3rd experimental approach of Origins of Life research  
        
   III) study of the Origins of Self-Replicating Chemical Systems...
              a key characteristic of life is self-replication...   Can molecules self-replicate???

     A) Evolution of an RNA WORLD*         (which came 1st DNA or RNA)
                   in 1989 Sidney Altman and Thomas Cech - received Nobel Prize
                  for demonstrating that RNA molecules may have Catalytic Activity  (RIBOZYMES*)
                  i.e., these RNA's catalyze hydrolysis & condensation rxs of phosphodiester bonds.

               If RNA can catalyze polymerization of like molecules maybe it can be a template also,
               akin to DNA --> figure*   i.e., replicate itself, then RNA molecules may have been the
                                                                                    1st SELF-REPLICATING living entity
.

        No self-replicating RNA molecules exists naturally today, but lab experimentation may
                  establish that it was feasible, and that RNA molecules can be selected for via
                  Darwinian evolutionary mechanisms (natural selection).
        In fact, a self-replicating spontaneously forming RNA may be a real statistical improbability.
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   Main criticisms of an RNA World...

       1. how did the 1st self-replicating RNA arise (actually an improbable chemical event for)?
            each nucleotide is a very complex molecule with 10-C's, 2to5-N's, 4or5-O's, & 1 P.
            spark discharge experiments (Miller 1953) DO NOT form nucleotides
            meteorites DO NOT contain great amounts of nucleotides
            some labs have made nucleotides, but only by elaborate & complex "prebiotic" chemistry
       &   even if random assembly of nucleotides was possible, it is likely that many might be
                       defective nucleotides & would prevent further replication-like activity.
So...
   B) a Metabolism First model - a thermodynamic replicator- for origins of life???
             
     a world based upon thermodynamic definitions of life rather than genetic

            it is likely that early forms of metabolism predated genetics. Metabolism here means a
            cycle of chemical reactions that produce energy in a pattern that can be harnessed by
            other processes. The idea is that once a primitive metabolic cycle was established, it began
            to produce ever more complex compounds, and possibly evolved replication ability.
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 5 Requirements for a Thermodynamic (Metabolism first) life system:
         
   1.  a boundary to separate life from non-life:       [compartmentation]
           life requires organization (a localized region where order can form and be maintained).
           2nd law of thermodynamics requires universe to move in a direction where disorder goes up.
           today's cells have membranes, but 1st system's may have had iron-sulfide surfaces*, i.e.,
           electrostatic minerals that segregated molecules from their environment
                (such as... iron pyrites at hydrothermal vents or lipid droplets on mineral surfaces).  

      A role for the hydrothermal vents in ?...   Origin of Metabolism
           Gunter Wachterschauser (German biochemist) has speculated that origins of
           bioorganic chemical reactivity may have originated near hydrothermal vents.
                  > earliest primordial biochemical cycles may have functioned there...
                   > metabolic cycles likely predate cells...
                      took place on mineral surfaces that acted as catalysts & surfaces for early reactions
       
  key reaction may have been a reductive reaction CO + oxidation vent minerals --> redox rx. ns
      
     in 1997 Wächtershäuser used CO, H2S, NiS, & FeS to make amino acids & peptides.
                                                 adding acetic acids  ---> pyruvate + NH3 ---> amino acids.
   next panel        Waechtershaeuser believes life emerged on a flat area where mineral surfaces
                                                  could spark chemical processes leading to living cells.

 

 

 
      Researchers including M.J. Russell, M.M. Hanczyc, D. Deamer, C. Huber, and G. Cody

      have synthesized amino acids and assembled/disassembled polypeptides under vent like conditions,
             thus reductive reactions of common metabolism (glycolysis & Krebs cycle) could have
             formed outside of cell membranes and eventually become encapsulated.
 

   2.  an energy source is needed to drive organization:      [an external energy supply - redox rx]
             redox reaction transfer electrons from energy rich molecules (reduced - H2S)
             to energy poor molecules (oxidized - CO).

   3.  a coupling mechanism to link energy release to organization processes (life):     
                                                                                       [driver reactions - coupled reactions]

   4.  a chemical network that is formed which permits adaptation and evolution:
                                                                               [metabolic reactions - compartment cycles]
             MetalH2 + BO -->  BH + CO --> CH + DO --> DH + MO --> MetalH2                 a cycle*
                    
metabolite D converts M back into the reduced mineral; a reaction sequence
                     that would be favored, for it replenished MH2 with a useful discharge of energy
 
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   5.  a chemical network that must grow & reproduce                    [mechanism of reproduction]
            metabolic pathway must gain material faster than it loses it; some diffusion out of
            compartmentalized cycle is entropically favored, but if the encapsulation is by a
            lipid membrane, we might envision "fission" after the encapsulated material grows in mass.

        Metabolism First systems are changing the Paradigm*:
                          these life systems are metabolically based, not genetically based,
                          and there is no information-like molecule that must be duplicated.

        Once formed replicating metabolic cycles could be favored by molecular Natural Selection:
             
        Carl Sagan: "Life is a localized region that increase in ORDER (decrease in Entropy)
                            through metabolic cycles driven by energy flow
."

 

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