UT AST 309L - Origin of Life (18 pages)

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Origin of Life



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Origin of Life

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Pages:
18
School:
University of Texas at Austin
Course:
Ast 309l - Search for Extraterrestrial Life
Search for Extraterrestrial Life Documents
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Origin of Life I Monomers to Polymers Synthesis of Monomers Life arose early on Earth within 0 7 109 y 1 Conditions 1 Liquid Water 2 Reducing or Neutral Atmosphere 3 Energy Sources 1 2 Originally thought atmosphere was NH3 CH4 H2O H2 Miller Urey Experiment Now Believe 3 CO2 H2O N2 Energy Sources Ultraviolet Light No Ozone Lightning Geothermal Lava Hot Springs Vents Miller Urey Experiment 2 COMPOUND Glycine Sarcosine Alanine N methylalanine Beta alanine Alpha amino n butyric acid Alpha aminoisobutyric acid Aspartic acid Glutamic acid Iminodiacetic acid Iminoacetic0propionic acid Lactic acid Formic acid Acetic acid Propionic acid Alpha hydroxybutyric acid Succinic acid Urea N methyl urea Relative Yield 270 21 145 4 64 21 0 4 2 2 66 6 133 1000 64 56 21 17 8 6 How did Amino Acids form in Miller Urey Experiment Strecker Synthesis CH4 H2 NH3 Energy e g Glycine Synthesis H2CO HCN HC3N Urea H2 NCONH2 Reactive H H H2CO NH3 HCN N H C C N H 2O H Aminoacetonitrile 3 H H N C H H N C N H2O H2O C C NH3 O H H O H H H glycine H H2CO C O form Aldehyde H More complex group other aldehydes more complex amino acids Lower yield if atmosphere was N2 CO2 H2O If H2 CO2 2 get good yield Problems with Miller Urey Atmosphere was N2 CO2 H2O NH3 CH4 would react N 2 CO2 Try N2 CO2 H2O in Miller Urey simulation Only get trace amounts of glycine Need CH4 to get more complex amino acids Need H2 CO2 2 to get much of any amino acid 4 Miller Urey with Cosmic Rays A group in Japan has obtained good yields of amino acids from slightly reducing gases CO2 CO N2 H2O When they used high energy protons simulate cosmic rays Apparently not Strecker Synthesis Low abundance of aminoacetonitrile Building Blocks of Nucleic Acids Not formed in Miller Urey But some intermediates were 1 Ribose Sugar 5 H2CO Heat H10C5O5 Clay Catalyst 2 Bases a Purines 5 HCN H5C5N5 Adenine b Pyrimidines HC3N Urea H5C4N3O Cytosine 1995 Cyanoacetaldehyde Urea Uracil 5 3 Phosphate Rock Erosion Less understood than amino acids Other Possibilities Seafloor Vents Interstellar Molecules Comets Alternative Delivery Molecular clouds strongly reducing contain many molecules used in Miller Urey H2 NH3 H2O CH4 and intermediates HCN H2CO HC3N and possibly glycine Problem These would not have survived in part of disk where Earth formed But interstellar ices comets Evidence from similar molecules e g C2H2 CH4 HNC Clearly indicates interstellar chemistry 6 Cratering record on moon fi heavy bombardment early in history Comets and their debris could have brought large amounts of organic matter to Earth and maybe oceans Some evidence for non biological amino acids in layer depostied after asteroid impact 65 million years ago Sources of Organic Molecules Quantitative comparison by Chyba Sagan Nature 1992 Vol 355 p 125 Currently Earth accretes 3 2 106 kg y 1 from interplanetary dust particles IDP 10 organic carbon fi 3 2 105 kg y 1 103 kg y 1 comets 10 kg y 1 meteorites 103 more at 4 5 109 yr ago cratering record UV reducing atmosphere 2 1011 kg y 1 But if H2 CO 0 1 IDP s dominant source 7 So if atmosphere very neutral IDP s Most of mass in IDP s in range of size 100 mm mass 10 5 g Complex structure composites of smaller grains some carbon rich Enhanced deuterium low T Also found in interstellar molecules fi connection back to interstellar chemistry 1 Interplanetary dust particles m 10 5 g 2 Smaller meteorites m 108 g Mass Accretion Rate on Earth 2 kinds mass ranges can supply organic matter 8 Alternative Sites Locally reducing environments 1 2 Ocean vents Sources of CH4 and H2S Current Vents have ecosystems based on energy from chemicals not photosynthesis H2S Bacteria Clams Tube Worms Pre biotic amino acid synthesis Inside Earth Many bacteria now known to live deep 2 miles in Earth Again on chemicals adapted to high temperature genetic makeup very ancient 3 Hot Springs Bacteria may be important in precipitating minerals again adapted to high T and ancient 9 Synthesis of Polymers M1 M 2 P H2O more likely in liquid H2O Solutions Remove H2O Drying Heat Sydney Fox Proteinoids Energy Releasing Reactions H2NCN or HC3N Catalysts Clays Monomers polymers A problem Peptide bond requires removal of H2O This would be hard in primordial sea Need special molecules to do what Ribosome does in living cells Input of energy or Dry environment dry land Imagine drying tidepool geothermal heat Heat amino acids peptide bonds Sidney Fox proteinoids or catalyst clay energy rich bonds 10 Problem greater for nucleic acids Sugar base heat some nucleosides Activated nucleosides phosphoric acid Zn 2 polymers up to 50 nucleotides linkages mostly correct Nucleic acids more complex nucleoside nucleotide Monomers of nucleic acids 11 Synthesis of Adenosine Base on 1 Carbon Adenine ribose sugar Also phosphates Why adenosine H 2O 3 5 carbons Otherwise Misalignment Leslie Orgel has had some success in getting high percentage of correct linkages in presence of Zinc ions 12 The Odds We need to get an interesting polymer Enzyme Self replicator Properties of polymer depend on Order in which monomers combine If we combine monomers at random How likely to get something interesting 13 Statistics of an unlikely event Random reactions in primordial soup Unlikely event versus many trials Probability Consider tossing 10 coins Probability of all heads product of prob 1 P 1 1 1 1 1 1 10 2 2 2 2 2 1024 2 Probability of getting 10 amino acids protein Chosen from 20 in a particular order 1 20 10 1 1 1013 But if you try many times the chance of success is higher P r n r n r r of successes pr 1 p n r p prob Of success on each trial n of trials n n n 1 n 2 1 e g make n 1 flip all 10 coins 1024 times p n 1 1 n P 1 1 1n 0 37 1 n 1 n Chance of one or more successes 0 63 For reasonable chance need n 1p 14 How many do we have to get right 1 How many atoms Lipids Enzymes RNA Bacterial DNA Bacterium Human Being 102 103 103 105 108 109 1011 1012 1027 1028 If we choose from H C N O probability of right choice 1 4 So for enzyme 41 103 10 600 ignore S P of trials Shapiro computes N 2 5 1051 surely an overestimate n 1 for simple enzyme p 2 What if we start with amino acids Need 1013 trials to get 10 amino acid protein To get 200 amino acids in right order 201 200 10 260 Hopeless Need something besides random combinations Selection Natural 15 Improving the Odds Many proteins composed of interchangeable segments Domains 10 250 amino acids One domain found in 70 different proteins Intermediate building blocks If so may only need to get


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