lecture_13 - Chemical Evolution Theory of Lifes Origins 1...

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Chemical Evolution Theory of Life’s Origins 1. the synthesis and accumulation of small organic molecules, or monomers, such as amino acids and nucleotides. Production of glycine (an amino acid) 3 HCN + 2 H 2 O energy -→ C 2 H 5 O 2 N + CN 2 H 2 . Production of adenine (a base): 5 HCN C 5 H 5 N 5 , Production of ribose (a sugar): 5 H 2 CO C 5 H 10 O 5 . 2. the joining of these monomers into polymers, including proteins and nucleic acids. Bernal showed that clay-like materials could serve as sites for polymerization. 3. the concentration of these molecules into droplets, called protobionts, that had chemical characteristics different from their surroundings. This relies heavily on the formation of a semi-permeable membrane, one that allows only certain materials to flow one way or the other through it. Droplet formation requires a liquid with a large surface tension, such as water. Membrane formation naturally occurs if phospholipids are present. 4. The origin of heredity, or a means of relatively error-free reproduction. It is widely, but not universally, believed that RNA-like molecules were the first self-replicators — the RNA world hypothesis. They may have been preceded by inorganic self-replicators. Lattimer, AST 248, Lecture 13 – p.1/14
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Abiogenesis Self-organization leads to more complex structure Big Bang atoms stars galaxies Crucial questions which did not have experimental answers up until now, but new evidence has become evident: Synthesis of nucleotides Polymerization of nucleotides Incorporation of a self-copying gene into single cells upon which natural selection could act “Gene-first” mechanism “Metabolism-first” mechanism Primitive metabolism provides environment for later emergence of RNA replication. Example: Wächtershäuser’s iron-sulfur world theory, De Duve thioester theory. But can’t explain the high specificity of chemical reactions. Thermosynthesis world, involving thermal cycling, suggests an ATP-like enzyme that promotes peptide bonds: the “First Protein”. Origin of homochirality Genesis of the protein translation mechanism Pieces are now coming together to support plausibility of spontaneous generation Minimal number of genes seems to be about 206 in theory, in experiment there seem to be 387 essential genes Evidence suggests that this complexity has evolved, step by step, from very simple beginnings Lattimer, AST 248, Lecture 13 – p.2/14
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Monomer Production: Step 1 is possible in the early Earth’s atmosphere if it was was highly reducing as opposed to oxidizing ( cf. Miller & Urey experiment). Later research cast doubt on the existence of a reducing atmosphere and pointed to a neutral atmosphere dominated by CO 2 . More recent evidence is that H escaped very slowly on early Earth and its abundance wasn’t negligible after all.
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