Origins of Life

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Origins of Life Chapter 21
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• Other theories
• comets may have delivered organic compounds
• probably will never prove how life began
• have narrowed down possibilities based on
  assumptions
• likely that life successfully only arose once
  under unique conditions

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Haldane and Operin (1930s)

• hypothesized that amino acids, sugars, and
  nucleotide bases could form spontaneously from
  molecules of ammonia, methane, and water under
  early (4.3 b.y.o) earth conditions
• pre-biotic earth probably had high temps., high
  UV levels, reducing atmosphere (no O2), and
  frequent lightning storms.
• finally proposed that amino acids ? polypeptides
  ? proteins ? enzymes (all happened very slowly)

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• 4. Early cells
• protobionts (IB) liposomes coacervates
• tiny spheres that form spontaneously when certain
  macromolecules (proteins and carbos.) are put in
  water at pH4.
• surround themselves with a layer that is
  selective in admitting certain molecules
• increase in size and then divide
• for the first timethere is an outside and an
  inside where molecules can be concentrated within
  a protected environment and rate of chemical
  reactions can increase!

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Miller and Urey (1950s)

• simulated primitive earth in the lab
• mixed water, methane, ammonia, water vapor,
  hydrogen, and then used electrical discharges to
  simulate lightning
• after a week the found aldehydes, carboxylic
  acids and 15 amino acids!

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Four processes necessary for spontaneous origin
of life on Earth (scientists still working on)

• 1. Synthesis of simple organic molecules
• the following monomers (building-block
  molecules) have been synthesized in the lab under
  primitive earth conditions
• all nucleotide bases of DNA and RNA
• sugars
• amino acids
• most vitamins

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• 2. Polymerization
• dehydration linkages between monomers is
  necessary to make polymers and only possible when
  in high concentrations or when enzymes are
  present
• monomers could have been concentrated inside of
  coacervates
• clay particles would act as catalysts for
  chemical reactions by allowing organic molecules
  stick to them
• RNA
• could have acted as a catalyst some reactions in
  ribosomes are still catalyzed by RNA

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• Sidney Fox (1950s) demonstrated that
  polymerization of amino acids can occur
• hot areas (volcanoes/pools) could concentrate
  amino acids in order to make polypeptides
• in the lab, he made polymers of 200 amino acids
  called thermal proteinoids
• when placed in water they can cluster together in
  bodies called proteinoid microspheres that
  automatically form two-layer membranes, grow, and
  even take up molecules from the surrounding
  environment

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When lipid and amino acid molecules mix in water
under the right conditions, some interesting
things happen. Tiny bubbles, called coacervates
or proteinoids, about the size of bacteria form.
But these are not just simple bubbles. They are
surrounded by a double membrane similar to cell
membranes. The bubbles can both get larger, by
adding more material from the surrounding water,
and bud off sections and divide. They may even be
able incorporate amino acids and carry on simple
chemical reactions similar to those found in
cells.
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• 3. Self-replicating systems
• a self-replicating system is not possible with
  simple molecules
• therefore, maybe RNA or DNA arose first... and
  maybe they became surrounded by a coacervate or
  protenoid microsphere (this would be similar to
  viruses of today)
• RNA can replicate (very slowly) through base
  pairing without enzymes (DNA needs enzymes)
• so what came firstnucleic acids or proteins?
  Need nucleic acids to make enzymes yet need
  enzymes to make nucleic acids?

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The Earliest Cells Prokaryotes?

• first genes were probably encoded in RNA, not DNA
• chemically simpler
• in the lab, can get spontaneous polymerization of
  RNA nucleotides
• finally DNA could have evolved from RNA with the
  help of reverse transcriptase
• membranes could have been formed from
  phospholipids that automatically form bilayers
  and then into cell-like spheres called micells.

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Possible scenarios....

• early autotrophic cells (phototrophic cells)
  started using water as the source of hydrogen and
  producing O2 as a waste product.
• many O2-sensitive organisms probably became
  extinct
• most of this early O2 was used up by oxidizing
  (rusting) metals such as Fe S
• finally (after about 2 billion years) O2 was left
  over enough to make the oxygen-rich atmosphere of
  today
• life evolves to utilize the abundant O2