Endosymbiotic Theory Short Lecture

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endosymbiotic theory for introductory college biology class

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Endosymbiotic Theory Short Lecture

  1. 1. The Endosymbiotic Theory and the Origin of Eukaryotic Cells BIO 101: General Biology I
  2. 2. Earth’s Earliest Inhabitants <ul><li>Earliest direct evidence of life dates from 3.5 billion years ago (BYA) </li></ul><ul><li>Prokaryotic organisms were the sole inhabitants of Earth until around 2.1 BYA </li></ul><ul><li>These anaerobic prokaryotic organisms transformed life on Earth </li></ul><ul><ul><li>Helped create hospitable environment for other types of organisms </li></ul></ul>
  3. 3. Oxygen Revolution <ul><li>Most atmospheric oxygen (O 2 ) comes from a biological source </li></ul><ul><li>Levels of atmospheric O 2 increased slowly from 2.7 to 2.3 BYA, then shot up quickly </li></ul><ul><ul><li>Ancestors of modern cyanobacteria (O 2 -releasing, photosynthetic bacteria) contributed to this O 2 increase </li></ul></ul>
  4. 4. The Changing Atmosphere and the Endosymbiotic Theory <ul><li>Some anaerobic prokaryotes adapted to the new environment </li></ul><ul><ul><li>Cellular respiration is a process in which oxygen is used to harvest energy from organic molecules </li></ul></ul><ul><li>How did aerobic prokaryotic cells living in this changed environment lead to eukaryotic cells? </li></ul><ul><li>The Endosymbiotic Theory </li></ul><ul><ul><li>Endosymbiont is a cell that lives within another cell, referred to as a host cell </li></ul></ul>
  5. 5. Development of Endosymbiosis <ul><li>Aerobic heterotrophic prokaryotes were engulfed by other cells </li></ul><ul><li>Provide oxygen utilization to an anaerobic host cell enabling it to survive in the more oxygen rich environment </li></ul><ul><li>Mitochondria - the site of cellular respiration </li></ul>
  6. 6. Development of Endosymbiosis <ul><li>Evidence supporting the Endosymbiotic Theory: </li></ul><ul><li>Enzymes and transport systems are homologous to those in living prokaryotes </li></ul><ul><li>Replicate by binary fission </li></ul><ul><li>Contain own DNA and ribosomes </li></ul>
  7. 7. Modern Model of Endosymbiosis & Eukaryotic Cell Origin <ul><li>Crithidia deanei – protozoan parasite of insects </li></ul><ul><ul><li>Harbors obligate intracellular bacterium, an endosymbiont </li></ul></ul><ul><li>Endosymbiont contains it own DNA but cannot survive and replicate outside of host protozoa </li></ul><ul><li>Endosymbiont is replicated and passed to daughter cells </li></ul>
  8. 8. Endosymbiont Passed to Daughter Cells <ul><li>Image legend: N=nucleus; S=symbiont </li></ul><ul><li>Model for studying eukaryotic cell evolution </li></ul><ul><li>Understanding could tell us something about how mitochondria and chloroplasts came to be eukaryotic organelles. </li></ul><ul><li>Motta, et al . PLoS ONE. 2010; 5(8) </li></ul>
  9. 9. References <ul><li>Reece, J., et al. Campbell’s Biology, 9 th ed. San Francisco: Pearson Benjamin Cummings; 2011. </li></ul><ul><ul><li>Source of all figures unless otherwise cited </li></ul></ul><ul><li>Motta MCM, Catta-Preta CMC, Schenkman S, Martins ACdA, Miranda K, et al . The bacterium endosymbiont of Crithidia deanei undergoes coordinated division with the host cell nucleus. PLoS ONE. 2010; 5(8):e12415. doi:10.1371/journal.pone.0012415. Accessed March 26, 2011. </li></ul>

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