Transcript Self-Replication

Origin of Life
History of Life on Earth
• Timeline:
Big Bang: Universe expanded explosively between 10 billion and 20
billion years ago, as evidenced by the red shift, which reflects the
velocity of galaxies moving away from each other.
The sun was formed 5 billion years ago, and the planets were formed
soon thereafter.
Evidence of life is found in ancient rocks in Australia: simple cells
similar to modern bacteria that are about 3.5 billion years old.
These may represent one of the early experiments in life or may be
in the ancestral line of modern life forms.
Current evidence suggests that the life forms now in existence had a
single common ancestor at least as far back in time as the
appearance of RNA-directed protein synthesis
What was the prebiotic Earth like?
• Stage 1 – no atmosphere protects the earth’s surface, so meteorite
impacts are abundant and cause the earth’s crust to melt –
radioactive decay heats the core -volcanoes erupt.
• Stage 2 – volcanic heat and meteorite impacts release geologic H2,
H2O, CO2, methane and ammonia – atmosphere with dense clouds
forms – rainfall begins – the Earth’s surface begins to cool.
• Stage 3 – The atmosphere and oceans are bombarded with energy
– volcanic heat – violent storms – lightning bolts – UV radiation.
Nucleotides and organics are concentrated in places where there is
water, high heat, and mineral catalysis – perhaps an environment
like deep-sea hydrothermal vents have today.
When and how did life begin on Earth?
• Hypothesis 1: conditions on the early Earth, as
described on the previous slide - favored the
evolutionary origin of prelife forms.
• Hypothesis 2: Life forms originated somewhere
else in the universe and migrated to the early
Earth in primitive, resistant forms (Panspermia –
this hypothesis was advanced by Arrhenius, the
scientist who discovered the correlation between
temperature and the rates of chemical reactions)
Although Hypothesis 1 is the reigning one at this
time, Hypothesis 2 cannot be refuted
Some Supporting Evidence:
• Comets are half frozen water containing organic compounds (and
cells?)
• Asteroid analysis yields organic molecules, e.g., the Murchison,
Australia (1969) asteroid contained 74 amino acids, 250
hydrocarbons, all 5 bases of DNA/RNA.
• Many contemporary bacterial species could survive interstellar
conditions indefinitely in the ensporulated form, so long as they had
some protection from cosmic rays
• Panspermia doesn’t solve the question of how life originated – it just
moves the question to another location.
Hypothesis and Experiment
Oparin (1924)
hypothesized that the
primordial atmosphere had
the components that could
have combined to produce
organic compounds….
Stanley Miller, a
graduate student in the lab
of Harold Urey at the
University of Chicago, began
a series of experiments that
showed that organic
synthesis under conditions
of the early atmosphere was
conceivable (1950s). (inset)
Spontaneous chemical synthesis
• After one week, the initial amino acids
began to be polymerized into simple
carbohydrates and peptides. What had
been transparent reactants took on a
reddish tinge – primordial slime….
The reactions of the citric
acid cycle are
conventionally known as
the intermediary reactions
of oxidative metabolism.
However, in the prebiotic Earth there was
little free oxygen, so instead of starting with
acetate and proceeding to CO2, the
reactions could have started with CO2 and
generated acetate – as well as
regenerating all the intermediates.
Now that we have some ingredients, what next?
“Metabolism-first world”
“RNA-first world”
Small organic molecules
Nucleic acids
Catalytic RNA
Networks of reactions –
non-enzymatic catalysts
Natural selection favors cyclic
reaction sequences that provide
pathways to lower energy levels for
high-energy electrons while
regenerating their own constituents
RNA-directed RNA
replication
DNA takes over information storage; proteins
take over catalytic functions
Evidence for the metabolism-first world
• Given appropriate physical conditions (reducing
atmosphere, heat, U.V. light, catalytic surfaces),
considerable organic synthesis can go on without
enzymatic catalysis – for example, the first 3 reactions of
the reverse citric acid cycle can be driven by UV light.
Evidence for RNA as the link between random organic
molecules and life…the RNA World hypothesis
Experiments that extend the Miller/Urey studies
revealed that RNA was produced from the
random oligonucleotides.
RNA complementarity elicits copy formation
(adenine-uracil, cytosine-guanine)
RNA has the ability to serve as a catalyst:
“ribozymes”, doing many of the things we
expect proteins to do. Promotion of synthesis
resulted from RNA catalysts that drew energy
from a triphosphate group (like ATP).
Self-Replication: Replication is an
intrinsic property of DNA and RNA
Evolution between RNA and amino acids led to
the Genetic Code
F
The 20 amino
acids used in
o
r nucleotides, e.g.:
triplets of
proteins are specified by
The genetic code that led to the RNA World evolved
very early – probably even before there were cells because it is essentially the same in all life forms.
Q: If there is RNA, capable of copying itself,
carrying out some life functions, and
ultimately, coding for proteins, what else is
needed?
A: ?
?
A: A membrane to keep the system
together, so that the enzymes do not
diffuse away from the reactants…
Voila – we have a prokaryotic cell!
Compartmentalization: The Role of
Membranes
• Systems of replicating RNA and the
proteins they produced needed to stay
together for the system to flourish…
• Either before or after the development of
the genetic code, a membrane-isolated
system may have developed.
• How?
Coacervates or proteinoids
• When lipids and amino acids mix in water,
tiny bubbles, called proteinoids or
coacervates, about the size of bacteria,
form. They are surrounded by a double
membrane similar to cell membranes; they
can grow by adding material and bud off or
divide. Simple chemical reactions can
occur inside them.
Phospholipid membranes are self-assembling
The intrinsic
property of
membrane
phospholipids that
drives selfassembly into
bubble-like spheres
is an amphipathic
structure, with
water-insoluble
hydrophobic
hydrocarbon chains
and water-soluble
heads that contain
phosphate.
Shifting gears…
Now that we have at least a hypothetical scheme for
cellular evolution, let’s look at the scale that we will be
operating in to examine cellular function:
Size Comparisons
Macromolecules: measured in Angstroms (10-10m)
Subcellular structures: measured in nanometers
(10-9m)
The largest viruses, bacteria and cells: measured
in micrometers or microns (10-6m)
Molecular structures
Size range
comparison
Relating
this to
visible
objects…
A BB pellet is an atom
A marble is a simple molecule
A cat is a chain molecule (like a microfilament or microtubule)
A tractor-trailer is a molecular structure (such as a lysosome or Golgi)
An oceanliner is a cell
A mountain range is an organ (like the heart)
And a continent is a whole body – around 5 trillion cells
Cells Alive clip
• http://www.cellsalive.com/howbig.htm
Cell Types
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1.
2.
3.
4.
•
1.
2.
3.
Archaea (Archaeobacteria): genetically distinct line that often
exists in extreme environments.
Eubacteria (Prokaryotes): These include the bacteria and
cyanophytes (blue-green algae)
no nucleus. genetic material (single circular DNA) not enclosed
within a membrane.
Recombination happens through transfer of plasmids (short circles
of DNA that pass from one bacterium to another).
Ribosomes, but few other organelles.
Cell wall made up of peptidoglycan.
Eukaryotes: cells found in multicellular organisms, algae, protozoa.
cells with a nucleus (the genetic material is surrounded by a
membrane)
multiple chromosomes, linear and complexed with regulatory
proteins.
Many organelles, with internal compartmentalization of functions.
Archaea are found in extreme environments such
as hot springs.
Prokaryotes
Cell types
What about viruses?
• Dogma: Non-living, replicating genetic material enclosed
in a protein coat, completely dependent on living cells for
their existence…
• Viruses infect all three branches of “life”.
• The overwhelming number of viruses are not harmful to
their hosts and peacefully co-exist – We have more viral
genes than human genes in us, if you take a whole body
and process it for genes.
• Viruses may have been a step in the sequence of
evolution of cells, or a parallel line of evolution that has
parasitized all three branches of cellular organisms. A
newly discovered virus called Mimi is visible with the light
microscope and possesses 10 times as many genes as
a typical DNA virus, including genes for DNA repair,
protein translation and other proteins.
Summary
• Whether life on earth resulted from reactions in
gaseous bubbles, deep in marine volcanoes, or
as a gift from outer space, it required molecules
that could store and pass on instructions,
mechanisms to utilize energy for growth and
replication, and membranes that could keep
functional elements together.
• The enormous diversity that has resulted is
exhibited by functional differences in the cells of
multicellular animals – the subject of this course.