14.3 The First Life Forms
Chapter 14 Origin of Life
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The Origin of Heredity
• Scientists still are investigating competing
hypotheses dealing with the possible
transitions from simple organic molecules to
cells (cell life).
• ? How could molecules be organized into
self-replicating systems?
– … in other words, to explain the origin of
heredity…
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The Origin of Heredity…
• Chapter 10 explained how hereditary
information affects the phenotype of cells.
• Hereditary information in DNA is
transcribed into mRNA (an RNA
message).
• RNA message is translated into a protein
• DNA template for RNA; RNA template for
specific proteins
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The Origin of Heredity…
• DNA doesn’t make proteins directly
•?
–RNA structures are unique for the
jobs involved in making proteins.
–mRNA; tRNA; rRNA
–DNA structure is less unique for this
function
–Double helix - replication
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The Origin of Heredity…
• RNA plays an important role in the
production of proteins
• RNA shapes require hydrogen bonds
between amino acids
• It has been speculated that some
RNA molecules might behave like
proteins and catalyze chemical
reactions.
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The Roles of RNA
• Thomas Cech (1947-) 1980s
• Type of RNA found in some unicellular eukaryotes is
able to act as a chemical catalyst (similar to an
enzyme)
– Ribozyme = RNA molecule that acts as a catalyst
to promote a specific chemical reaction
– Later studies indicated that ribozymes could act as
catalysts for their own replication.
– Self-replicating systems of RNA molecules have
been created in the laboratory.
– This supports the hypothesis that life could have
started with self-replicating molecules of RNA.
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The RNA World
• RNA vital for
– DNA replication
– Protein synthesis
– + other basic biochemistry
– Maybe the chemistry and genetics of early
cells was based on RNA
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The RNA World
• First case of heredity and competition???
• Self-replication might involve competing
with other RNA molecules for a limited
number of available nucleotides.
• An RNA molecule with a slight advantage,
maybe in form, might be capable of outcompeting other RNA molecules without the
advantage.
– Over time, the more efficient RNA molecules
would survive, and the world of RNA would
change – slowly.
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The RNA World
Several competing hypotheses:
• Certain kinds of minerals formed a template on
which organic molecules lined up to form polymers.
• Self-replicating RNA began to evolve inside cell-like
structures such as microspheres or coacervates.
– The self-replicating RNA could have provided the
hereditary information that the cell-like structures
lack.
– If the RNA molecules were able to direct the
assembly (the manufacture, making) of the
structures that carried them, a cell-like system
would be formed.
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The First Cells
• No direct evidence of the first cells
• Scientists make inferences…the process of
arriving at some conclusion based on some degree
of probability relative to the premises.
Scientists believe that little or no oxygen existed
on early earth.
The oldest fossils that are thought to be cells are
the size and shape of some living prokaryotes.
The first cells might have developed in an
environment filled with organic molecules for
food.
The first cells were probably anaerobic,
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heterotrophic prokaryotes.
The First Cells
• A growing population of heterotrophs that
depended on spontaneously formed organic
molecules for food eventually would have
removed these molecules from the
environment.
• Now, autotrophs would have begun to have
an advantage.
• The first autotrophs probably did not
depend on photosynthesis, as they do now.
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Chemosynthesis
• Archaea = related group of unicellular
organisms
– Harsh environmental conditions
– Methanosarcina barkeri
• Anaerobic conditions
• Chemosynthesis
– CO2 serves as carbon source
• Assembly of organic molecules
– Energy comes from the oxidation of various
inorganic substances, such as sulfur.
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Photosynthesis & Aerobic Respiration
• Some forms of life had become
photosynthetic ~ 3 billion years ago.
– Geological evidence
• Chemical traces of photosynthetic activity
• Cyanobacteria
– Photosynthetic, unicellular prokaryotes
– Similar to oldest fossils found
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Photosynthesis and Aerobic Respiration
• Oxygen – byproduct of photosynthesis
– Harmful to many unicellular organisms
– Could destroy their coenzymes
– These enzymes essential to cell function
• In some organisms, O2 bonded to other
compounds.
– Prevented it from doing harm
– This was one of the first steps in aerobic
respiration.
– An early functionof a.r. may have been to
preserve important organic compounds.
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Photosynthesis & Aerobic Respiration
• ~ 1 billion + years for oxygen gas levels to reach
current levels
• O2 eventually reached the upper levels of the
atmosphere
• Bombarded by sunlight
• Some wavelengths of sunlight can split O2
• This forms highly reactive single O atoms.
• O + O2 = O3 (ozone)
– Poisonous to plant and animal life
– In upper atmosphere, absorbs much UV
radiation from sun (necessary because…)
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The First Eukaryotes
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•
•
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Eukaryotic cells are larger than prokaryotic cells.
DNA organized into chromosomes within a nucleus
Cytoskeleton and membrane-bound organelles
These features unique to eukaryotes are thought to have
evolved from prokaryotes.
– i.e., E.R. and a nuclear membrane formed from the
infolding of the plasma membrane of a prokaryotic cell.
• Lynn Margulis – researcher
– Proposed further evolved from a mutually beneficial
relationship between primitive eukaryotes and the
prokaryote it engulfed.
– The theory of endosymbiosis
• Figure 14.11, page 290 (add slide)
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The First Eukaryotes
• Aerobic prokaryotes that were engulfed by
eukaryotic cells evolved into the mitochondria.
– Performs aerobic respiration
• Engulfed photosynthetic cyanobacteria evolved
into chloroplasts.
– Performs photosynthesis in modern plant and
algae cells
• The eukaryote provided a beneficial
environment for the prokaryote.
• Likewise, the prokaryote provided a source of
energy for the eukaryote.
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The First Eukaryotes
• Strong evidence supports this theory.
• Chloroplasts and mitochondria replicate
independently from the replication of the cell that
contains them.
• Chloroplasts and mitochondria contain some of
their own genetic material.
– Which is more similar to that of prokaryotes
than it is to the rest of the cell
• Finally, the DNA of these organelles is found in a
circular arrangement, which is characteristic of
prokaryotic cells.
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