The Endosymbiotic Theory

This is to supplement the Week 3 notes.

There is a very cool theory called the Endosymbiotic Theory that was proposed in the early 1970's by cell biologist Lynn Margulis to explain the origin of two types of organelle: the mitochondrion and the chloroplast.

Dr. Margulis was studying the evolution of eukaryotic cells (the more complex cells with nuclei and membrane-bound organelles...like the cells in our bodies). Her idea was that mitochondria (the ATP producing organelles) and chloroplasts (the photosynthesizing organelles) were originally free-living prokaryotic organisms (like bacteria) that were engulfed by (taken in by) other prokaryotic organisms.

Dr. Margulis wondered if, rather than being digested by their engulfers, these precursors to mitochondria and chloroplasts might have set up a long-term symbiotic relationship with their hosts.

Symbiosis: a prolonged close relationship between two organisms that often (but not always) benefits both organisms

Dr. Margulis was widely criticized for this idea when it first came out, but since then, a large and well-accepted body of evidence has been discovered to support her idea.

Remember from Lab 1... a theory in science is not a guess! An hypothesis is a guess, a theory is supported by a vast body of evidence.

The evidence for this theory includes:

---Mitochondria and chloroplasts have their own separate DNA...and the DNA is circular, like bacterial DNA.

---Mitochondria and chloroplasts divide independently from the rest of the cell

---Mitochondria and chloroplasts have their own ribosomes, and these ribosomes resemble bacterial ribosomes

---Mitochondria and chloroplasts have two membranes with slightly different structures (the inner membrane is more similar to bacterial membranes)

---The few differences in the DNA code or "DNA language" found in living things are found in mitochondria and chloroplasts (this will make more sense later in the course).

---Mitochondria and chloroplasts are around the same size as bacteria.

Mitochondria most closely resemble modern day Rickettsia bacteria...like those that cause Rocky Mountain Spotted Fever and typhus.

Below is an image of Rickettsia invading a human cell. Interestingly, these modern bacteriahave to be engulfed by a host cell in order to survive...

Chloroplasts resemble modern day cyanobacteria...which are photosynthetic bacteria.

Below is a picture of colonies of cyanobacteria: