Experimental Approaches
to Origins of Life Research
[J.
Lucentini, "Darkness before Dawn", The Scientist 17:(23) 28-29, 2003]
Ultimate Goal is
"creation of an artificial cell, as a model of a life system"
3
experimental approaches are active in today's research
...Search for sources of early precursor
organic molecules of Life ???
I)
classical chemical
evolution approach
search for
bioorganic precursor molecules of life...
A) formed from a chemically reactive soup... in early
oceans of Earth
1922 - Oparin & Haldane:
suggest
early Earth had a reducing atmosphere
early Earth rich in
NH3, CH4, H2, & H2O
1953 - Miller & Urey ---> organics
> abiotic origin of organic molecules from simple inorganics
> H2O, NH3, CH4,
& H2 in reducing atmosphere
> produce HCN and formaldehyde which leads to...
> amino acids, nucleotides, and sugars.
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CRITICISM of research: was not
really a reducing atmosphere,
so chemically is unlikely
but, experiments
recently redone by J.L.
Bada
& S.L. Miller using a weakly reducing
atmosphere (irradiated with protons)... they obtained bioorganic compounds
in amounts comparable to those of Miller/Urey type
experiments.
organic syntheses and origins of life timeline*
B)
Deep dwelling (ocean)
hydrothermal vents
(discovered in 1979)
with minerals spewing up from pressurized, hot springs...
vent
areas are full of organically rich molecules --->
life
(tube worms @ vents).
and bacteria living in hydrothermal vents
speculation is that life may have originated in vents regions.
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Origins of Metabolism (a role of hydrothermal vents?)
Gunter Wachterschauser (german biochemist) has
speculated that origins of
bioorganic chemical reactivity may have originated
at hydrothermal vents.
> earliest primordial biochemical
cycles may have functioned there
> metabolic cycles likely predate cells...
took place on mineral surfaces
that acted as catalysts & surfaces for early reactions
> key reaction may have been a
reductive reaction like citric acid cycle
CO + oxidation of vent minerals --> reduction
Groups including M.J. Russell,
M.M. Hanczyc,
D. Deamer,
C. Huber, and G. Cody
have synthesized amino acids and assembled/disassembled polypeptides
all crucial metabolic processes that might have arisen via hydrothermal vents.
Origins Cellular Life by Deamer
essay
but maybe,
there was a possible Role for Minerals* in Earth's chemical evolution
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Source of Bioorganic molecules
(the precursors to life?)
a) Miller &
Urey - abiotic synthesis
b)
hydrothermal vents (spewed up from "middle
Earth")
C)
SPACE
DEBRIS: (as seen thru today's living
microbes)
chemical evolution vs.
meteorite
dust, meteorites, asteroids, & comets may have
deposited
organics on newly formed planet Earth.
Comets
are mostly ice crystals on cores of silicates & carbon
contain about 10% CO, CO2, CH4, CH3OH, and NH3
Asteroids rocky objects, that hit Earth & contain molecules such as...
kerogen [a PAH], nucleobases, quinones, carboxylic acids,
amines & amides
= some 70 amino acids, including
8 of common 20 aa's.
? while Miller & Urey's soup had 50%/50% mix of
D & L aa's optical
isomers,
asteroid aa's have a surplus of
L-aa's, as in today's
proteins
(argues for asteroids).
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Max Berstein
(NASA - 1999)
has modeled
cryochemistry
reactions in the lab,
to look at molecular events within comets...
he has observed...
> chemical reactions can occur even in very low temps [25K
(-400oF)],
> UV light can break
chemical bonds
- forming ketones, esters, alcohols, and quinones
(for e- transport).
> some molecules formed spherical, capsule-like droplets when
exposed to water, with properties akin to cell membranes.
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some other Astrobiology & Extraterrestrial
models
PANSPERMIA - idea that living microbes drifted in from space & colonized Earth;
Svante Arrhenius (1908) radiation emitted
by stars carried
microbes thru
space; (also supported by
Francis Crick)
SETI - Universe some 1020 stars
with properties similar to our sun's [?]
if 10% have planetary systems &
if 1/10,000 has a planet equal in size & properties to earth
then life as we known it
(carbon based life forms) may still
occur on as many as 1015 planets
(Carl Sagan &
Fred Hoyle)
a speculation ??? extraterrestrial aa's built 1st proteins ?
(optional: Some historical perspective: Ideas on the origins of Life.)
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2nd experimental approaches of origins of life research
II) Evolution of an RNA world... (which came 1st DNA or RNA)
in 1989 Sidney Altman and Thomas Cech - received
Nobel Prize
for demonstrating that RNA molecules (RIBOZYMES)
have CATALYTIC ACTIVITY
i.e., these RNA's catalyze hydrolysis & condensation rxs
of phosphodiester bonds.
If
RNA can be a template and also
catalyze polymerization of like molecules,
i.e., replicate itself, then RNA
molecules may have been the 1st
SELF-REPLICATING living entity.
No self-replicating
RNA molecules exists naturally today, but lab experimentation may
establish that it was feasible, and that
RNA molecules can be selected for via
Darwinian evolutionary mechanisms (natural selection).
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3rd
experimental approaches of origins of life research
III)
Knock-out Cells...
By
modifying a simple microbe, scientists hope to create a
new form of single cell life.
>
J. Craig Venter, a principle investigator (P.I.) of
the Human Genome Project is attempting
to make a new type of bacterium using DNA manufactured in the lab.
> using the sequenced the genes of a
bacterium called
Mycoplasma
genitalium, a gram-positive
parasitic bacterium, whose primary infection site may be the human urogenital
tract.
It probably causes
non-gonococcal
urethritis.
It is one of the simplest microbes known
with only one chromosome and
517 genes.
> researchers began systematically
removing genes to determine how many genes
were essential for life. In 1999, they published a paper that narrowed the needs
of M. genitalium to between 265 and 350 genes.
> a genomic goal will be to learn on
a molecular level the minimum genes a cell needs to
thrive
and reproduce and how to artificially make those and other genes.
