D.1 origin-of-life-on-earth

Option D.1
12 IB Biology 2011
Miss Werba

• The conditions of pre-biotic Earth
• Experiments of Miller and Urey
• Hypothesis regarding first catalysts
• Theory that regarding RNA and replication
• Possible origin of membranes and prokaryotic
cells
• Endosymbiotic theory for the origin
of eukaryotes
MISS J WERBA – IB BIOLOGY 2

D.1.1

• There are processes that were needed for the
spontaneous generation of life on Earth:
– Non-living synthesis of simple organic molecules
– Assembly of these molecules into polymers
– Inheritance possible once self-replicating
molecules originated
– Packaging of these molecules into membranes


D.1.1
D.1.3
D.1.4

• 15 billion years after the “Big Bang”, the planets
began to form.
• The atmosphere on Earth at this time probably
contained a variety of inorganic molecules:
– Water vapour
– Methane
– Ammonia
– Hydrogen
– Carbon dioxide


D.1.1
D.1.3
D.1.4

• The energy for forming the organic molecules was
provided by:
– frequent thunder storms and lightning strikes
– volcanic activity
– meteorite bombardment
– high temperatures due to
greenhouse gases
– UV radiation
(no ozone so was extreme)


D.1.1
D.1.3
D.1.4

• These elements and inorganic molecules are
presumed to have been sufficient for life to begin.
• The organic molecules may have been generated on
Earth or introduced from space.


D.1.1
D.1.3
D.1.4

• The hypothesis that life on Earth originated by
introduction of complex organic chemicals or even
bacteria via comets is called panspermia.
• A shower of comets about
4 thousand million years ago
could have introduced complex
organic molecules and water to
the Earth and initiated chemical
evolution.


D.1.1

• There was little to no oxygen in the atmosphere at
the time, as any oxygen was absorbed by rocks.
• This meant that there was no oxygen to steal
electrons away from other atoms (ie. oxidise them).
• This would have resulted in a ‘reducing atmosphere’
which would have made the joining of simple
molecules to form more complex ones more likely.


D.1.1

• Experiments have shown that it is possible to form
organic molecules in a reducing atmosphere
• However it is very difficult to do when there is
oxygen in the atmosphere
• This polymerisation process would allow the larger
chemicals needed by cells to form.


D.1.1

• How were polymers — the basis of life itself —
assembled????
• In solution, hydrolysis of a growing polymer would
soon limit the size it could reach.
• This has led to a theory that early polymers were
assembled on solid, mineral surfaces that protected
them from degradation.
• In lab experiments they have been synthesized on
clay.

D.1.1
D.1.5

• In current cells, DNA can replicate but it needs the
help of enzymes (proteins) to do this.
• The proteins are assembled based on information
carried on the DNA and transcribed into RNA.

• So what came first.....
the DNA to make proteins or
the proteins to make the DNA?!?!?!?!?


D.1.1
D.1.5

• The synthesis of DNA and RNA requires proteins.
• So:
– proteins cannot be made without nucleic acids
and
– nucleic acids cannot be made without proteins

• Wrong!


D.1.1
D.1.5

• The synthesis of nucleotides and their bases could
have happened easily.
• Once this had occurred, it is not hard to see how a
single strand of RNA could have formed.
• Once this had occurred, complementary base pairing
could have resulted in the non-enzymatic replication
of RNA.


D.1.1
D.1.5

SOURCE: Purcell, D. (2009)


D.1.1
D.1.5

• Self-replicating molecules are molecules that are
able to undergo replication.
• They are able to act as a template for copies of
themselves to be made.
• The only biological molecules capable of self-
replication are DNA & RNA.
• Unlike DNA, RNA sequences are capable of self-
replication: it can catalyse its formation from
nucleotides in the absence of proteins.

D.1.1
D.1.5



D.1.1
D.1.5

• The discovery that certain RNA molecules have
enzymatic activity provides a possible solution.
• These RNA molecules — called ribozymes—
incorporate both the features required of life:
– storage of information
– the ability to act as catalysts
• Active ribozymes can be easily assembled from
shorter olignonucleotides (strands of nucleotides).


D.1.1
D.1.5

• Ribozymes have been synthesized in the laboratory
and can catalyze exact complements of themselves.

• The ribozyme serves as both:
– the template on which short lengths of RNA
("oligonucleotides“) are assembled, following the
rules of base pairing and
– the catalyst for covalently linking these
oligonucleotides.


D.1.1
D.1.5



D.1.1
D.1.5

• Evidence for this ideas is provided by the fact that
many of the cofactors that play so many roles in life
are based on ribose:
ATP
NAD
FAD
coenzyme A
cyclic AMP
GTP

D.1.1
D.1.6

• The development of the lipid bilayer was imitated in
the laboratory by Fox and his co-workers
• They heated amino acids without water and
produced long protein chains
• When water was added and
the mixture cooled, small
stable microspheres or
coacervates were formed


D.1.1
D.1.6

• The coacervates seemed to be able to accumulate
certain compounds inside them so that they became
more concentrated than outside
• They also attracted lipids and
formed a lipid-protein layer
around them


D.1.1
D.1.6

• If we assume that the coacervates also combined
with self-replicating molecules such as RNA, we are
looking at a very primitive organism...
• This is thought to have happened about 3.8 billion
years ago


D.1.1
D.1.6



D.1.1
D.1.6

• The aggregates or coacervates are also known as
protobionts or proto cells.
• The most successful liposomes (protobiont in
presence of lipids) at surviving would have passed on
their characteristics and developed into early
prokaryotes!


D.1.1
D.1.6

SOURCE: McFadden, G. (2009)

D.1.1
D.1.6


D.1.2

• Stanley Miller and Harold Urey worked on trying to
confirm some of these ideas regarding pre-biotic
Earth.
• In 1953, Miller set up an apparatus to simulate
conditions on the early Earth.
• The apparatus contained a warmed flask of water
simulating the primeval sea and an atmosphere of
water, hydrogen gas, CH4 (methane), and NH3
(ammonia).

D.1.2

• Sparks were discharged in the synthetic atmosphere
to mimic lightning.
• Water was boiled, while a condenser cooled the
atmosphere, raining water and any dissolved
compounds back to the miniature sea.
• The simulated environment produced many types of
amino acids and other organic molecules leading
them to conclude the pre-biotic synthesis of organic
molecules was possible.

D.1.2


D.1.2

• This spontaneous generation of organic molecules
was supported by investigation of meteorites.

• In 1970, a meteorite was found
to contain 7 different amino
acids, 2 of which are not found
in living things on Earth.


D.1.7

MISS J WERBA – IB BIOLOGY SOURCE: McFadden, G. (2009) 42

D.1.7


D.1.7

• Prokaryotes had the planet to themselves for about 2
billion years!
• Oxygen began to gradually accumulate in the
atmosphere on Earth.
• Bacteria evolved naturally to contain a form of
chlorophyll, which then allowed a simple form of
photosynthesis to occur.


D.1.7

• This caused an explosive rise in the levels of
atmospheric oxygen known as the oxygen
catastrophe.
• This had an irreversible effect on the subsequent
evolution of life.
• The remaining chemicals in the “chemical soup” in
the oceans were broken down into carbon dioxide
and oxidised sediments.


D.1.7

• In addition, a layer of ozone (O3) began to form in
the upper atmosphere.
• This protected the planet
from UV radiation from
the Sun and blocked the
production of new organic
chemicals in the
“chemical soup”.


D.1.8



D.1.8

• Grypania is ~2mm in diameter, so it is too big to be a
prokaryotic cell.
• Tappania is definitely too big and complicated to be
prokaryotic.
• Bangiomorpha had 3D structure! Definitely too
complicated to be prokaryotic!


D.1.8

• The oldest fossils of eukaryotic cells have been found
to be approximately 1.5 billion years old.
• The endosymbiotic theory from Lyn Margulis (1967)
tries to explain how eukaryotic cells may have
evolved.
• Endosymbiosis: the condition in which one organism
lives inside the cell of another organism
• Both cells benefit from this - the cells no longer can
live separately from each other

D.1.8


D.1.8

• Mitochondria and chloroplasts were once free living
bacteria cells:
– Mitochondria aerobic bacteria
– Chloroplasts photosynthetic bacteria
• These cells were “swallowed up” by other cells by
endocytosis  cells engulfed but not eaten


D.1.8


D.1.8

• Mitochondria:
– additional energy (aerobic respiration) and
receives protection

• Chloroplast:
– provide food by photosynthesis and receives
protection


D.1.8

• Prokaryotes are similar to mitochondria and
chloroplasts:
– Similar size
– Similar ribosomes (70S)
– Contain DNA that is different from the nucleus
– Surrounded by double membrane
– Formation of new organelles resembles binary
fission


D.1.8

• The four eukaryotic kingdoms are:
– Protoctista
– Fungi
– Plantae
– Animalia
• Eukaryotic cells have some advantages over
prokaryotic cells so the early eukaryotes survived
and proliferated
• Hence the wide diversity of species we know today!

D.1 origin-of-life-on-earth

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