D.1 Origin of Life on Earth
1. Describe four processes needed for the spontaneous origin of life on Earth
Process 1: producing organic molecules: synthesis of simple organic molecules from environmental precursors
- chemical reactions to produce simple organic molecules, such as:
- amino acids: 20 types
- nucleotides: purines & pyrimidines
- monosaccharides: glucose, ribose
- fatty acids, glycerol
- from inorganic molecules, such as:
- water
- carbon dioxide
- ammonia
Process 2: polymerization: assembly of simple organic molecules into polymers
- polypeptides from amino acids
- nucleic acids from nucleotides
Process 3: forming a genetic material: formation of polymers that can self-replicate
- RNA has two key abilities that make it the likely original genetic material
- genetically: self-replication
- RNA has been experimentally shown to have the ability to self-replicate
- RNA nucleotide sequence is variable,
- allowing for inheritance of information coding for amino acid sequences in polypeptides
- enzymatically: catalyzing chemical reactions
- RNA ribozymes are found in modern cell
- RNA almost certainly preceded DNA as the genetic material = "RNA world"
Process 4: producing membranes: packaging the above molecules inside membranes creating an internal chemistry different from their surroundings, including polymers that held the genetic information
- coacervates
- a spherical aggregation of lipid molecules making up a colloidal inclusion
- held together by hydrophobic forces
- form spontaneously from certain dilute organic solutions
- 1 to 100 micrometers in diameter
- possess osmotic properties
- microspheres
- small spherical aggregations of proteins
- 2 micrometers in diameter
- form spontaneously from heated and cooled amino acids
- exhibit some properties associated with life:
- response to the environment
- basic metabolism
- simple reproduction
- liposomes
- a spherical vesicle composed of a bilayer membrane
- form spontaneously from phospholipids
Result: "protobionts:" the product of the above four processes is likely to have been cell-like structures
- natural selection is likely to have acted on variants of protobionts competing for resources
- selecting for:
- stability: homeostasis, produced by enzymes controlling metabolic reactions
- longevity: survivorship
- fidelity: transmitting genetic information with minimal error
- fecundity: rate of reproduction
2. Outline the experiments of Miller and Urey into origin of organic compounds
Simulate reducing atmosphere
- Miller/Urey: water vapor, hydrogen, methane, ammonia
- Others: various mixtures w/ carbon dioxide, carbon monoxide, nitrogen, phosphates, etc., and even small amounts of oxygen
Simulate high energy sources
- Miller/Urey: electric spark simulates lightning
- Others: UV radiation, heat, etc.
Products
- Miller/Urey: mixture of amino acids
- Others: various mixtures including all 20 amino acids, sugars, lipids, purine and pyrimidine bases of DNA and RNA nucleotides, ATP
3. State that comets may have delivered organic compounds to Earth.
- comets contain a variety of organic compounds
- Murchison meteroite contents are similar to the organic compounds produced by the Miller/Urey experiments
- heavy comet/meteorite bombardment of Earth about 4 billion years ago
- could have supplied earth with organic compounds and water
Figure represents the abundance of various amino acids in the Murchison meteorite. Each number represents an amino acid: 1=alanine
4. Discuss possible locations where conditions would have allowed the synthesis of organic compounds.
- deep-sea hydrothermal vents
- energy source: heat
- spontaneously produces reduced compounds such as iron sulfide
- which can be oxidized to synthesize organic molecules
- provides a source of energy for assembly of polymers from monomers
- volcanoes
- extraterrestrial sources
5. Outline two properties of RNA that would have allowed it to play a role in the origin of life.
- RNA has two key abilities that make it the likely original genetic material
- genetic: self-replication
- RNA has been experimentally shown to have the ability to self-replicate
- individual RNA nucleotides self-assemble into RNA polymers
- RNA polymers attract complementary nucleotide bases
- transmitting genetic information between genertions
- RNA nucleotide sequence is variable,
- allowing for inheritance of information coding for amino acid sequences in polypeptides
- enzymatic: catalyzing chemical reactions
- RNA can act as an enzyme, catalyzing various reactions, producing polymers from monomers
- in eukaryotic organisms today, RNA regulates numerous cellular functions, including protein synthesis and genetic control
- for example, RNA ribozymes are found in modern cell
- Therefore, RNA almost certainly preceded DNA as the genetic material = "RNA world"
- RNA, like DNA, is a sequence of nucleotides that can carry a genetic code
- RNA is structurally simpler than DNA
- RNA can self-assemble from nucleotides available from the environment
- RNA can self-replicate using an existing RNA molecule as a template, adding free nucleotides available from the environment
- copying mistakes = mutations
- RNA can enzymatically catalyze metabolic reactions
- competition between various RNA varieties selects for most efficient variety
6. State that living cells may have been preceded by protobionts, with an internal chemcial environment different from their surroundings.
Origins of prokaryotic cells
- random collections of organic molecules do not qualify as living, whereas cells do
- life organized as cells requires both organization and stability over time
- cellular structure allows for compartmentalization
- protobionts form compartments which in turn might contain molecules such as RNA capable of reactions, catalysis and replication
- selection would favor the most successful protobionts in terms of stability/survivorship and replication/progeny
- genetic material, probably RNA, would be needed as information for control which could be copied to progeny
- protobionts
- possible origin of membranes
- from aggregates of abiotically produced organic molecules
- coacervates
- a spherical aggregation of lipid molecules making up a colloidal inclusion
- held together by hydrophobic forces
- form spontaneously from certain dilute organic solutions
- 1 to 100 micrometers in diameter
- possess osmotic properties
- microspheres
- small spherical aggregations of proteins
- 2 micrometers in diameter
- form spontaneously from heated and cooled amino acids
- forms a selective membrane
- maintains an osmotic potential
- liposomes
- a spherical vesicle composed of a bilayer membrane
- form spontaneously from phospholipid molecules in turbulent water
- self-assemble into a lipid bilayer forming membrane-bound droplets
- capable of growth and division
- Result:protobionts
- spherical structures containing RNA
- maintaining an internal chemical environment different from their surrounding
- natural selection is likely to have acted on variants of protobionts competing for resources
- selecting for:
- stability: homeostasis, produced by enzymes controlling metabolic reactions
- longevity: survivorship, produced by combinations of metabolic enzymes
- fidelity: transmitting genetic information with minimal error
- fecundity: rate of reproduction
7. Outline the contribution of prokaryotes to the creation of an oxygen-rich atmosphere.
- early cells competing for energy sources are likely to have provided a selection pressure favoring the evolution of photosynthesis
- some photosynthetic prokaryotes use sources of electrons other than water
- photosynthetic prokaryotes that live today in hot springs use hydrogen sulfide as an electron source
- photosynthetic prokaryotes that use water as an electron source are highly successful and produce large quantities of atmospheric oxygen through photolysis of water
- rocks in Greenland dated from 3.7-3.8 billion years ago, called banded iron formations, provide evidence of atmospheric oxygen, suggesting the presence of photosynthetic prokaryotes
- many species of photosynthetic prokaryotes survive today
- cyanobacteria contribute large amounts of oxygen to the oceans and atmosphere
- initially the oxygen produced by photosynthetic prokaryotes would have been consumed in chemical reactions, or remain dissolved in the oceans
- with additional photosynthesis beyond this level, oxygen would likely have accumulated in the atmosphere
- with the emergence of eukaryotes with chloroplasts, the rate of oxygen production likely increased
- further oxygenating the atmosphere
- since oxygen is toxic to some organisms, it is likely to have selected against some species, leading to their extinction
- an abundance of oxygen is likely to have lead to oxidative cellular respiration
8. Discuss the endosymbiotic theory for the origin of eukaryotic cells.
- eukaryotic cells appear to have evolved from prokaryotic ancestry as a smaller prokaryote within a larger prokaryote
- in other situations besides endosymbiosis, the outcome would be either:
a. larger host cell digests smaller invader
b. smaller invader multiplies and kills larger host
- with endosymbiosis there is a third outcome: coexistence
- coexistence is only likely if it is mutually beneficial
- a scenario of mutuall beneficial coexistence:
a. larger host cell: Eater consuming other, smaller cells
restricted to anaerobic environments
b. smaller invader: Eliminator eliminates oxygen (poison)
thus occupying environments where Eater is absent
c. if Eater consumes eliminator, and eliminator avoids digestion,
eater benefits from low oxygen levels & eliminator benefits
from predation avoidance
d. the resulting host becomes the eukaryotic cells with
eliminator as mitochondria
e. a similar process involves coexistence with an additional
consumed cell, photosynthetic Sunshine
f. sunshine produces oxygen as a byproduct and therefore
benefits from coexistence with mitochondria/eliminator
g. sunshine also benefits from protection within eater, who
benefits from the food produced by sushine/chloroplast
- Evidence for endosymbiotic origin of eukaryotic cells
a. mitochondria & chloroplasts both have double membrane
the second outer membrane from the host, eater
b. mitochondria & chloroplasts both have a loop of naked DNA
lacking histone proteins, as do prokaryotes
c. mitochondria & chloroplasts both divide by binary fission
independent of nuclear division
d. mitochondria & chloroplasts both have smaller 70S ribosomes
similar to prokaryotes, & different from 80S eukaryotic
ribosomes
e. chloroplast thylakoids are similar to cyanobacterial
photosynthetic structures
f. chlorophyll a is the main photosynthetic pigment for both
chloroplasts and prokaryotes
g. mitochondrial cristae are similar to bacterial mesosomes
D.2 Lect. Notes
