NICHOLLS BIOL 370 - Life’s Origin and History

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1Life’s Origin and HistoryThe age of the earth is estimated at about 4.5 billion yearsbased on: the age of meteorites, moon rocks, and the oldest rocks on earth (3.9 billion years old).The earliest sedimentary rocks are about 3.8 billion years old and these contain organic deposits.The first definite microfossils date from 3.5 to 3.6 billion yearsresemble modern stromatolites - mats of blue-green algaeIt is clear that life got started very early in Earth’s history.2How did life begin?Life - a highly organized chemical system that is able to maintain itself and reproduce.One hypothesis is that life came here from elsewhere - another l t ith i l t t id l t Thiplanet either in our solar system or outside our solar system. This is the “Panspermia Hypothesis.” • life came here unaided as a spore from another solar system (several thousand solar systems are within 100 light years)• life came here from another solar system aided by an intelligent life form (e.g. Voyager)•life came here from another planet in our solar system (e g•life came here from another planet in our solar system (e.g. Mars rocks)This hypothesis begs the question of how life got started elsewhere and, at this point, can’t be investigated scientifically Working hypothesis: Life originated from chemical building blocks on earth.If life organized itself according to physical laws it can be investigated scientifically. If it can be shown that one of the properties of life, or one of the basic building blocks of living systems, could not have become organized on its own under conditions that existed in the past, then we would have to reject the working hypothesis. It is clear that it would be difficult for amino acids, proteins, and nucleic acids to become organized today because of the high concentration of oxygen in the atmosphere and surface waters. Any high-energy compound will donate its electrons to any ready electron acceptor. Oxygen is a good electron acceptor. So, the environment of early earth must have been different than today. It must have not have had many oxidizing agents. The environment must have been a reducing environment.3Is there evidence that early earth had a reducing environment?Other planets in our solar system (Venus, Mars, Jupiter, Saturn, and Uranus) all have reducing atmospheres - rich in CH4, CO2, CO, H2S, NH32,3Sediments that were laid down prior to 2.5 bya contain reduced chemical compounds. After 2.2 bya the chemicals deposited in sediments tend to be oxidized.Reduced Iron (Ferrous Iron - Fe++) is soluble in waterOxidized Iron (Ferric Iron - Fe+++) is insoluble in waterThe“redbeds”or“banded iron formations”were created asThe red-beds or banded iron formations were created as soluble iron was oxidized and precipitated out of solution in the earth’s oceans.It appears that oxygen was rare in the earth’s atmosphere until about 2.2 bya.4Can complex chemicals form in a reducing atmosphere?1953 - Miller and Urey created an apparatus to address the question. Used atmosphere of CH4, NH3, H2, and H2O and used electrical discharges as a source of energy (simulated lightning).Later experiments showed that heat or UV radiation and other mixtures of gases lacking oxygen produced similar results.After a few days the mixture yielded some amino acids, HCN, H2CO (formaldehyde), and these subsequently reacted to produce sugars, purines, and pyrimidines.Current estimates of the amounts of methane and ammonia that were present in Earth’s early atmosphere suggest that pggover time oceans would have been rich in organic molecules with a concentration similar to weak chicken broth. That broth has been labeled the “prebiotic soup” or “primordial soup.”5Macromolecules are necessary for life. Could polymers (proteins, nucleic acids) of the building blocks have formed?This is a potential problem because polymers form through dehydration syntheses. In aqueous environments hydrolysis is lik lmore likely.Fox found that dry mixtures of amino acids spontaneously polymerize at 130°CAmino acid adenylates (amino acids charged with ATP) form random polymers spontaneously at 60°CHuber found if CO was added to the mixture there was aHuber found if CO was added to the mixture there was a preference for stable peptide bonds between amino acids.The primordial soup may have been dried on hot rocks or in evaporating pools to become a “primordial pizza” where spontaneous polymerization would have been favorable.CTP, GTP, TTP, UTP when placed together at 55°C spontaneously form polymers.However, the polymers are 5’→3’ and 5’→2’ in orientation.The addition of zinc results in only 5’→3’polymerizationThe addition of zinc results in only 5→3 polymerization.(Today DNA polymerase and RNA polymerase require zinc as a cofactor.)DNA and RNA are autocatalytic with or without polymerases when placed in mixtures of triphosphate nucleotides. Some RNA molecules are catalytic (ribozymes) and can promoteSome RNA molecules are catalytic (ribozymes) and can promote the formation of complimentary copies of themselves.So if nucleic acids were present they would spontaneously create complimentary copies of themselves.6Read: Experimental Evidence on the Origins of Natural SelectionSpiegelman and colleagues showed that tttbRNAa test tube RNA replication systems evolves in response to different selective regimes.A theoretical model for a self-replicating systemThe R3C ribozyme can catalyze the formation of copies of itself.7Life requires isolation from the surrounding environment for the maintenance of order. Can compartmentalized systems form spontaneously?Phospholipids spontaneously form lipid bilayers and micelles.pp p y p yLipid bilayers exhibit many properties of cell membranes - selective permeability and accumulation of some chemicalschemicals.Mixtures of polymers also form coacervates -polymer rich droplets that can be stable in solution.Various models suggest how chemical systems could have initially become isolated from the surrounding medium.Droplet systems have been created that can grow and split into new droplets.8Mutualism between replicating systems has been suggested as a mechanism for the evolution of the first cells.If one of the systems also causes the formation of membranes surrounding the mutualists then natural selection can act on their combined abilities to create more copies of themselves.The field of prebiotic chemistry is still young and there


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