic theory proposes that these organelles were once prokaryotic cells, living inside larger host cells. The prokaryotes may initially have been parasites or even an intended meal for the larger cell, somehow escaping digestion.

Whatever the cause of their initial internment, these prokaryotes might soon have become willing prisoners to a grateful warden. The prisoner prokaryotes might have provided crucial nutrients (in the case of the primitive chloroplast) or helped to exploit oxygen for extracting energy (in the case of the primitive mitochondrion). The prokaryotes, in turn, would have received protection and a steady environment in which to live.

 

 

This picture diagrams the endosymbiotic theory, how it evolved and how it continued. 

 

Key to the success of eukaryotic cells have been two powerful, mutually supportive organelles: the mitochondrion and the chloroplast:

  • The mitochondrion consumes oxygen to efficiently extract energy from carbon sources like glucose, producing carbon dioxide and water in the process.
  • The chloroplast consumes water and carbon dioxide as it captures energy from light and funnels it into the chemical energy of glucose, releasing oxygen in the process.

 

This picture is a very simplistic view of the endosymbiotic process, including the meeting, consumption, and retainment of the prokaryote.

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This is a video explaining endosymbiotic theory in an over-simplified way. It's easy to understand because it was made by students, and the sidebar has a nice explanation of the theory as well. 

 

This is another view outlining the theory of endosymbiotic and why its important for understanding evolution

YouTube plugin error  

This website contains a fun webquest that shows every step of the endosymbiotic process, as well as some information about the theory's founder. http://www.sumanasinc.com/webcontent/animations/content/organelles.html 

 

 

This website has a lot of information about the history of endosymbiosis as well as pictures and diagrams detailing the process: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/Endosymbiosis.html

 

 

 

The Endosymbiotic Theory

 

The Endosymbiotic Theory was first proposed by former Boston University Biologist Lynn Margulis in the 1960's and officially in her 1981 book "Symbiosis in Cell Evolution".  Although now accepted as a well-supported theory, both she and the theory were ridiculed by mainstream biologists for a number of years.  Thanks to her persistance, and the large volumes of data that support this hypothesis gathered by her and many other scientists over the last 30 years, biology can now offer a plausible explanation for the evolution of eukaryotes.

 

Dr. Margulis was doing reserarch on the origin of eukaryotic cells. She looked at all the data about prokaryotes, eukaryotes, and organelles. She proposed that the similarities between prokaryotes and organelles, together with their appearance in the fossil record, could best be explained by "endo-symbiosis".

[Endo = "within"]

[Endocytosis = (cyto = cell) a process of 'cell eating' - cells are engulfed, but then usually digested as food....]

[Endosymbiosis = cells are engulfed, but not digested...cells live together is a mutually benefitting relationship, or symbiosis]

 

Her hypothesis originally proposed that:

  • mitochondria are the result of endocytosis of aerobic bacteria
  • chloroplasts are the result of endocytosis of photosynthetic bacteria
  • in both cases by large anaerobic bacteria who would not otherwise be able to exist in an aerobic environment.
  • this arrangement became a mutually beneficial relationship for both cells (symbiotic).

 

Margulis' original hypothesis proposed that aerobic bacteria (that require oxygen) were ingested by anaerobic bacteria (poisoned by oxygen), and may each have had a survival advantage as long as they continued their partnership.

 

 

 

 

Each would have performed mutually benefiting functions from their symbiotic relationship.  The aerobic bacteria would have handled the toxic oxygen for the anaerobic bacteria, and the anaerobic bacteria would ingested food and protected the aerobic "symbiote"..

The result = a cell with a double-membrane bound organelle. The inner lipid bilayer would have been the bacterial cell's plasma membrane, and the ouler lipid bilayer came from the cell that engulfed it.

 

Other evidence that supports this hypothesis:

1. . The timeline of life on Earth:

a. Anaerobic bacteria: Scientists have fossil evidence of bacterial life on Earth ~3.8 billion years ago. At this time, the atmosphere of the Earth did not contain oxygen, and all life (bacterial cells) was anaerobic.

 

b. Photosynthetic bacteria: About ~3.2 billion years ago, fossil evidence of photosynthetic bacteria, or cyanobacteria, appears. These bacteria use the sun's energy to make sugar. Oxygen, released as a byproduct, began to accumulate in the atmosphere. However, oxygen is actually pretty toxic to cells, even our cells! As a result, anaerobic cells were now a disadvantage in an oxygen-containing atmosphere, and started to die out as oxygen levels increased.

 

c. Aerobic cells appear in the fossil record shortly after that (~2.5 Billion years ago). There cells were were able to use that 'toxic' oxygen and convert it into energy (ATP) and water. Organisms that could thrive in an oxygen-containing atmosphere were now 'best suited to the environment'.

 

2. Organelles have their own DNA, and divide independently of the cell they live in: When Margulis initially proposed the Symbiotic Theory, she predicted that, if the organelles were really bacterial (prokaryotic) symbionts, they would have their own DNA.  If her theory DID best explain the origin of eukaryotic cells, she reasoned, organelles would have DNA that resembled bacterial DNA and be different from the cell's DNA (located in the nucleus membrane).  Amazingly, in the 1980's this was proven to be the case for two classes of organelles, the mitochondria and chloroplasts.  Further, in the late 1980's a team of Rockefeller University investigators announced their similar discovery regarding centrioles, structures that provide the eukaryotic cell with the ability of locomotion and cell division. 

 

Thanks to over 30 years of additional evidence, The Endosymbiotic Theory provides the most plausible explanation for the development of organelles within the eukaryotic cell.

An anaerobic organism or anaerobe is any organism that does not require oxygen for growth, could possibly react negatively and may even die in its presence. There are three types: obligate anaerobes, which cannot use oxygen for growth and are even harmed by it; aerotolerant organisms, which cannot use oxygen for growth, but tolerate the presence of it; and facultative anaerobes, which can grow without oxygen but can utilize oxygen if it is present.

 

An aerobic organism or aerobe is an organism that can survive and grow in an oxygenated environment.

 

Evidence

  • Both mitochondria and chloroplasts can arise only from preexisting mitochondria and chloroplasts. They cannot be formed in a cell that lacks them because nuclear genes encode only some of the proteins of which they are made.
  • Both mitochondria and chloroplasts have their own genome and it resembles that of bacteria not that of the nuclear genome.
    • Both genomes consist of a single circular molecule of DNA.
    • There are no histones associated with the DNA.
  • Both mitochondria and chloroplasts have their own protein-synthesizing machinery, and it more closely resembles that of bacteria than that found in the cytoplasm of eukaryotes.
    • The first amino acid of their transcripts is always fMet as it is in bacteria  that is the first amino acid in eukaryotic proteins).
    • A number of antibiotics that act by blocking protein synthesis in bacteria also block protein synthesis within mitochondria and chloroplasts. They do not interfere with protein synthesis in the cytoplasm of the eukaryotes.
    • Conversely, inhibitors (e.g., diphtheria toxin) of protein synthesis by eukaryotic ribosomes do not — sensibly enough — have any effect on bacterial protein synthesis nor on protein synthesis within mitochondria and chloroplasts.
    • The antibiotic rifampicin, which inhibits the RNA polymerase of bacteria, also inhibits the RNA polymerase within mitochondria. It has no such effect on the RNA polymerase within the eukaryotic nucleus.

 

 

  Prokaryotes Eukaryotes Mitochondria of
Eukaryotic cells
Chloroplasts of Photosynthetic eukaryotes
DNA 1 single, circular chromosome Multiple linear chromosomes
compartmentalized in a nucleus
1 single, circular
chromosome
1 single, circular chromosome
Replication Binary Fission
(1 cell splits into 2)
Mitosis Binary Fission
(1 cell splits into 2)
Binary Fission
(1 cell splits into 2)
Ribosomes "70 S" "80 S" "70 S" "70 S"
Electron Transport Chain Found in the plasma membrane around cell Not found in the plasma membrane
around cell (found only in the cell's mitochondria and chloroplasts)
Found in the plasma membrane around mitochondrion Found in the plasma membrane around chloroplast
Size (approximate) ~1-10 microns ~50 - 500 microns ~1-10 microns ~1-10 microns
Appearance on Earth Anaerobic bacteria:
~3.8 Billion years ago
Photosynthetic bacteria:
~3.2 Billion years ago
Aerobic bacteria:
~2.5 Billion years ago
~1.5 billion years ago ~1.5 billion years ago

~1.5 billion years ago

 

 

 

 

 

 

 

Endosymbiosis and The Origin of Eukaryotes

The endosymbiosis theory postulates that

  • The mitochondria of eukaryotes evolved from aerobic bacteria (probably related to the rickettsias) living within their host cell.
  • The chloroplasts of eukaryotes evolved from endosymbiotic cyanobacteria.

The Evidence

  • Both mitochondria and chloroplasts can arise only from preexisting mitochondria and chloroplasts. They cannot be formed in a cell that lacks them because nuclear genes encode only some of the proteins of which they are made.
  • Both mitochondria and chloroplasts have their own genome and it resembles that of bacteria not that of the nuclear genome.
    • Both genomes consist of a single circular molecule of DNA.
    • There are no histones associated with the DNA.
  • Both mitochondria and chloroplasts have their own protein-synthesizing machinery, and it more closely resembles that of bacteria than that found in the cytoplasm of eukaryotes.
    • The first amino acid of their transcripts is always fMet as it is in bacteria (not methionine [Met] that is the first amino acid in eukaryotic proteins).
    • A number of antibiotics (e.g., streptomycin) that act by blocking protein synthesis in bacteria also block protein synthesis within mitochondria and chloroplasts. They do not interfere with protein synthesis in the cytoplasm of the eukaryotes.
    • Conversely, inhibitors (e.g., diphtheria toxin) of protein synthesis by eukaryotic ribosomes do not — sensibly enough — have any effect on bacterial protein synthesis nor on protein synthesis within mitochondria and chloroplasts.
    • The antibiotic rifampicin, which inhibits the RNA polymerase of bacteria, also inhibits the RNA polymerase within mitochondria. It has no such effect on the RNA polymerase within the eukaryotic nucleus.

The Mitochondrial Genome

The genome of human mitochondria contains 16,569 base pairs of DNA organized in a closed circle. These encode:

  • 2 ribosomal RNA (rRNA) molecules
  • 22 transfer RNA (tRNA) molecules (shown in the figure as yellow bars; two of them labeled)
  • 13 polypeptides

The 13 polypeptides participate in building several protein complexes embedded in the inner mitochondrial membrane.

All these gene products are used within the mitochondrion, but the mitochondrion also needs proteins encoded by nuclear genes. These proteins (e.g., cytochrome c and the RNA and DNA polymerases used within the mitochondrion) are synthesized in the cytosol and then imported into the mitochondrion.

 

Secondary Endosymbiosis: Eukaryotes Engulfing Eukaryotes

The Nucleomorph

Once both heterotrophic and photosynthetic eukaryotes had evolved, the former repeatedly engulfed the latter to exploit their autotrophic way of life. Many animals living today engulf algae for this purpose [Link to examples]. Usually the partners in these mutualistic relationships can be grown separately.

However, a growing body of evidence indicates that the chloroplasts of some algae have not been derived by engulfing cyanobacteria in a primary endosymbiosis like those discussed above, but by engulfing photosynthetic eukaryotes. This is called secondary endosymbiosis. It occurred so long ago that these endosymbionts cannot be cultured away from their host.

In two groups, the eukaryotic nature of the endosymbiont can be seen by its retention of a vestige of a nucleus (called its nucleomorph).

  • A group of unicellular, motile algae called cryptomonads appear to be the evolutionary outcome of a nonphotosynthetic eukaryotic flagellate (i.e., a protozoan) engulfing a red alga by endocytosis.
  • Another tiny group of unicellular algae, called chlorarachniophytes, appear to be the outcome of a flagellated protozoan having engulfed a green alga.

The result in both cases: a motile, autotrophic cell containing:

  • its own nucleus
  • its own mitochondria
  • its own endoplasmic reticulum, which contains the endosymbiont with
    • its own plasma membrane
    • its own cytoplasm, the periplastid space
    • its own ribosomes
    • its own chloroplast, and
    • its nucleomorph - only a vestige of its original nucleus, but still
      • surrounded by a nuclear envelope perforated with nuclear pore complexes and
      • containing a tiny but still-functioning genome.

The Four Genomes of Guillardia theta

The cryptomonad Guillardia theta contains four different genomes:

  • its own nuclear genome; by far the largest with ~350x109 base pairs (bp) of DNA;
  • the genome of its mitochondria (48,000 bp);
  • the genome of the chloroplast in its endosymbiont (121,000 bp);
  • the genome of the nucleomorph (551,264 bp).

Susan Douglas and her colleagues reported (in the 26 April 2001 issue of Nature) the completely-sequenced genome of the nucleomorph.

It contains 3 small chromosomes with

  • 47 genes for nonmessenger RNAs (rRNA, tRNA, snRNA)
  • 464 genes for messenger RNA; that is, encoding proteins such as
    • 65 proteins for its own ribosomes
    • 30 for its chloroplast (a small fraction of the hundreds needed)
    • a variety of proteins needed within the nucleomorph, including
      • DNA licensing factors
      • histones
      • proteins needed for DNA replication (but no genes for DNA polymerases, which must be translated by and imported from the host ribosomes)

The genes are crowded closely on the three chromosomes. In fact, 44 of them overlap each other. Only 17 genes contain introns, and these are very small.  

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