a brief tour of life. calendar of events event origin of earth 1 st evidence of life (crystals only...
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Calendar of Events
EventOrigin of Earth1st Evidence of Life (crystals only produced by organisms)Fossils (Bacteria)Origin of Photosynthesis (Bacteria)Last Banded Iron FormationsOrigin of EukaryotesOrigin of Multicellular Organisms
Time (Gya)4.5-4.63.9-4.13.7-3.83.52.4-1.81.50.6
Origin of Life
1. Panspermia (pan = expanded; sperm = seed)• Life “seeded” from space• Did not originate on Earth
2. Life Started on Earth — 4 stagesa. Abiotic synthesis of simple organic moleculesb. Assembly of simple molecules into polymersc. Origin of self-replicating molecules (= inheritance)d. Packaging molecules inside membranes
Abiotic synthesis of simple organic molecules
2 possible sources• Extraterrestrial• Meteors• Comets• Evidence: found amino acids inside meteorites
• Synthesis on Earth• Miller-Urey experiment
Miller-Urey Experiment
• Purpose
• Gases
• Spark
• Results
• Conclusion
Test whether abiotic synthesis of organic molecules is possible
NH3, CH4, H2, H2O
Energy source—UV, lightening
After 1 week, brown soup containing simple organic molecules (some amino acids, building blocks of nucleotides)
Abiotic synthesis of simple organic molecules is possible
Origin of Life
1. Panspermia (pan = expanded; sperm = seed)• Life “seeded” from space• Did not originate on Earth
2. Life Started on Earth — 4 stagesa. Abiotic synthesis of simple organic moleculesb. Assembly of simple molecules into polymersc. Origin of self-replicating molecules (= inheritance)d. Packaging molecules inside membranes
Assembly of simple molecules into polymers
• At edges of warm pools, water evaporates• Warm: energy• Evaporation: concentrates molecules• Clays: catalytic• Result: polymers
• Problem? Intense UV• Why intense UV?
• Ozone (O3) absorbs UV• No ozone because no O2
• UV = high energy• Breaks down polymers faster than can form
• Alternatives?• Polymerization in protected areas:
hydrothermal vents, continental volcanoes
Origin of Life
1. Panspermia (pan = expanded; sperm = seed)• Life “seeded” from space• Did not originate on Earth
2. Life Started on Earth — 4 stagesa. Abiotic synthesis of simple organic moleculesb. Assembly of simple molecules into polymersc. Origin of self-replicating molecules (= inheritance)d. Packaging molecules inside membranes
Origin of self-replicating molecules (= inheritance)
• What is the replicator today?• 1st replicator not DNA. Why?
• DNA synthesis requires enzymes (= proteins)• Proteins synthesis requires DNA• Chicken & egg problem: Which came first?
• Conclusion• DNA/Protein system too complex• 1st replicator not DNA, but some simpler system
• What replicator came before DNA?• Unknown• Requirements: Same molecule must …
• Store information for its own synthesis (genetic information)• Catalytic activity to synthesize itself from monomers
• What molecule satisfies these requirements?
DNA
RNA
RNA: A Closer Look• Eukaryote Genes Contain
• Exons• Sequences coding for proteins
• Introns• ”Junk” sequences not coding
for protein• Removed before translation
• The Process• DNA transcribed to pre-mRNA
containing exons & introns• Pre-mRNA processed to mature
RNA transcript by …• Removing introns• Splicing exons
RNA Satisfies Requirements for Replicator• Stores genetic information
• Where?
• Has catalytic activity• Some RNA molecules catalyze reactions
mRNA self-catalyzes removal of its own introns
Sequence of nucleotides
RNA World• RNA Replicator
• Contains genetic information in its nucleotide sequence
• Catalyzes its own synthesis
• Was RNA 1st replicator?
• Too complex to synthesize abiotically
• But probably preceded DNA
Probably not
Origin of Life
1. Panspermia (pan = expanded; sperm = seed)• Life “seeded” from space• Did not originate on Earth
2. Life Started on Earth — 4 stagesa. Abiotic synthesis of simple organic moleculesb. Assembly of simple molecules into polymersc. Origin of self-replicating molecules (= inheritance)d. Packaging molecules inside membranes
Packaging molecules inside membranes
• When some lipids mixed with water, spontaneously form droplets
• Lipids outside; water inside• Called coacervates
Calendar of Events
EventOrigin of Earth1st Evidence of Life (crystals only produced by organisms)Fossils (Bacteria)Origin of Photosynthesis (Bacteria)Last Banded Iron FormationsOrigin of EukaryotesOrigin of Multicellular Organisms
Time (Gya)4.5-4.63.9-4.13.7-3.83.52.4-1.81.50.6
Review: Metabolic Pathways
Catabolic—break down• Cellular Respiration (aerobic)• Glycolysis• Link Reaction• Krebs Cycle• Oxidative Phosphorylation
• Fermentation (anaerobic)
Anabolic—build up (synthesize)• Photosynthesis• Light Dependent Reactions• Light Independent Reactions
Cellular Respiration: Overview
• Cytosol• Glycolysis
• Glucose Pyruvate
• Mitochondrial Matrix• Link Reaction
• Pyruvate Acetyl CoA• Krebs (Citric Acid) Cycle
• Acetyl CoA CO2
• Inner Membrane• Oxidative Phosphorylation
• Most ATP synthesis
• NAD+/NADH connects
Cellular Respiration: Oxidative Phosphorylation• NADH drops off
electrons on ETC• Electrons pass down
ETC, releasing energy• Energy used to pump
H+ to intermembrane space• Electrons move back
to matrix through ATP synthase, making ATP
Fermentation • In absence of O2 cannot break down pyruvate• Synthesize ethanol or
lactate to regenerate NADH
Photosynthesis: Overview
• Light Dependent Reactions• Thylakoid membrane• ETC & chemiosmosis
• Light Independent Reactions• Stroma• Fix carbon
Photosynthesis: ETC & Chemiosmosis• Photon boosts
electrons• Electrons pass down
ETC, releasing energy• Energy used to pump
H+ inside thylakoid• Electrons move back to
stroma through ATP synthase, making ATP
Early Metabolic Pathways
• Early Earth’s atmosphere contained no O2
• Organisms “ate” simple organic molecules made abiotically
• 1st metabolic pathways: • How do we know?• Most widespread in organisms• Inherited from common ancestor
Glycolysis & Fermentation
Photosynthesis
• After a while, “free lunch” over—organic molecules used up• Organisms needed source of food
• Idea: make food (organic compounds) from energy in sunlight• 2nd metabolic pathway:
• How do we know? • Fossilized Photosynthetic Prokaryotes• Banded Iron Formations
Photosynthesis
Fossilized Photosynthetic Prokaryotes
Fossilized Stromatolites (New York State)
Modern Stromatolites (Shark Bay, Australia)
made by cyanobacteria
Historical Atmospheric Oxygen Levels
• Photosynthesis began 3.5 Gya• But atmospheric O2 levels did
not begin to rise until 1.5 Gya
• Why the delay?• Where did the O2 produced
between 3.5-1.5 Gya go?
Banded Iron Formations• How Do We Know Early Earth’s
Atmosphere Contained No O2?• Fe2+ dissolved in oceans• In presence of O2, Fe2+ Fe3+
• Precipitates as Fe2O3 (rust, insoluble)• Produced banded iron formations
• So, Fe2+ absorbed all O2 as it was produced
• How much Fe2+? Enormous amount—continued for 1.5-2 Gy!
Calendar of Events
EventOrigin of Earth1st Evidence of Life (crystals only produced by organisms)Fossils (Bacteria)Origin of Photosynthesis (Bacteria)Last Banded Iron FormationsOrigin of EukaryotesOrigin of Multicellular Organisms
Time (Gya)4.5-4.63.9-4.13.7-3.83.52.4-1.81.50.6
Last Banded Iron Formations
• So, when last Fe2+
precipitated, atmospheric O2 levels began to rise• 1.5 Gya• Great Oxygenation Event!
Great Oxygenation Event
• Good, right?• All organisms at that time anaerobic (lived in absence of O2)• O2 highly reactive• Kills living organisms
• Result: catastrophic extinction
New Opportunities
• O2 imposed strong selection pressure• Any organism that tolerated O2 = at
evolutionary advantage
• Solution: detoxify O2 • 4H+ + 4e- + O2 (toxic) 2H2O (non-toxic)• Familiar?• So, ETC evolved to detoxify O2
electron transport chain of cellular respiration
New Opportunities
• 4H+ + 4e- + O2 2H2O releases LOTS of energy• Some bacteria coupled this
release of energy with ETC & ATP synthesis• Where from?
• 3rd metabolic pathway:
Photosynthesis
Cellular Respiration
Calendar of Events
EventOrigin of Earth1st Evidence of Life (crystals only produced by organisms)Fossils (Bacteria)Origin of Photosynthesis (Bacteria)Last Banded Iron FormationsOrigin of EukaryotesOrigin of Multicellular Organisms
Time (Gya)4.5-4.63.9-4.13.7-3.83.52.4-1.81.50.6
Origin of Eukaryotes
• History of life on Earth: >60% just prokaryotes• How did eukaryotes arise?• 2 theories• Autogenous Model• Endosymbiotic Model
Autogenous Model
• Auto = self; gen = produce, birth• Idea: Cell membrane folds in to form inner membrane structures• Used to explain origin of• Nuclear membrane• Endomembrane system (smooth ER, rough ER, Golgi apparatus, vesicles)
• Supporting evidence?• Nuclear membrane, ER, & cell membrane continuous• Nuclear membrane = double membrane (2 bilayers)
Endosymbiotic Model
• Endo = inside; sym = together; bio = living• Idea: • Large prokaryote engulfed
smaller prokaryote (endocytosis)• Smaller prokaryote survived =
parasite• Over time, two organisms
developed mutual dependence
Endosymbiotic Model• Smaller prokaryote = heterotroph (cellular respiration)• Became “leaky” to ATP• Mutual dependence
• Large cell provides home, sugar, O2
• Small cell provides ATP
• Organelle?
• Smaller prokaryote = autotroph (photosynthesis)• Became “leaky” to sugar• Mutual dependence
• Large cell provides home• Small cell provides sugar (food)
• Organelle?
Mitochondrion
Chloroplast
Endosymbiotic Model: Supporting Evidence?Chloroplasts & Mitochondria have …• DNA• Only organelles (except nucleus) with DNA• DNA similar to prokaryote DNA
• Not membrane-bound (in nucleus)• Single chromosome• Circular (eukaryote DNA = linear) (Fig 1)• Naked DNA = no histone proteins (eukaryote DNA = with histones) (Fig 2)
• As expected if were free-living prokaryotes
Fig 1
Fig 2
Endosymbiotic Model: Supporting Evidence?Chloroplasts & Mitochondria have …• Ribosomes• Contain own ribosomes• Ribosomes similar to prokaryote ribosomes
• Smaller ribosomes (larger ribosomes in eukaryotes)• (Remember, both eukaryote and prokaryote
ribosomes contain large and small subunits; however, the whole ribosomes is smaller in prokaryotes.)
• As expected if were free-living prokaryotes
Endosymbiotic Model: Supporting Evidence?Chloroplasts & Mitochondria have …• Double Membranes• Membrane chemical composition• Outer• Inner
• As expected if engulfed by endocytosis
: like prokaryote: like eukaryote