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Section 22.14 Overlapping

StagesLECTURE

SLIDES

Prepared by

Aditya AiyerRaritan Valley Community

College

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

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

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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