B.sc.(micro) i em unit 1.2 origin of life
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Transcript of B.sc.(micro) i em unit 1.2 origin of life
• Universe formed 15 billion years ago (Big Bang)
• Galaxies formed from stars, dust and gas
• Earth formed 4.6 billion years ago
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• Suns energy stripped away 1st atmosphere
• 2nd atmosphere formed from volcanic outgassing
• Primitive atmosphere: CO2, water vapor, lesser amts
of CO, N2, H2, HCl, and traces of NH3 and CH4 (3.5 bya)
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• O2 came in 1.5 bya
• Autotrophic Organisms: photosynthesis
• Another environmental change
• Result in evolution
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• 0.5 billion years ago
• Atmosphere O2 to 1% current
• Compare to present: 78% N2, 21% O2, 0.04% CO2, + trace gasses
• Relatively small, most single cell
• Start of multicellularity
• Increase in cell complexity5
Life began~ 3.5 bya
Organic molecules (C H O N P S) swimming in shallow seas
Stage 1: Abiotic synthesis of organic molecules such as proteins, amino acids and nucleotides
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Stage 4: packaging these molecules into pre-cells, droplets of molecules with membranes that maintained an internal chemistry
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MAJOR EPISODES IN THE HISTORY OF LIFE
– Earth was formed about 4.6 billion years ago.
– Prokaryotes
• Evolved by 3.5 billion years ago
• Began oxygen production about 2.7 billion years ago
• Lived alone for almost 2 billion years
• Continue in great abundance today
– Single-celled eukaryotes first evolved about 2.1 billion years ago.
– Multicellular eukaryotes first evolved at least 1.2 billion years ago.
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Resolving the Biogenesis Paradox
– All life today arises by the reproduction of preexisting life, or biogenesis.
– If this is true, how could the first organisms arise?
– From the time of the ancient Greeks until well into the 19th century, it was commonly believed that life regularly arises from nonliving matter, an idea called spontaneous generation.
– Today, most biologists think it is possible that life on early Earth produced simple cells by chemical and physical processes.
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A Four-Stage Hypothesis for the Origin of Life
– According to one hypothesis, the first organisms were products of chemical evolution in four stages.
– Stage 1: Abiotic Synthesis of Organic Monomers
• The first stage in the origin of life has been the most extensively studied by scientists in the laboratory.
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The Process of Science: Can BiologicalMonomers Form Spontaneously?
– Observation: Modern biological macromolecules are all composed of elements that were present in abundance on the early Earth.
– Question: Could biological molecules arise spontaneously under conditions like those on the early Earth?
– Hypothesis: A closed system designed in the laboratory to simulate early Earth conditions could produce biologically important organic molecules from inorganic ingredients.
– Prediction: Organic molecules would form and accumulate.
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The Process of Science: Can BiologicalMonomers Form Spontaneously?
– Experiment: An apparatus was built to mimic the early Earth atmosphere and included
• Hydrogen gas (H2), methane (CH4), ammonia (NH3), and water vapor (H2O)
• Sparks were discharged into the chamber to mimic the prevalent lightning of the early Earth
• A condenser to cool the atmosphere, causing water and dissolved compounds to “rain” into the miniature “sea”
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The Process of Science: Can BiologicalMonomers Form Spontaneously?
– Results: After the apparatus had run for a week, an abundance of organic molecules essential for life had collected in the “sea,” including amino acids, the monomers of proteins.
– Since Miller and Urey’s experiments, laboratory analogues of the primeval Earth have produced
• All 20 amino acids
• Several sugars
• Lipids
• Purine and pyrimidine bases (found in DNA, RNA & ATP)
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Stage 2: Abiotic Synthesis of Polymers
– Researchers have brought about the polymerization of monomers to form polymers, such as proteins and nucleic acids, by dripping solutions of organic monomers onto
• Hot sand
• Clay
• Rock
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Stage 3: Formation of Pre-Cells
– A key step in the origin of life was the isolation of a collection of abiotically created molecules within a membrane.
– Laboratory experiments demonstrate that pre-cells could have formed spontaneously from abioticallyproduced organic compounds.
– Such pre-cells produced in the laboratory display some lifelike properties. They:• Have a selectively permeable surface
• Can grow by absorbing molecules from their surroundings
• Swell or shrink when placed in solutions of different salt concentrations
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Inorganic compounds
Abiotic synthesisof organic monomers
Abiotic synthesisof polymers
Formationof pre-cells
Self-replicatingmolecules
Membrane-enclosed compartment
Complementarychain
Polymer
Organic monomers
Figure 15.UN0420
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Stage 4: Origin of Self-Replicating Molecules
– Life is defined partly by the process of inheritance, which is based on self-replicating molecules.
– One hypothesis is that the first genes were short strands of RNA that replicated themselves without the assistance of proteins, perhaps using RNAs that can act as enzymes, called ribozymes.
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From Chemical Evolution to Darwinian Evolution
– Over millions of years• Natural selection favored the most efficient pre-cells
• The first prokaryotic cells evolved
– Prokaryotes lived and evolved all alone on Earth for 2 billion years before eukaryotes evolved.• Are found wherever there is life
• Far outnumber eukaryotes
• Can cause disease
• Can be beneficial
– Prokaryotes live deep within the Earth and in habitats too cold, too hot, too salty, too acidic, or too alkaline for any eukaryote to survive.
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References:
• Images (1 to 4) : http://www.tutorvista.com/content/biology/biology-iii/origin-life/origin-life-steps.php
• Image 5: http://www.bio.utexas.edu/faculty/sjasper/Bio213/earthhist.html
• Image 6: http://en.wikipedia.org/wiki/Miller%E2%80%93Urey_experiment#mediaviewer/File:Miller-Urey_experiment-en.svg
• Image ( 7 & 8) : http://www.slideshare.net/gdot204/evolutionary-theory-11965379
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