Transcript Origin of Life

Origin of Life
What is Life?
• Life resists a simple, one-sentence definition because it is associated
with numerous emergent properties - properties that emerge as a
result of interactions between components
• But, we can recognize life without defining it, by recognizing its
properties:
• Order
• Reproduction
• Growth and Development
• Energy Utilization
• Response to the Environment
• Homeostasis
• Evolutionary Adaptation
When did life arise on Earth?
• The Earth is thought to be approximately 4.6 billion years old, but
life is believed to have occurred approximately 4 billion years ago
(bya)
What were the conditions like on Earth when life arose?
• Up to about 4 bya, asteroid impacts and volcanic eruptions resulted
in the release of various gases that began to form an atmosphere
• It consisted mainly of CO2, with some nitrogen, water vapor and
sulfur gases; hydrogen quickly escaped into space
• CO2 in the atmosphere trapped solar radiation, making the Earth’s
surface rather warm
• Earth was cool enough to form a crust, and water vapor condensed
to form oceans
• Oceans in turn helped to dissolve CO2 from the atmosphere and
deposit it into carbonate rocks on the seafloor
What were the conditions like on Earth when life arose? cont.
• Organic molecules were undoubtedly being formed on the Earth’s
surface
• Lightening and ultraviolet radiation from the Sun acted on the
atmosphere to forms small traces of many different gases, including
ammonia (NH3), methane (CH4), carbon monoxide (CO) and ethane
• Also, cyanide (HCN) probably formed easily in the upper
atmosphere, from solar radiation and then dissolved in raindrops
What is the simplest living cell that one can imagine?
A universal minimal cell must
contain the following::
 Cell membrane
 Cytoplasm
 DNA and RNA
 Proteins
 Enzymes
 Ribozymes
Conditions that are necessary if life is to evolve from nonlife
 Energy – energy to form complex organic molecules
 Protection – continued energy input will destroy complex organic
molecules that form in reactions; they must, therefore, be protected after
they are formed
 Concentration – Chemical reactions run better at high concentrations,
but most reactions give rather low yields
 Catalysis – All reactions inside our cells are aided by the necessary
activity of enzymes
Where did the basic building blocks come from?
Miller-Urey Experiment
• A mixture of methane, ammonia, water
vapor, and hydrogen was circulated through
a liquid water solution and continuously
sparked by a corona discharge elsewhere in
the apparatus.
• After several days of exposure to
sparking, the solution changed color.
• Subsequent analysis indicated that several
amino and hydroxy acids had been
produced by this simple procedure.
Additional experimental evidence
• Carl Sagan, and his colleagues made amino acids by long
wavelength ultraviolet irradiation of a mixture of methane,
ammonia, water, and H2S.
• It is quite remarkable that amino acids can be made so readily
under simulated primitive conditions.
• However, when laboratory conditions become oxidizing, no
amino acids are formed, suggesting that reducing conditions were
necessary for prebiological organic synthesis.
Extraterrestrial delivery
• Comets and some meteorites are rich in amino acids, sugars, and
fatty acids.
• However, the survival of organic matter during large impacts may
be small.
• Interplanetary dust particles can have the same composition; about
10,000 tons of dust falls to Earth every year.
Hydrothermal Vents
On the sea floor, where new ocean crust is forming, hot mineral-rich
water is venting into the ocean; many fresh mineral surfaces occur
in these vents.
These surfaces catalyze the conversion of carbon dioxide and
nitrogen gas to methane and ammonia, which are good ingredients
from which to make the basic building blocks.
Is it possible to simulate the production of early organic polymers?
The Dilemma: Organic polymers
such as proteins are synthesized by
dehydration reactions
(condensation) that remove
hydrogen and hydroxyl (-OH)
groups from the monomers,
forming water as a by-product
• Also, enzymes within the cell are
responsible for catalyzing these
kinds of reactions
•Abiotic synthesis of polymers on the early Earth would have had to
occur without the help of these enzymes
• Moreover, the monomers would have been present in dilute
concentrations, making spontaneous condensation reactions rather
unlikely
Potential Solutions
• Polymerization has been demonstrated in lab experiments when
dilute solutions of organic monomers are dripped onto hot rocks
• The process appears to vaporize water and concentrate the
monomers on the substrate
• On the early Earth, waves or rain may have splashed dilute solutions
of organic monomers onto hot rocks and subsequently rinsed
polymers back into the water
• Clay may have served as a substratum for the polymerization of
monomers
• Various monomers bind to charged sites on clay particles; clay may
have concentrated various organic monomers present in dilute
solutions
• At some of the binding sites, metal atoms, such as iron and zinc,
function as catalysts facilitating the reactions that link monomers
The formation of an early cell
Review
• Cells exist in a watery world.
• A water molecule can behave
as if charged because of its
polar structure.
• This polar structure is the
basis for an interesting
relationship between water
molecules and lipids
• The lipid’s charged polar head
(hydrophyllic) can form a weak bond
with a water molecule, but the
uncharged, nonpolar tail
(hydrophobic) cannot.
• In a membrane, lipids are
usually arranged in sheets made
of two layers, with the lipids in
each layer pointing in opposite
directions.
• The water-loving heads contact
water both inside and outside the
cell, while the water- loathing
tails stay tucked safely within the
wall’s oily interior.
• Arranged this way, lipids make
surprisingly good barriers.
The Formation and Significance of Liposomes
• When Alex Bangham (circa 1960) extracted lipids from egg yolks
and threw them into water, he found that the lipids would naturally
organize themselves into double-layered bubbles roughly the size of
a cell; these bubbles became known as liposomes.
• This discovery led Bangham and Deamer to speculate that
liposomes may have predated life.and may have provided life’s first
shelter
• Deamer took mixtures of fatty acids, glycerol, and phosphates and
found that in the right concentrations they formed into lipids, and in turn,
the lipids spontaneously assembled into liposomes.
Question: How could macromolecules have gotten inside them?
• Deamer extracted lipids from egg yolk, and mixed some of it into a
small test tube of water
• He then extracted a few drops from the mixture and put them on a glass
slide.
• To this he added a some fluorescently stained DNA
• The slide on a hot plate to simulate primordial tide pool; after a few
minutes, the lipids and DNA on the slide dried into a thin film.
• Deamer later added a few drops of water and put it under a fluorescent
microscope
• He noticed the lipids swelled into bubbles; some containing fluorescent
DNA
• Provided proof that as the planes of lipids curled up into vesicles, the
DNA that had been sandwiched in between them got trapped inside.
An Extraterrestrial Solution?
• Deamer also wondered whether outerspace could have supplied
early membranes
• He examined a 200-pound meteorite that had fallen in Murchison,
Australia, with the interest in determining whether there were any
things in the meteor that form bilayers?
• Deamer ground a piece of the Murchison meteorite and extracted
the organic carbon, made it into a slurry, dried it, and then added
water again.
• He took the extract and put it on a slide and noticed that the whole
slide began to fill with little vesicles.
Early Sources of Cellular Energy
• Meteorites are comprised of a group of
chemicals named polycyclic aromatic
hydrocarbons (PAHs) that are made of
hexagons of carbon and hydrogen atoms
linked in various arrangements.
• PAHs may have made life possible on early Earth because the give
off electrons when exposed to light
• These electrons could have supplied energy to early cells.
Question
How did the early organic molecules and other
biological molecules become self-replicating and
self-regulating?
The Central Dogma: A Brief Review
• DNA is replicated when cells divide and when sex cells are formed.
• Genes are transcribed to produce single strands of RNA
• RNA (messenger RNA) provides the template from which protein
synthesis is carried out.
• Strands of messenger RNA are translated to produce a sequence of
amino acids (=protein).
• Which came first, DNA, RNA or protein?
The First Genetic Material: The RNA World Hypothesis
• The Idea: Primitive RNA molecules may have assembled themselves
randomly from building blocks in the primordial ooze and performed
simple chemical chores.
• The Evidence: In the early 1980's, Sidney Altman and Thomas Cech,
discovered a kind of RNA - a ribozyme - that could edit out
unnecessary parts of the message it carried before delivering it to the
ribosome.
• Long before there were enzymes or DNA, RNA molecules may have
been capable of self-replication
• But skeptics argued that an RNA's being able to cleave itself was all
well and good, but what about all the other chemical reactions that But
could RNA serve as the sole information molecule and enzyme of early
cells?
RNA and Translation
• Harry Noller attempted to map ribosomes and figure out which of its
proteins were responsible for translation of mRNA.
• He treated the ribosomes with protein-digesting enzymes to show
that the rest of the ribosome couldn't translate mRNA.
• Despite these efforts, translation persisted; it suggested that RNA
was doing the translating.
• Noller's et al. Later identified a few crucial locations in ribosomal
RNA that allow translation.
RNA Speed Rate of Reaction
• RNA was also hypothesized to help catalyze the synthesis of new
RNA (e.g., it was acting like a type of enzyme)
RNA Speed Rate of Reaction cont.
• Interestingly, Charles Wilson was successful in getting RNA to
speed a reaction that doesn't involve DNA or RNA
• Wilson found and cultivated ribozymes that could carry out
alkylation a hundred times faster than the protein that's normally
responsible for it in a series of experiments designed to mimic
evolution.
• He began with billions of messenger RNAs, random sheets torn
from volumes of DNA , and presented them with carbon and nitrogen
atoms.
• Some were able to stick one of each atom together.
RNA Speed Rate of Reaction cont.
• Although RNAs can't reproduce like animals or plants, given the
right materials, they can make copies of themselves that are more or
less identical.
• Surprisingly, it's the less-identical ones - those that have errors in
them - that win over time.
• Subtle differences that may make an RNA better able to put carbon
and nitrogen together - or render it completely useless, which is
usually what happens.
• By selecting the RNAs that could speed alkylation and then letting
them reproduce, generation after generation, Wilson eventually
wound up with a group of RNAs that were really good at sticking
the atoms together.
Conclusions
• The rudiments of RNA-directed protein synthesis may have been the
weak binding of specific amino acids to bases along RNA molecules,
which functioned as templates holding a few amino acids together
long enough for them to be linked.
• If RNA happened
to synthesize a short
polypeptide chain
that in turn behaved
as an enzyme
helping the RNA
molecule to
replicate, then the
early chemical
dynamics included
molecular
cooperation and
competition
Conclusions
• The first steps toward replication and translation of genetic
information may have been taken by molecular evolution even before
RNA and polypeptides became packaged within membranes
• Once primitive genes and their products became confined to
membrane enclosed compartments the units could have evolved
collectively