Bio 211 Intro Molecular and Cell Biology
Lecture 9 “Chemical Evolution”
Reading: Campbell Chap. 26 pp. 490-498
In today’s lecture, we will take a brief look at the evolution of single cells. What’s
a possible scenario for how life arose? This will be sort of a retroactive view, based on
what we know about the molecules in cells; how might they have gotten together to
generate “life”.
Outline:
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Chemical evolution
Synthesis of the building blocks of life
RNA was the first genetic material
Scenario for the origin of life
One of the most important theories that underscores biology is the theory of
evolution. All life evolved from pre-existing life forms. Nature selects among those
variants that already exist for the forms best adapted to their particular environments.
Many of the ideas that lead to our current view of evolution originated from Charles
Darwin and his contemporaries.
1. Chemical evolution
Sometime between 4 billion years ago and 3.5 billion years ago, the first
organisms came into being. What was their origin? We don’t have fossils or other
evidence to explain what the earliest life was like, therefore much of what I will present
today is hypothetical. Experiments can be done that produce results consistent with
different stages in the scenario. Let’s briefly look at some of the key experiments that
shed light on these stages.
2. Synthesis of the building blocks of life
The setting:
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Planet earth containing water, gases like methane, ammonia and hydrogen,
very little oxygen.
Much more volcanic activity, lightening, meteorite bombardment and
radiation (X-rays and UV light) than in the present day.
Early atmosphere favored formation of organic compounds.
Energy sources such as light from sun, lightening would have been needed to
make or break chemical bonds.
Organic molecules accumulated--no microorganisms to break them down.
Macromolecules could be assembled from smaller building blocks.
Experiments of Miller, 1953:
Test the effects of electrical discharges on a simulated reducing atmosphere.
Apparatus for experiment (Wolfe Fig. 26.2)
Key components of experiments:
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Reduced gases in closed atmosphere (methane, ammonia, water and
hydrogen).
Expose to energy source--sparking electrodes.
Water vapor added to atmosphere.
Organic molecules carried down by condensation.
Results:
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Many organic molecules produced including amino acids, organic acids
such as lactic acid and acetic acid and urea.
If atmosphere also includes HCN, additional amino acids, purines,
pyrimidines (precursors of nucleotides) and porphyrins (cofactors) are
produced.
Experiments produce many non-biological amino acids, D-forms as well
as the biologically active L forms--not readily explained.
If atmosphere includes formaldehyde, many types of sugars are produced
and some fatty acids.
These experiments require the absence of oxygen to succeed.
3. Which came first, DNA, RNA or proteins? RNA was the first genetic material.
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Spontaneous combination of organic subunits into larger molecules.
Condensation reactions link building blocks into larger molecules.
Synthesis of larger molecules perhaps occurred by absorption of reacting
materials on clays, which can concentrate organic molecules on their
surfaces. Clays attract positively charged ions to their surface which can
act as catalysts.
Scientists have used clays to demonstrate spontaneous synthesis of RNAlike molecules from nucleotide precursors.
RNA has been found to be both catalytic (synthesizing peptide bonds in
protein synthesis, carrying out RNA splicing) and informational (many
viruses have RNA as their genetic material).
Thus RNA probably came first.
4. The general scenario
Cells evolved from aggregates of molecules derived from non-living materials.
Stages in the scenario:
(1) synthesis of small organic molecules
(2) join small molecules (monomers) into big molecules (polymers)
(3) aggregrate molecules into droplets which have different properties than their nonliving constituents.
(4) origin of heredity, so “droplets” can pass on their molecules to offspring
Evolution of cells: Ancestors of Prokaryotes. Requires the acquisition of a surrounding
membrane. Prokaryotes all have DNA as their genetic material, not RNA, and proteins
carry out most catalytic reactions, so information transfer mechanisms had to evolve.
Evolution of Eukaryotes: Acquisition of organelles and the shift to an atmosphere with
more oxygen.