Download ORIGIN OF CELLS

ORIGIN OF CELLS
Summary
•Bio-organic chemicals can be formed by simple inorganic
processes involving basic C, H, O, S, and N compounds
and a source of energy
•Concentration of substrates for synthetic reactions may
involve membranes or adsorptive surfaces
•Self-reproducing systems can be envisioned as starting with
either a “replicator” or with a complex metabolic system
Geological stratigraphy,
together with radioactive
dating, show the
sequence of events in
the history of the Earth.
Note the entry for
“cyanobacteria” and
“stromatolites” only one
billion years after the
formation of the Earth and
very soon after the last
Impact heating.
Where did the materials for
making cells come from?
Was heating necessary? Last impact heating
~3500
Or deleterious?
Were bacteria really the first
life?
Life starts with chemistry
Chemical experiments, initiated by Stanley Miller and Harold Urey,
showed that some biochemicals could be formed naturally
(but not all--where did the rest come from?)
The Miller-Urey experiment was repeated with several modifications:
The gas mixture was varied
Initially, H2, NH3, CH4, H2O
Later, less or no H2, added CO2, H2S, SO2
The energy source was varied
Spark (lightning)
UV lamp (sunlight)
Sonic energy (thunder)
The Miller experiment lives on
New amino acids detected in old vials
(Johnson et al., Science 322:404, 17 Oct 2008)
Applying the Miller experiment to extraterrestrial conditions
Researchers used radio-frequency radiation—a more convenient substitute for
ultraviolet sunlight—to turn methane, nitrogen, and carbon monoxide (the main
constituents of Titan's atmosphere) into glycine and alanine, the two smallest amino
acids, and cytosine, adenine, thymine, and guanine, the four most basic components
of DNA, and uracil, a precursor of RNA. The researchers said that because they
achieved the reactions without the presence of liquid water, it's possible life could
have sprung forth on Earth not in the seas, as commonly assumed, but perhaps in the
planet's early atmosphere—a considerably thinner version of the fog enveloping Titan
today (Science 330, 307, 2010)
Notice that the Miller-Urey compounds are similar to those
found in the Murchison meteorite--extraterrestrial synthesis?
(Murchison meteorite: Australia, 1969)
Life requires a boundary
Hypothesis:
Metabolism (energy, reproduction) needs a
way of concentrating intermediates.
Membranes
Adsorptive surfaces
Protobionts: a step
toward living cells?
In 1957, Sidney Fox demonstrated that dry mixtures of amino acids
could be encouraged to polymerize upon exposure to moderate heat.
When the resulting polypeptides, or proteinoids, were dissolved in hot
water and the solution allowed to cool, they formed small spherical
shells about 2 μm in diameter—microspheres. Under appropriate
conditions, microspheres will bud new spheres at their surfaces.
A different kind of protobiont?
Tiny compartments in mineral structures can shelter simple
molecules, while mineral surfaces can provide the scaffolding
on which those molecules assemble and grow. Beyond
these sheltering and supportive functions, crystal faces
of certain minerals can actively select particular molecules
resembling those that were destined to become biologically
Important [eg. L-amino acids, D-sugars]. The metallic ions in other
minerals can jumpstart meaningful reactions like those that must
have converted simple molecules into self-replicating entities.
--R.M. Hazen, “Life’s Rocky Start” Scientific American, April, 2001
Some have taken the term “protobiont” a step too far.
How does a
replicating system
arise from passive
adsorption?
Two hypotheses:
•Replicator first
•Metabolism first
From R. Shapiro, “A
Simpler Origin of Life”
Scientific American
June, 2007
From R. Shapiro, “A
Simpler Origin of Life”
Scientific American
June, 2007
1.
2.
3.
4.
5.
Boundary
Energy source
Energy coupled to chemical reaction
Network; adaptation; evolution
Growth; reproduction
Shapiro, in proposing the
metabolism-first hypothesis,
accepts general ideas about
what life requires:
1.
2.
3.
4.
5.
Boundary
Energy source
Energy coupled to chemical
reaction
Network; adaptation; evolution
Growth; reproduction
[From my first lecture:
•
Catalysis
•
Complex organization
•
Requirement for energy
•
Homeostasis
•
Inheritance
•
Natural selection]
From R. Shapiro, “A
Simpler Origin of Life”
Scientific American
June, 2007
Summary
•Bio-organic chemicals can be formed by simple inorganic
processes involving basic C, H, O, S, and N compounds
and a source of energy
•Concentration of substrates for synthetic reactions may
involve membranes or adsorptive surfaces
•Self-reproducing systems can be envisioned as starting with
either a “replicator” or with a complex metabolic system