Transcript Cell evolution

• Origin of Life
• In 1862, Louis
Pasteur conducted
experiments that
rejected the idea
of spontaneous
generation.
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• All life today arises only by the reproduction of
preexisting life, the principle of biogenesis.
• Conditions on the early Earth were very different.
• little atmospheric oxygen
• Energy sources, such as lightning, volcanic activity,
and ultraviolet sunlight
• Sometime between about 4.0 billion years ago, when
the Earth’s crust began to solidify, and 3.5 billion years
ago the first organisms came into being.
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• Hypothesis- chemical and physical processes in
Earth’s primordial environment eventually
produced simple cells.
• (1) the abiotic synthesis of small organic
molecules;
(2) joining these small molecules into polymers:
(3) origin of self-replicating molecules;
(4) packaging of these molecules into “protobionts.”
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Abiotic synthesis of organic molecules is
a testable hypothesis
• In the 1920’s, A.I. Oparin and J.B.S. Haldane
independently postulated that conditions on the
early Earth favored the synthesis of organic
compounds from inorganic precursors.
• They reasoned that this cannot happen today because
high levels of oxygen in the atmosphere attack
chemical bonds.
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• The reducing environment in the early
atmosphere would have promoted the joining of
simple molecules to form more complex ones.
• The considerable energy required to make organic
molecules could be provided by lightning and the
intense UV radiation that penetrated the primitive
atmosphere.
• Young suns emit more UV radiation and the lack of an
ozone layer in the early atmosphere would have
allowed this radiation to reach the Earth.
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• Oparin-Haldane hypothesis - conditions on the early Earth favored the
synthesis of organic compounds from inorganic precursors
• In 1953, Stanley Miller and Harold Urey tested this hypothesis by creating,
in the laboratory, the
conditions that
had been postulated
for early Earth.
• They discharged sparks
in an “atmosphere” of
gases and water vapor.
H2O, H2, CH4, and NH3
The experiments produced
a variety of amino acids and
other organic molecules.
• Alternate sites
volcanoes & deep-sea vents
meteorites
Laboratory simulations of early-Earth
conditions have produced organic
polymers
• The abiotic origin hypothesis predicts that
monomers should link to form polymers without
enzymes and other cellular equipment.
• Polymers (polypeptides)form from monomers on
hot sand, clay, or rock.
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RNA may have been the first genetic
material
• Cells store their genetic information as DNA,
transcribe genes into RNA, and translate the RNA
messages into enzymes and other proteins.
• Many researchers have proposed that the first
hereditary material was RNA, not DNA.
• RNA can also function as an enzymes, it helps resolve
the paradox of which came first, genes or enzymes.
• RNA catalysts, Ribozymes catalyze the synthesis of new RNA polymers.
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• Short polymers of ribonucleotides can be
synthesized abiotically in the laboratory.
• Ribozymes also help catalyze the synthesis of new
RNA polymers.
• zinc can act as a catalyst - the copied sequences may
reach 40 nucleotides with less than 1% error.
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Fig. 26.11
• In the pre-biotic world, RNA molecules may have
been fully capable of ribozyme-catalyzed replication.
• RNA-directed protein synthesis - binding of
specific amino acids to bases along RNA
molecules -This is one function of rRNA in
ribosomes.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
self-assembly of abiotically produced
molecules
• Living cells may have been preceded by
aggregates of abiotically produced molecules
which maintain an internal chemical environment
from their surroundings and may show some
properties associated with life, metabolism, and
excitability.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Liposomes behave dynamically, growing by
engulfing smaller liposomes or “giving birth” to
smaller liposomes.
liposomes form a molecular bilayer, much like
the lipid bilayer of a membrane
Fig. 26.12a
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• If enzymes are included among the ingredients, they are incorporated into
the droplets.
•
Some molecules produced abiotically have weak catalytic capacities.
Fig. 26.12b
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• Evolution - replacement of RNA as the genetic
information by DNA, a more stable molecule.
• Laboratory simulations cannot prove but describe
steps that could have happened to create life on
the primitive Earth.
• organic monomers synthesized on early Earth or from
comets and meteorites?
• Location - deep-sea vents?
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Oldest multicellular fossil
Oldest fossil
prokaryotic cell
• The oldest fossils that have been uncovered were
embedded in rocks from western Australia that are
3.5 billion years ago.
• The presence of these fossils, resembling bacteria,
would imply that life originated much earlier.
• This may have been as early as 3.9 billion years ago,
when Earth began
to cool to a
temperature at
which liquid
water could
exist.
Fig. 26.3a
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Multicellular eukaryotes evolved by 1.2
billion years ago
• A great range of eukaryotic unicellular forms
evolved into the diversity of present-day
“protists.”
• Multicellular organisms,
differentiating from a
single-celled precursor,
appear 1.2 billion years
ago as fossils, or perhaps
as early as 1.5 billion
years ago from molecular
clock estimates.
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Fig. 26.6