Download Section 22.1 4 Overlapping Stages LECTURE SLIDES Prepared by

Section 22.1
4 Overlapping
Stages
LECTURE
SLIDES
Prepared by
Aditya Aiyer
Raritan Valley Community
College
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Section 22.1 Overview
Origin of Life
 Stage 1*
 Reduced Atmosphere Hypothesis
 Extraterrestrial Hypothesis
 Deep Sea Vent Hypothesis
 Stage 2*
 Stage 3*
 Stage 4*

2
Origin 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

3
Stage 1: Origin of Organic
Molecules
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”

4
Stage 1 cntd
Scientists 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
6
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Electrical discharge
To vacuum
Electrodes
Gases
H2O
H2
CH4
NH3
Cold water
Condenser
Precipitating
droplets
Boiling water
Trap
Sample containing
organic molecules
such as amino acids
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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
8
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Hot H2S gas
Cold H2O
Hot
H2O temperature H2O
suitable for organic
chemistry
Vent
Ocean floor
Crack in
Earth’s crust
(a) Deep-sea vent hypothesis
(b) A deep-sea vent community
b: © Woods Hole Oceanographic Institution/Visuals Unlimited
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Stage 2: Complex Organic Molecule
Synthesis

Experimentally, prebiotic synthesis of
polymers is not possible in aqueous
solutions
◦ Hydrolysis competes with polymerization

Certain experiments have shown
formation of complex nucleic acids and
proteins on a solid surface, particularly
clay
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Stage 3: Formation of boundaries

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 Two
Different Protobiont Structures

Coacervates
◦ 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
13
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Skin of water
(a) Coacervates
57 µm
Solid droplet of protein
and carbohydrate
Hollow sphere
of phospholipid
filled with water
(b) Liposomes
Phospholipid
bilayer
200 nm
a: © A. I. Oparin. From the Origin of Life, New York: Dover, 1952; b: © Mary Kraft
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Stage 4: RNA was Inner Content of
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 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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