Viroids and Origins of Life Quiz

Origin of Life on Earth

• Africa, a material collected from an asteroid's surface, contains clues about the origin of life on Earth
• Synthesis at deep-sea hydrothermal vents: energy source is chemical gradient, and vent microbes are some of the most primitive living organisms (Archaea)
• Deep ocean was protected from meteorite bombardment and lethal radiation

Synthesis by Destruction

• Energy source: impact from meteorites
• Inorganic molecules in the presence of clay can be converted to organic molecules

Macromolecule Assembly

• Monomers → polymers
• Enzymes are responsible for assembly in cells today, but there's a "chicken-and-egg" problem
• Mineral surfaces such as clay act as catalysts and templates for formation of polymers
• Release of electrons to provide energy to form chemical bonds
• Act as catalysts to speed reactions
• Polypeptides and polynucleotides (RNA molecules) have been linked together on clay surfaces

Reproduction of Macromolecules

• Identical copies of macromolecules are necessary for continuity of life from one generation to the next
• RNA stores information, but some RNA can also act as a catalyst for chemical reactions (ribozymes)
• RNA World hypothesis: a single macromolecule could be both the genetic carrier and the catalyst to copy itself
• Natural selection: best replicators would produce greater numbers in each succeeding generation

System Assembly

• Machinery of life must be separate from its surroundings
• Building blocks are held in higher concentration to facilitate chemical reactions
• Research has shown that fatty acids and other molecules synthesized under prebiotic conditions can form lipid bilayers and spontaneously assemble into enclosed vesicles
• Supports primordial soup hypothesis

Development of Life on Earth

• Hypotheses on the origin of organic compounds:
  • Hypothesis 1: Organic molecules were synthesized from inorganic compounds in the atmosphere
  • Hypothesis 2: Organic molecules rained down on the Earth from outer space
  • Hypothesis 3: Organic molecules were synthesized at hydrothermal vents on the ocean floor
  • Hypothesis 4: Organic molecules were synthesized when comets or asteroids struck the early Earth

Oparin-Haldane Hypothesis

• Life arose from inorganic molecules: gradual chemical evolution
• Primordial soup: inorganic molecules + energy = building blocks such as amino acids and nucleotides
• Oxygen-poor atmosphere
• Building blocks combined to form larger, more complex molecules
• Polymers assembled into structures capable of sustaining and replicating themselves

Miller-Urey Experiment

• Tested the Oparin-Haldane hypothesis
• Recreated environmental conditions of early Earth
• Energy source: electricity to simulate lightning
• Produced organic molecules such as amino acids, sugars, and lipids

Exogenous Delivery

• Organic molecules seeded by comets, meteorites, space dust
• Evidence: Meteorite in Australia in 1969 contained dozens of different amino acids
• 3.3 billion year old extraterrestrial organic matter found in sediment in S. Viroids

Conditions on Early Earth

• Almost no atmosphere: volcanic gases such as hydrogen sulfide, methane, and carbon dioxide
• Intense solar radiation
• Strong volcanic activity
• Constant bombardment by meteorites and comets
• History of Earth:
  • Earth is ~4.5 billion years old
  • Scientists think Earth developed conditions for life ~4.3 billion years ago
  • Break in the asteroid bombardment
  • Cooler temps allowed condensation to form oceans
  • Second round of bombardment ~3.9 billion years ago
  • Oldest fossils are ~3.5 billion years old
  • Sulfur-metabolizing bacteria
  • Life may have evolved multiple times during the billion year gap