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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Section 22.14 Overlapping
StagesLECTURE
SLIDES
Prepared by
Aditya AiyerRaritan Valley Community
College
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Section 22.1 OverviewSection 22.1 OverviewOrigin of LifeStage 1*Reduced Atmosphere HypothesisExtraterrestrial HypothesisDeep Sea Vent HypothesisStage 2*Stage 3*Stage 4*
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Origin of LifeOrigin of Life
The universe created 13.7 billion years ago
Our solar system created 4.6 billion years ago
The Earth formed 4.55 billion years ago
50 million years - Earth cooled enough for outer layers
to solidify and oceans to form- Life begins to emerge
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Stage 1: Origin of Organic Stage 1: Origin of Organic MoleculesMolecules
There were specific conditions on early Earth may conducive to prebiotic formation of organic molecules
In prebiotic/abiotic synthesis, methane, carbon dioxide, water vapor, and hydrogen gases formed where no oxygen oxidized these gases or living organisms metabolized these gas
Scientists referred to this accumulation as a “prebiotic soup”
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Stage 1 cntdStage 1 cntdScientists formulated hypotheses
on where and how organic molecules first formed
Widely debated hypotheses include the Reducing Atmosphere, Extraterrestrial, and Deep Sea Vent Hypotheses
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Reducing atmosphere hypothesis◦Reducing atmosphere rich in water vapor, H2,
CH4, and NH3 gases ◦Ammonia and Methane gases reduced other
molecules◦Oxygen cannot oxidize gases and no living
organisms can metabolize molecules◦Formed organic monomers essential to
macromolecule formation: amino acids, sugars and nitrogenous bases
◦First attempt to apply scientific experiments to understand origin of life
◦Since 1950s, about early Earth atmosphere changed where prebiotic synthesis could be replicated under neutral conditions
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ElectrodesElectrical discharge
GasesH2OH2
CH4
NH3
To vacuumCold water
Condenser
TrapBoiling water
Sample containingorganic moleculessuch as amino acids
Precipitatingdroplets
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Extraterrestrial hypothesis◦ Asteroids and comets brought organic carbon
compounds to Earth Includes amino acids and nucleic acid nitrogen bases
◦ Some scientists argue that most of organic content would be disintegrated in intense heating and collision against atmosphere
Deep-sea vent hypothesis◦ Biologically important molecules may have been
formed in the temperature gradient between extremely hot vent water and cold ocean water
◦ Most tested and accepted among the three hypotheses
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(a) Deep-sea vent hypothesis
(b) A deep-sea vent community
Vent
Ocean floor
Cold H2O
H2O temperaturesuitable for organicchemistry
HotH2O
Hot H2S gas
Crack inEarth’s crust
b: © Woods Hole Oceanographic Institution/Visuals Unlimited
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Stage 2: Complex Organic Stage 2: Complex Organic Molecule SynthesisMolecule Synthesis
Experimentally, prebiotic synthesis of polymers is not possible in aqueous solutions◦Hydrolysis competes with
polymerizationCertain experiments have shown
formation of complex nucleic acids and proteins on a solid surface, particularly clay
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Stage 3: Formation of Stage 3: Formation of boundariesboundaries Protobiont
◦Collection of prebiotically synthesized organic macromolecules surrounded by a boundary, such as a lipid bilayer or membrane, that allowed it to have chemical properties different from that of its surrounding environment
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4 defining characteristics of protobionts- Inner contents of protobiont and
environment separated by a boundary like membrane
- Nucleic acid polymers like RNA inside protobionts store information
- Nucleic acid polymers like RNA had enzymatic functions within protobionts
- Nucleic polymers like RNA allow protobionts the capability of self-replication
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Living cells may have evolved from Living cells may have evolved from Two Different Protobiont StructuresTwo Different Protobiont StructuresCoacervates
◦ Clusters of charged polymers◦ Inner enzymes in coarcervates could
perform rudimentary metabolism Liposomes
◦ Vesicles surrounded by a lipid bilayer◦ Clay, or solid surface, can catalyze
formation of liposomes that grow and divide
◦ Liposomes can enclose RNA, as RNA is considered to be inner contents of protobionts
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(a) Coacervates
(b) Liposomes
57 µm
200 nm
Skin of water
Solid droplet of proteinand carbohydrate
Hollow sphere of phospholipidfilled with water
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a: © A. I. Oparin. From the Origin of Life, New York: Dover, 1952; b: © Mary Kraft
Phospholipidbilayer
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Stage 4: RNA was Inner Content Stage 4: RNA was Inner Content of Protobiontsof Protobionts
Most scientists believe RNA to be the first macromolecule contained in protobionts
RNA has 3 major functions- Ability to store information- Capacity for self-replication- Enzymatic function as ribozymes
DNA and proteins do not share all 3 of these important functions
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Chemical selectionChemical selection Original RNA population gradually
changes over time by mutations, ultimately producing a new mutant RNA population with a different chemical composition
One example comprises of 2 mutations- One mutation caused first mutant RNA
population to acquire enzymatic function to attach nucleotides, thereby increasing rate of replication
- Another mutation caused new mutant RNA population to acquire enzymatic function to create nucleotides, which does not require prebiotic synthesis of nucleotides
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