The Origin Of Life Dual Origin Hypothesis


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The Origin Of Life Dual Origin Hypothesis

  1. 1. The Origin of Life: A Dual Origin Hypothesis Heather E. Jordan Bryant Lab, 232 S. Frear The Pennsylvania State University University Park, PA 16802 MICRB 497-B 3/25/2004
  2. 2. Where did I come from? <ul><li>Cells? Our genes? What did we descend from? </li></ul><ul><li>Stardust </li></ul><ul><li>The first life form on Earth </li></ul><ul><ul><li>Progenote </li></ul></ul><ul><ul><li>Cenancestor(s) </li></ul></ul><ul><ul><li>LUCA </li></ul></ul><ul><li>Are we alone? </li></ul><ul><li>Nobody knows for sure (but there may be clues!) </li></ul>
  3. 3. Past Attempts to Answer the Question <ul><li>Pagan Folklore </li></ul>
  4. 4. Past Attempts to Answer the Question <ul><li>Pagan Folklore </li></ul><ul><li>Religion </li></ul>
  5. 5. Past Attempts to Answer the Question <ul><li>Pagan Folklore </li></ul><ul><li>Religion </li></ul><ul><li>Spontaneous Generation (Pasteur) </li></ul>Totora, G., Funke, B. and Case, C.. Microbiology: An Introduction . Pg 8. Redwood City: The Benjamin/Cummings Publishing Company. 1995.
  6. 6. Past Attempts to Answer the Question <ul><li>Pagan Folklore </li></ul><ul><li>Religion </li></ul><ul><li>Spontaneous Generation (Pasteur) </li></ul><ul><li>Directed Panspermia (Crick) </li></ul>
  7. 7. Vital Force or Vital Farce? <ul><li>Wöhler (1828) accidentally synthesized urea while trying to make Ammonium cyanate (evaporated instead of allowing crystalization @ room temp.) </li></ul><ul><li>Letter to Berzelius: &quot;I can no longer, so to speak, hold my chemical water and must tell you that I can make urea without needing a kidney, whether of man or dog; the ammonium salt of cyanic acid is urea &quot;. </li></ul><ul><li>AgCNO + NH 4 Cl  AgCl + NH 4 CNO </li></ul>(Minard, PSARC Presentation)
  8. 8. The Miller-Urey Experiment <ul><li>Original: </li></ul><ul><ul><li>H 2 </li></ul></ul><ul><ul><li>H 2 O </li></ul></ul><ul><ul><li>NH 3 </li></ul></ul><ul><ul><li>CH 4 </li></ul></ul><ul><li>Modified </li></ul><ul><li>Experiments: </li></ul><ul><ul><li>CO </li></ul></ul><ul><ul><li>CO 2 </li></ul></ul><ul><ul><li>CH 2 =CH 2 </li></ul></ul><ul><ul><li>HC = CH </li></ul></ul><ul><ul><li>N 2 </li></ul></ul><ul><ul><li>HCN </li></ul></ul> <ul><li>Energy Sources: </li></ul><ul><ul><li>Photochemical </li></ul></ul><ul><ul><li>Shock waves </li></ul></ul><ul><ul><li>Heat </li></ul></ul><ul><ul><li>Electrical </li></ul></ul><ul><li>Products: </li></ul><ul><ul><li>Amino Acids </li></ul></ul><ul><ul><li>Nitrogenous Bases </li></ul></ul><ul><ul><li>Aldehydes </li></ul></ul><ul><ul><li>Alcohols </li></ul></ul><ul><ul><li>Hydrocarbons </li></ul></ul><ul><ul><li>Amines </li></ul></ul><ul><ul><li>3-6 C Sugars </li></ul></ul><ul><ul><li>Esters </li></ul></ul><ul><ul><li>Carboxylic Acids </li></ul></ul><ul><ul><li>Amides </li></ul></ul><ul><ul><li>Ketones </li></ul></ul><ul><ul><li>Ethers </li></ul></ul><ul><ul><li>HCN, CO, CO2, H2O2, H2CO3, NH2CONHCONH2, etc. </li></ul></ul>
  9. 9. The Biochemistry of Titan (Maybe?) <ul><ul><li>Protection of Organic Polymers : </li></ul></ul><ul><ul><ul><li>Thick Atmosphere </li></ul></ul></ul><ul><ul><ul><ul><li>Saturn’s Cosmic Rays & e-s + Solar UV : N 2 + CH 4  free radicals </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Yield : Hydrocarbons, Acetylene, HCN (& polymers) </li></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Drifts down to the lunar surface </li></ul></ul></ul></ul></ul><ul><ul><ul><li>Extreme Cold (-178 o C) </li></ul></ul></ul><ul><ul><ul><ul><li>Liquid H 2 O could result transiently from: </li></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Volcanic Activity </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Impacts </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><li>Heat could also be provided for reactions this way </li></ul></ul></ul></ul><ul><ul><ul><ul><li>NH 3 is an antifreeze for H 2 O (pooled on surface) </li></ul></ul></ul></ul>(Lunine,
  10. 10. The Biochemistry of Titan (Maybe?) <ul><ul><li>Protection of Organic Polymers : </li></ul></ul>(Lunine, <ul><ul><ul><li>O from liquid H 2 O is </li></ul></ul></ul><ul><ul><ul><li>donated to hydrocarbon </li></ul></ul></ul><ul><ul><ul><li>chains on surface </li></ul></ul></ul><ul><ul><ul><li>Heat Dissipates </li></ul></ul></ul><ul><ul><ul><li>Organics are deep-frozen </li></ul></ul></ul><ul><ul><ul><li>& preserved </li></ul></ul></ul><ul><ul><ul><li>Additional shielding from </li></ul></ul></ul><ul><ul><ul><li>atmosphere </li></ul></ul></ul>
  11. 11. Strecker Synthesis <ul><li>The methane and nitrogen in the atmosphere reacted to form hydrogen & hydrogen cyanide </li></ul><ul><li> CH 4 + N 2  2 HCN + 3H 2 </li></ul><ul><li>The hydrogen cyanide in turn, reacted with the formaldehyde </li></ul><ul><ul><li>HCN + HCHO  H 2 N-CH 2 OH </li></ul></ul><ul><li>Other products resulted from this </li></ul><ul><li>H 2 N-CH 2 OH  HN=CH 2 + H 2 O </li></ul><ul><li>HN=CH 2 + HCN  H 2 NCH 2 CN </li></ul><ul><li>H 2 NCH 2 CN + H 2 O  H 2 NCH 2 COOH + NH 3 </li></ul><ul><li>HCN + NH 3  Adenine </li></ul><ul><ul><li>Successive wetting & freeze-drying under UV </li></ul></ul><ul><ul><li>Clusters of HCN formed 5-member ring </li></ul></ul>
  12. 12. Incredible HCN! <ul><li>Other purines & pyrimidines produced in smaller amounts </li></ul><ul><li>Strecker Synthesis also forms Gly </li></ul><ul><li>HCN spontaneously polymerizes </li></ul><ul><ul><li>xHCN HCN x </li></ul></ul><ul><ul><li>2 Forms : (Polymerization?) </li></ul></ul><ul><ul><ul><li>Orange (water soluble) </li></ul></ul></ul><ul><ul><ul><li>Black (water insoluble) </li></ul></ul></ul><ul><ul><li>Acid hydrolysis </li></ul></ul><ul><ul><ul><li>20%+ amino acids </li></ul></ul></ul><ul><ul><ul><li>Mostly Gly </li></ul></ul></ul><ul><ul><ul><li>Trace Ala, Asp, Glu, Ser, β-Ala & α-amino isobutyric acid. </li></ul></ul></ul><ul><ul><ul><li>Urea, Adenine & more! </li></ul></ul></ul>Base Catalyst (Minard, HCN Photos)
  13. 13. Great but, how else can they be protected from degradation? <ul><li>Preserved by absorption into minerals (meteorites!) </li></ul><ul><li>Amino Acids Diversity : </li></ul><ul><ul><li>Miller-Urey (30+ in original, 90+ in variations) </li></ul></ul><ul><ul><li>Murchison (69+) </li></ul></ul><ul><ul><li>HCN x (7) </li></ul></ul>
  14. 14. Biological Triad <ul><li>3 elements are required by life: </li></ul><ul><ul><li>Water </li></ul></ul><ul><ul><li>Energy </li></ul></ul><ul><ul><li>Atmosphere </li></ul></ul><ul><li>Which planets/moons in our solar system have these elements? </li></ul><ul><ul><li>Titan </li></ul></ul><ul><ul><ul><li>2/3 (very little water) </li></ul></ul></ul><ul><ul><li>Mars, Ganymede, Europa, Callisto </li></ul></ul><ul><ul><ul><li>Have complete triad </li></ul></ul></ul>
  15. 15. Biological Triad <ul><li>Ganymede </li></ul><ul><ul><li>Light Atmosphere : H, O 2 , CO 2 & traces </li></ul></ul><ul><ul><li>H corona </li></ul></ul><ul><ul><li>Embedded in Jupiter’s magnetosphere (intense radiation & charged particles) </li></ul></ul><ul><ul><li>Ozone at the poles (e-s travel along field lines & hit polar ice) </li></ul></ul><ul><li>Europa </li></ul><ul><ul><li>Light Atmosphere : H, O 2 , CO 2 & traces </li></ul></ul><ul><ul><li>Significant O 2 levels in atmosphere (HST emission measurements) </li></ul></ul><ul><ul><li>Embedded in Jupiter’s magnetosphere (intense radiation & charged particles) </li></ul></ul><ul><li>Callisto </li></ul><ul><ul><li>Light Atmosphere : H, O 2 , CO 2 & traces </li></ul></ul><ul><ul><li>H corona </li></ul></ul><ul><ul><li>Embedded in Jupiter’s magnetosphere (intense radiation & charged particles) </li></ul></ul>
  16. 16. Planetary Atmospheres <ul><li>Venus </li></ul><ul><ul><li>CO 2 96.0% </li></ul></ul><ul><ul><li>N 2 3.5% </li></ul></ul><ul><ul><li>H 2 O trace </li></ul></ul><ul><li>Mars </li></ul><ul><ul><li>CO 2 95.0% </li></ul></ul><ul><ul><li>N 2 3.0% </li></ul></ul><ul><ul><li>O 2 /H 2 O trace </li></ul></ul><ul><li>Earth Signature of Life? </li></ul><ul><ul><li>N 2 78.1% </li></ul></ul><ul><ul><li>O 2 21.0% </li></ul></ul><ul><ul><li>Ar 0.9% </li></ul></ul><ul><ul><li>H 2 O 0.1-3.0% </li></ul></ul><ul><ul><li>CO 2 0.03% </li></ul></ul>
  17. 17. The Parsimonious Conclusion <ul><li>Currently, most think that early Earth’s atmosphere was composed primarily of CO 2 & N 2 </li></ul><ul><li>&quot;This type of atmosphere is neutral for oxidation and reduction reactions and does not allow an easy and direct formation of long chains of organic molecules,“ ( Jonathan I. Lunine, University of Arizona planetary sciences Professor). </li></ul> *
  18. 18. The Parsimonious Conclusion <ul><li>Large moons of giant, Jovian planets are likely to have thick atmospheres </li></ul><ul><li>Proximity of moons to giants provides a source for various gasses </li></ul><ul><li>Simulations of KBO impacts demonstrate why Titan was the only large moon to retain a thick atmosphere in our solar system * </li></ul><ul><li>Escape Velocities: </li></ul><ul><ul><li>Titan-Saturn: 35.489 km/s </li></ul></ul><ul><ul><li>Ganymede-Jupiter: 2.74 km/s </li></ul></ul><ul><ul><li>Europa-Jupiter: 2.02 km/s </li></ul></ul><ul><ul><li>Callisto-Jupiter: 2.45 km/s </li></ul></ul>*
  19. 19. The Parsimonious Conclusion <ul><li>The “Miller-Urey” atmosphere did not exist on Earth (not enough CH 4 to make the organic precursors) </li></ul><ul><li>Earth was seeded with organics from space (asteroids & comets) </li></ul><ul><li>Must have been many small impacts of organic-rich material (and 1 really big one?) </li></ul><ul><li>What types of molecules could have fallen here? </li></ul>
  20. 20. The Origin of Organic Molecules <ul><li>Starting Materials: </li></ul><ul><ul><li>Methane </li></ul></ul><ul><ul><li>Nitrogen </li></ul></ul><ul><ul><li>Formaldehyde </li></ul></ul><ul><ul><li>Water </li></ul></ul>http:// <ul><li>From a moon or planet with a reducing atmosphere : A dead solar system (?) </li></ul><ul><li>Thick atmosphere of NH 3 , CH 4 & N 2 </li></ul><ul><li>Covered in tar and formaldehyde </li></ul>
  21. 21. * CRASH!!! * <ul><li>Frozen organic planet (Mars-sized?) crashes into Earth 4.5 bya </li></ul><ul><li>Earth crust melts on impact but much debris is scattered in space (some of which forms the moon) </li></ul><ul><li>Earth cools & crust re-forms </li></ul><ul><li>Atmosphere restored via geothermal processes </li></ul>
  22. 22. * CRASH!!! * <ul><li>Many asteroids fall to Earth, carrying organics from the invading body </li></ul><ul><li>Organics are concentrated at the surface but protected from degradation until erosion releases them from the rocks. </li></ul>
  23. 23. <ul><li>Any planets orbiting α-Centauri A or B; 4.35 ly away may have worked their way into the Oort cloud surrounding that system </li></ul><ul><ul><li>Both stars are ~ 5-6 billion years old </li></ul></ul><ul><li>All 3 stars of the triad are older than our sun and 1 of them (closest) is a brown dwarf (Proxima) </li></ul><ul><li>Proxima is spectral type M5 </li></ul><ul><li> </li></ul><ul><ul><li>This star has a MS lifespan of 2 x 10 12 years ( 1 st generation! ) </li></ul></ul><ul><ul><li>Stars in any given region of the galaxy tend to be about the same age so… where are Proxima’s solar siblings? </li></ul></ul><ul><ul><li>Is the material in our solar system (including pre-biotic organics) leftover from the event that created α-Centauri A & B? </li></ul></ul>
  24. 24. How plausible is this? <ul><li>Pluto, Quaoar (kwa-whar) & Sedna </li></ul><ul><ul><li>Sedna is 0.014 ly from sun </li></ul></ul><ul><ul><li>Oort cloud extends 3 ly from sun </li></ul></ul><ul><ul><ul><li>Oort clouds of other stars can perturb it. </li></ul></ul></ul><ul><ul><li>Captured from another solar system? </li></ul></ul>
  25. 25. Precious Cargo <ul><ul><li>Body exposed to a supernova shockwave  Propulsion! </li></ul></ul><ul><ul><li>May not have traveled far… </li></ul></ul><ul><ul><li>May have been in our own cosmic backyard! </li></ul></ul>
  26. 26. The Chirality Riddle <ul><li>More L-amino acids in Murchison & Murray meteorites </li></ul><ul><ul><li>Extraterrestrial cause </li></ul></ul><ul><li>Neutron star synchrotron radiation from supernova </li></ul><ul><ul><li>Circularly polarized radiation (Counterclockwise from the star's northern hemisphere and clockwise from the south  chiral radiation!) </li></ul></ul><ul><ul><li>Polarization too low </li></ul></ul>
  27. 27. The Chirality Riddle <ul><li>High circular polarizations in star-forming regions of reflection nebulae </li></ul><ul><ul><li>Orion OMC1 (a region in the Orion nebula M42) </li></ul></ul><ul><ul><li>NGC 6334. </li></ul></ul>
  28. 28. The Chirality Riddle <ul><li>High circular polarizations in star-forming regions of reflection nebulae </li></ul><ul><ul><li>Orion OMC1 (a region in the Orion nebula M42) </li></ul></ul><ul><ul><li>NGC 6334. </li></ul></ul><ul><li>Prediction : “Circular polarization should also be present at the UV wavelengths needed for asymmetric photolysis of molecules such as amino acids.” </li></ul><ul><li>Our own solar system may have formed in a region of high circular polarization </li></ul> Anglo-Australian Telescope IR Image Highest circular polarization in white
  29. 29. More stuff! <ul><li>Ammonia (imported) + water vapor (native)  hydroxyl acids </li></ul><ul><li>H 2 S gas (UV absorbing gas) + lightning  cysteine </li></ul><ul><li>Possibly over 90 other amino acids (imported) </li></ul><ul><li>Same processes that synthesized also degraded them </li></ul><ul><ul><li>Aldehydes & cyanides continued to react </li></ul></ul>
  30. 30. More organics form… <ul><li>Rocks which preserved imported organics (meteorites) eroded and released them into environment </li></ul><ul><li>Formaldehyde combined with organics to form sugars and amidinium carbodiimide </li></ul><ul><ul><li>Catalyzes formation of peptide bonds at 70 o C in dilute solutions </li></ul></ul>
  31. 31. High Tide <ul><li>Tides created by the gravitational pull of the moon </li></ul><ul><li>Cool water rushes in and suspends dried, polymerized organics </li></ul>
  32. 32. The Moon <ul><li>Washed organics onto rocks (mostly iron with some zinc, etc.) like pyrite </li></ul><ul><ul><li>Bound there & aligned by charge </li></ul></ul><ul><ul><li>Zn facilitated polymerization </li></ul></ul><ul><ul><li>Most fell apart but a few didn’t </li></ul></ul><ul><li>Baked under UV during the day </li></ul>
  33. 33. Sunscreen for Pre-biotic Molecules?! <ul><li>Evolution: </li></ul><ul><ul><li>5 classes of chlorophyll-protein complexes share a common ancestor </li></ul></ul><ul><ul><li>Derived from a large pigment-carrying protein with more than 10 transmembrane spans </li></ul></ul><ul><ul><li>Complex protects (cell?) against UV light </li></ul></ul><ul><ul><li>Dissipative chemistry  photosynthesis </li></ul></ul> Mulkidjanian, A.Y. & Junge, W. (1997) On the origin of photosynthesis as inferred from sequence analysis. Photosynthesis Research 51:27-42.
  34. 34. The UV-Protector Hypothesis <ul><li>Hypothesis: Monomers of reaction centers were pigment-carrying antenna proteins </li></ul><ul><li>Evidence: sequence alignments revealed clustered UV-absorbing residues </li></ul><ul><ul><ul><li>With each other </li></ul></ul></ul><ul><ul><ul><li>With chlorophyll & cofactor ligands </li></ul></ul></ul><ul><ul><li>Clusters most highly conserved in RC1 </li></ul></ul><ul><ul><ul><li>Most ancient </li></ul></ul></ul><ul><ul><ul><li>(Baymann et al., 2001) </li></ul></ul></ul>Baymann, F., Brugna M., Muhlenhoff, U. & Nitschke, W. (2001) Review: Daddy Where Did PSI Come From? Biochimica et Biophysica Acta 1507: 291-310.
  35. 35. Advantages of Clustering <ul><li>↓ Liklihood of photocleavage by: </li></ul><ul><li>↓ Lifetime of the excited state, which then: </li></ul><ul><li>↑ Photostability </li></ul><ul><li>Then: </li></ul><ul><ul><li>Excitation  pigments (rapid internal conversion to lowest excited singlet state) </li></ul></ul><ul><ul><li>Thermal energy release & dissipation </li></ul></ul>
  36. 36. UV Trapping <ul><li>3 layers </li></ul><ul><ul><li>Each α-helix helps pack residues with pigments </li></ul></ul><ul><ul><li>Increases absorption cross section of the membrane </li></ul></ul><ul><li>Arrangement helped dissipate energy </li></ul>
  37. 37. Does this make sense? <ul><li>Glycine + Acetate  Protoporphyrin IX </li></ul><ul><ul><li>Precursors were available to make this </li></ul></ul><ul><ul><li>Intermediate in heme biosythesis </li></ul></ul><ul><li>Elasticity of α-helices (where porphyrins are attached) </li></ul><ul><li>Chlorophyll favored due to lower energy singlet state production (in comparison) </li></ul><ul><ul><li>Natural selection  crucial component in photosynthesis later </li></ul></ul>
  38. 38. Lots of chemistry going on it’s all still inanimate… what’s missing ? <ul><li>Chemistry needs a container! </li></ul><ul><ul><li>Brings reagents together </li></ul></ul><ul><li>The ocean is too dilute... puddles are too risky… what the heck are all these bubbles? All those white flakes can’t be soap! </li></ul><ul><ul><li>Polymerization of washed up organics, dried on the shore are reclaimed by the tide & voila! </li></ul></ul>
  39. 39. The First “Container”? <ul><li>Spontaneously form proteinoid microspheres (electrostatic interactions) </li></ul><ul><ul><li>Able to take up molecules & have electrical potentials across “membranes” </li></ul></ul><ul><ul><li>Respond to changes in osmotic pressure </li></ul></ul><ul><li>High Tide = Dinner time!! </li></ul>
  40. 40. Microsphere Meals <ul><li>Phosphate also taken up (some already contained A & sugars)  AMP  ATP </li></ul><ul><li>Contained up to 250 amino acids </li></ul><ul><li>Wide variety of proteins & enzymes produced </li></ul><ul><ul><li>Proteolytic </li></ul></ul><ul><ul><li>Michaelis-Menten kinetics </li></ul></ul><ul><ul><li>pH optimum </li></ul></ul><ul><ul><li>ATPase activity </li></ul></ul>
  41. 41. Elusive Phosphorus <ul><li>Most P trapped in insoluble minerals </li></ul><ul><li>Heat from volcanic activity released it into oceans </li></ul><ul><li>Combined with amino acids  acyl phosphates (in vivo) + phosphoramidates (in vitro) </li></ul><ul><li>Phosphoric acid  more efficiently catalyzes formation of peptide bonds @ 70 o C </li></ul>
  42. 42. What a Little Phosphate Can Do <ul><li>Phosphates + Glycerol + Fatty Acids </li></ul><ul><li> Phospholipids </li></ul><ul><li>Clumped together </li></ul><ul><ul><li>Phospholipid Bilayers </li></ul></ul><ul><ul><li> Liposomes </li></ul></ul><ul><ul><ul><li>Acquired many different solutes while drying </li></ul></ul></ul><ul><li>Nucleotides! </li></ul>
  43. 43. Fatty Acids <ul><li>Many different chain lengths </li></ul><ul><li>14-C chain had special properties : </li></ul><ul><ul><li>No large proteins could pass </li></ul></ul><ul><ul><li>Small enzymes & nucleotides enter but can’t exit! </li></ul></ul>
  44. 44. Creation of Protocells <ul><li>Microspheres picked up everything … even liposomes </li></ul><ul><li>ATP + nucleotides  oligonucleotides (inside ingested liposome) </li></ul><ul><ul><li>Began to base pair with itself  RNA? </li></ul></ul><ul><ul><li>Ribose : 2 1 -5 1 linkage </li></ul></ul><ul><li>Liposomes + hollow proteins  “membrane” pores </li></ul><ul><ul><li>Now oligonucleotides can leave </li></ul></ul>
  45. 45. A Pre -RNA World? <ul><li>RNA is simpler than DNA but still complex </li></ul><ul><li>Possible RNA precursors: </li></ul><ul><ul><li>PNA </li></ul></ul><ul><ul><li>pNA </li></ul></ul><ul><ul><li>TNA </li></ul></ul><ul><ul><ul><li>Best candidate </li></ul></ul></ul>
  46. 46. A Pre -RNA World? <ul><li>PNA </li></ul><ul><ul><li>Peptide/Polyamide Nucleic Acid </li></ul></ul><ul><ul><li>L-arabinopyranosyl-(4 1  2 1 ) oligonucleotides </li></ul></ul><ul><ul><ul><li>Strongest base-pairing system of pentopyranosyl family </li></ul></ul></ul><ul><ul><li>An HCN Polymer </li></ul></ul><ul><ul><li>Hybridizes to cDNA, RNA * PNA oligomers </li></ul></ul><ul><ul><li>Can cause steric hindrance of RT, Telomerase & the Ribosome </li></ul></ul><ul><ul><li>Uncharged (neither repelled by – charged cell walls nor by high inside – membrane potential </li></ul></ul><ul><ul><li>Promising for anti-microbial drug development </li></ul></ul>(Dr. B. Minard, PSARC Presentation) Cherny, D.Y., et al. , 1993, Proc. Natl. Acad. Sci. USA 90 , 1667-70; Wittung, P. et al. , 1994, Nature , 368 , 561-63.
  47. 47. PNA: A self-complimentary Sequence!
  48. 48. A PNA World <ul><li>‘ T’s attached to aminoethylglycine backbone </li></ul><ul><li>Bind selectively to ‘A’s of oligos & double-stranded DNA </li></ul><ul><ul><li>Strand displacement: PNA  A-strand & T-strand (single)  excluded </li></ul></ul>(Dr. B. Minard, PSARC Presentation) Cherny, D.Y., et al. , 1992, Proc. Natl. Acad. Sci. USA 90 , 1667-70;
  49. 49. A PNA World <ul><li>‘ T’s attached to aminoethylglycine backbone </li></ul><ul><li>Bind selectively to ‘A’s of oligos & double-stranded DNA </li></ul><ul><ul><li>Strand displacement: PNA  A-strand & T-strand (single)  excluded </li></ul></ul><ul><li>Binding to closed, circular DNA  unwinding of double-helix (1 turn/10bp) </li></ul><ul><li>DNA.PNA complex: </li></ul><ul><ul><li>Forms @ low [salt] </li></ul></ul><ul><ul><li>Kinetically stable & cannot be dissociated by ↑ [salt] up to 500 nM. </li></ul></ul>Arrow = unwound DNA by PNA Cherny, D.Y., et al. , 1992, Proc. Natl. Acad. Sci. USA 90 , 1667-70; ↑ ↓
  50. 50. A T NA World? <ul><li>(L)- α –threofuranosyl-(3 1  2 1 ) oligonucleotide </li></ul><ul><ul><li>Threose is the sugar </li></ul></ul><ul><li>Simplest nucleic acid alternative </li></ul><ul><ul><li>Possible ancestor of RNA </li></ul></ul><ul><ul><li>Possible protector/regulator of RNA (binds to it) </li></ul></ul><ul><li>Forms base pairs </li></ul><ul><ul><li>G = C & T/U=A </li></ul></ul><ul><ul><li>Informational in anti-parallel </li></ul></ul><ul><ul><li>Cross-pairs with RNA & DNA </li></ul></ul><ul><li>A & T nucleobase analogs: </li></ul><ul><ul><li>2’-amino-(2’-NH2 TNA) </li></ul></ul><ul><ul><li>3’-amino-(3’-NH2 TNA) </li></ul></ul><ul><li>Bst PolI, bacteriophage T7 DNA Pol (exo-) & MMLV-RT </li></ul>Chaput, J.C., Ichida, J.K & Szostak, J.W. (2002) DNA Polymerase-Mediated DNA Synthesis on a TNA Template . J. AM. CHEM. SOC. 125, 856-857.
  51. 51. A T NA World? <ul><li>Easily forms hairpins </li></ul><ul><li>much more stable to hydrolytic cleavage than are RNAs and may be as stable as DNAs </li></ul><ul><li>TNA strands can be synthesized by template-controlled ligation with either complementary TNA or RNA strands as templates </li></ul><ul><li>corresponding formation of RNA sequences by ligation on a TNA template does also occur, although with less efficiency </li></ul>
  52. 52. RNA Replication <ul><li>Sequence that allowed it to copy itself </li></ul><ul><li>Not advantageous in itself unless– the sequence copied has some advantageous parts in it. </li></ul><ul><li>More protocells with L-amino acids than R so more D-sugars were used </li></ul>
  53. 53. The Protonucleus <ul><li>Did not originally divide </li></ul><ul><ul><li>Streamed through pores, filled up cell & inhibited metabolism </li></ul></ul><ul><ul><li>Clogged pores  breakdown of liposome/protonucleus </li></ul></ul>
  54. 54. The Protonucleus <ul><li>Ribozymes </li></ul><ul><ul><li>Excised fragments of RNA (introns?) </li></ul></ul><ul><ul><li>Crowded protonucleus burst </li></ul></ul><ul><ul><ul><li>2 tangles of folded RNA (replicated) </li></ul></ul></ul><ul><ul><ul><li>Intron scraps </li></ul></ul></ul> U1A SPLICEOSOMAL PROTEIN/HEPATITIS DELTA VIRUS GENOMIC RIBOZYME COMPLEX
  55. 55. Protonuclear Division <ul><li>Liposome phospholipids attracted to each other but pushed apart by tangles of RNA </li></ul><ul><ul><li>Reform encircling RNA (some mistakes, i.e., picking up introns or nothing) </li></ul></ul><ul><ul><li>RNA folding ensured that everything stuck together (daughter cell received 1 copy) </li></ul></ul>
  56. 56. Primitive Protocell Metabolism <ul><li>Precursors needed to maintain “membranes” </li></ul><ul><ul><li>Proteins, lipids & carbohydrates </li></ul></ul><ul><ul><li>First chemoorganotroph (popular; simple metabolism) </li></ul></ul><ul><ul><li>Protocells died when starved, became toxic, got too big or in wrong environment </li></ul></ul><ul><ul><li>Some grew faster than others, made products that facilitated growth, etc. </li></ul></ul>
  57. 57. Fortunate mistakes <ul><li>RNA has a high error rate </li></ul><ul><li>Enough nucleic acid precursors? </li></ul><ul><ul><li>Multi-protonuclei </li></ul></ul><ul><ul><li>Less nutrients coming in & available (volume increased faster than surface area) </li></ul></ul><ul><ul><li>Few cells acquired a mutation that provided the protocell with a cytoskeleton </li></ul></ul><ul><ul><ul><li>Contracted along equatorial plane in response to diminished nutrient levels </li></ul></ul></ul><ul><ul><ul><li>Cleaved cell & split up protonuclei </li></ul></ul></ul>$1349
  58. 58. So… what is it? <ul><li>Self-replicating, Gram negative chemoorganotroph with ability to pass on genetic information to progeny </li></ul><ul><li>Competition: </li></ul><ul><ul><li>Food </li></ul></ul><ul><ul><li>Advantageous mutations  Zn-containing polymerases (from washing on rocks & still used today) </li></ul></ul>
  59. 59. A Cenancestor? <ul><li>Retention of favorable mutations required higher fidelity </li></ul><ul><ul><li>DNA (with deoxyribose, now have 5 1 -3 1 linkage) </li></ul></ul><ul><ul><li>But RNA not completely out of the picture (just in different niches as it diversified) </li></ul></ul><ul><ul><li>Ribosomes from protonucleus? </li></ul></ul><ul><ul><li>Viruses still have the protein coats (lost genes?) </li></ul></ul><ul><ul><ul><li>Genome shortened as it took up residence in larger protocells  faster replication top priority </li></ul></ul></ul>
  60. 60. But… there is one other possibility! <ul><li>The first life form may have been photosynthetic! You’re kidding, right? </li></ul>
  61. 61. What is agreed upon <ul><li>Anaerobic environment </li></ul><ul><li>1 st photosynthesizers used H 2 or H 2 S as substrates </li></ul><ul><li>Microbes still do this (H 2 S  H 2 + S) </li></ul><ul><ul><li>Purple & green sulfur bacteria </li></ul></ul><ul><ul><li>PSI has 11 transmembrane domains </li></ul></ul>
  62. 62. UV-Protector  Energy Producer <ul><li>Mutations that prevented chlorophyll binding </li></ul><ul><ul><li>Cavity exposed charges of ligands </li></ul></ul><ul><ul><li>Attracted cofactors (FeS complexes/quinones) </li></ul></ul><ul><ul><ul><li>Re-stabilized the polypeptide </li></ul></ul></ul><ul><ul><ul><li>↑ UV absorbance & dissipation </li></ul></ul></ul><ul><li>Mutations : </li></ul><ul><ul><li>Gene fission </li></ul></ul><ul><ul><li>Loss of α-helical domains (1 o e- binding site) </li></ul></ul>
  63. 63. Exhibit A: Careful! It’s 3.5 billion years old! <ul><li>3.5 billion year-old cyanobacteria-like fossils </li></ul><ul><ul><li>Already photosynthesizing </li></ul></ul><ul><ul><li>Pigment-carrying antenna protein may have evolved concurrently with life </li></ul></ul><ul><ul><li>200 million-year window from the solidification of Earth’s crust & this fossil! </li></ul></ul><ul><ul><ul><li>Much of this time, too warm for the right chemistry so… </li></ul></ul></ul>
  64. 65. Acknowledgements <ul><li>Dr. Bob Minard </li></ul>Dr. Greg Ferry Dr. John Golbeck Sabrina Zimmerman
  65. 66. ??Questions??