1. Online Counseling Resource YCMOU ELearning Drive… School of Architecture, Science and Technology Yashwantrao Chavan Maharashtra Open University, Nashik – 422222, India

2. SBT/SBI/SGS011-CP2-03 Introduction Programmes and Courses  SEP – SBT011 – CP2-U2  SEP – SGS011– CP2-U2

3. School of Science and Technology, Online Counseling Resource… Credits  Academic Inputs by Sonali Alkari Faculty YCMOU Nagpur Centre, Faculty LAD college P.G. D of Biotechnology Research officer Ankur Seeds Pvt Ltd sonalisa_alkari@yahoo.co.in Sonalisaal@rediffmail.com

4. School of Science and Technology, Online Counseling Resource… © 2007, YCMOU. All Rights Reserved.4 How to Use This Resource  Counselor at each study center should use this presentation to deliver lecture of 40-60 minutes during Face-To-Face counseling.  Discussion about students difficulties or tutorial with assignments should follow the lecture for about 40-60 minutes.  Handouts (with 6 slides on each A4 size page) of this presentation should be provided to each student.  Each student should discuss on the discussion forum all the terms which could not be understood. This will improve his writing skills and enhance knowledge level about topics, which shall be immensely useful for end exam.  Appear several times, for all the Self-Tests, available for this course.  Student can use handouts for last minutes preparation just before end exam.

5. School of Science and Technology, Online Counseling Resource… © 2007, YCMOU. All Rights Reserved.5 Learning Objectives After studying this module, you should be able to: Describe postulates of Endosymbiotic hypothesis. Discuss how endosymbiosis happened Explain evidence in support of endosymbiotic hypothesis

6. School of Science and Technology, Online Counseling Resource… Introduction: 1  The theory was first proposed by Andreas Schimper in 1883, but was neglected until the discovery of mitochondrial DNA in the 1960s.  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.  The Endosymbiotic theory states that the cellular organelles mitochondria and chloroplasts (more generally, plastids) of eukaryotic cells were originally free living prokaryotic organisms.

7. School of Science and Technology, Online Counseling Resource… Introduction: 2  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]

8. School of Science and Technology, Online Counseling Resource… Endosymbiotic Hypothesis :1 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).

9. School of Science and Technology, Online Counseling Resource… Endosymbiotic Hypothesis:2  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.

10. School of Science and Technology, Online Counseling Resource… Endosymbiotic Hypothesis:3  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.

11. School of Science and Technology, Online Counseling Resource… Evidence for Endosymbiotic Theory  The endosymbiotic hypothesis might be called a theory, but experimental evidence can't be provided to test it.  Only circumstantial evidence is available in support of the proposal, which is the most likely explanation for the origin of mitochondria.  Evidences like the timeline of life on Earth, Photosynthetic bacteria, Aerobic support this theory.  The evidence needed to change the model from hypothesis to theory is likely forever lost in antiquity.

12. School of Science and Technology, Online Counseling Resource… How It Happened:1  A common ability of eukaryotic cells is "phagocytosis", the ability to eat things by pulling them in, engulfing them, and letting their cell membranes become bubbles around them ("food vacuoles").  The contents are then digested by having digestive-enzyme-containing lysosomes fuse with them.  After the digestion is finished and desired molecules absorbed, this bubble can be pushed toward the cell membrane and its contents released ("exocytosis").

13. School of Science and Technology, Online Counseling Resource… How It Happened:2

14. School of Science and Technology, Online Counseling Resource… How It Happened?:3  Many protozoans (animal-like protists) eat by phagocytosis, and various animal immune- system cells, called hemocytes in invertebrates and phagocytes in vertebrates, also do this.  But consider the fate of a victim of phagocytosis that escapes being digested.  It can live inside its "eater" and even proliferate, as the likes of Buchnera and Rickettsia do.  And it is only a small step from there to a closer relationship.

15. School of Science and Technology, Online Counseling Resource… How It Happened?:4  Origin by phagocytosis also explains the double membranes of mitochondria and many chloroplasts;  The inner membrane is the organelle's original membrane, while the outer membrane is the original food-vacuole membrane.

16. School of Science and Technology, Online Counseling Resource… Genetic and Metabolic Comparisons:1  An apparent difficulty with the endosymbiosis hypothesis is that several mitochondrial and chloroplast proteins are made using nuclear genes and then shipped to those respective organelles.  At first sight, this would imply that these organelles could never have been free-living.  However, there is a solution: the organelles have lost genes to the nuclei.  Gene loss also explains the variable sizes of these organelles' genomes; it has gone further in some organisms than in others.

17. School of Science and Technology, Online Counseling Resource… Genetic and Metabolic Comparisons:2  For example, the jakobid flagellate protist Reclinomonas americana has 69,034 base pairs in its mitochondrial genome, while human mitochondrial genome has around 16,568 bp.  Gene loss is also known to have happened in other endosymbiotic organisms, like Buchnera aphidicola, a symbiont of aphids that supplies "essential" amino acids and other nutrients to their hosts, and Rickettsia prowazekii, which causes the disease typhus.

18. School of Science and Technology, Online Counseling Resource… Vestigiality:1  The genomic apparatus and multiple membranes of mitochondria and chloroplasts are reasonably interpreted as vestigial features.  They have only limited function and a structure that suggests an origin from more- highly-developed features.  And these organelles themselves can become vestigial.  An interesting example is hydrogenosomes, structures in certain mitochondrion-less ("amitochondriate") protists that release hydrogen.

19. School of Science and Technology, Online Counseling Resource… Vestigiality:2  These organelles have no internal genomes, but they have proteins which are recognizably related to mitochondrial ones.  This suggests that they are mitochondria that have lost their genomes, and thus have only a broken respiratory chain that combines electrons directly with hydrogen ions, instead of extracting energy from them and combining them with oxygen.

20. School of Science and Technology, Online Counseling Resource… What You Learn … You have learnt : Similarities between prokaryotes and organelles, together with their appearance in the fossil record, could best be explained by "endo-symbiosis”. The Endosymbiotic theory states that the cellular organelles mitochondria and chloroplasts (more generally, plastids) of eukaryotic cells were originally free living prokaryotic organisms. The organelles have lost genes to the nuclei.

21. School of Science and Technology, Online Counseling Resource… Critical Thinking Questions 1.State the role of mitochondria and chloroplasts in eukaryotic evolution. 2.Explain Endosymbiotic theory with evidences. © 2007, YCMOU. All Rights Reserved.21

22. School of Science and Technology, Online Counseling Resource… Hints For Critical Thinking Question 1.mutually beneficial relationship for both cells, phagocytosis, endosymobiosis. 2.Theory postulates, evidences: circumstantial evidence, Genetic and Metabolic Comparisons, Ve stigiality © 2007, YCMOU. All Rights Reserved.22

23. School of Science and Technology, Online Counseling Resource… Study Tips  Book1 Title:The Living World Author: George Johnson  Book2 Title: ABC Of Biology Publisher: Holy Faith  Book3 Title: Biological Science Author: Taylor, Green & Stout

24. School of Science and Technology, Online Counseling Resource… Study Tips www.en.wikipedia.org Microsoft Encarta Encyclopedia http://en.wikipedia.org/wiki/ Wikipedia the free encyclopedia

25. School of Science and Technology, Online Counseling Resource… End of the Presentation Thank You !