How did life begin?

Conditions on early Earth made the origin of life possible 4 main stages could have produced very simple cells: The abiotic synthesis of small organic molecules Joining of these small molecules into macromolecules (proteins, nucleic acids) Packaging of these macromolecules into protocells, droplets with membranes that maintained internal chemistry different from their surroundings Origin of self-replicating molecules that eventually made inheritance possible 2

Synthesis of Organic Cpds on early Earth Planets of our solar system formed ~ 4.6 billion yrs ago 1st few hundred million yrs conditions would not have allowed life on Earth

1st Atmosphere Collisions would have vaporized any water preventing seas from forming Atmosphere thick with gases released from volcanic activity

1st Atmosphere 1920’s: Oparin (Russian chemist) and Haldane (British scientist) each came to conclusion early atmosphere was reducing environment (gain e-) in which organic cpds could have formed from simpler molecules

In 1953, Stanley Miller and Harold Urey conducted lab experiments that showed that the abiotic synthesis of organic monomers (amino acids) in a reducing atmosphere is possible. Rain brings to earth and sea © 2011 Pearson Education, Inc. 6

Miller & Urey’s Experiment

1st Organic Compounds Energy sources: Lightening Thermal energy Intense UV radiation

The key building blocks of life are not hard to come by… Amino acids have also been found in meteorites RNA monomers have been produced spontaneously from simple molecules In water, lipids and other organic molecules can spontaneously form vesicles with a lipid bilayer Vesicles exhibit simple reproduction Resultprotocells © 2011 Pearson Education, Inc. 9

Miller & Urey’s Results

Miller & Urey’s Results Have been repeated using same or similar ingredients, different recipes for the atmosphere and they also produced organic compounds Still ?s about amounts of methane, ammonia (was there really enough to make it a reducing environment?) Some repeated experiment in non-reducing, non-oxidizing conditions & still produce organic cpds

Miller-Urey Experiment demonstrates: Abiotic synthesis of organic molecules is possible under various assumptions about the composition of Earth’s early atmosphere Meterorites may also have been source of minerals and organic molecules Contain amino acids, lipids, simple sugars, uracil

Murchison Meteorite Fell to Earth in so named town in Australia in 1969 large (100 kg) and was quickly retrieved 2010 article published in Scientific American: results of mass spectrometry (separating cpds based on charge & size) have revealed at least 14,000 unique molecules

Abiotic Synthesis of Macromolecules 2009 study showed the abiotic synthesis of RNA monomers can occur spontaneously from simpler precursor molecules Drip solutions with amino acids (aa) or RNA nucleotides onto hot sand, rock, or clay  polymers of aa & RNA (w/out using enzymes or ribosomes)

Protocells Basic characteristics of life : reproduction & metabolism: So 1st cells would have had to be able to reproduce which would have required them to have a source of nitrogenous bases, sugars, phosphate groups Now complex enzymes make this all happen

Vesicles as 1st step? When lipids & other organic molecules added to water  vesicles spontaneously form lipid bilayer (separation of hydrophiloic & hydrophobic molecules) These abiotically produced vesicles “reproduce” and grow on their own. clay like from early Earth will be absorbed into the vesicles some vesicles demonstrate semi-permeability

Self-Replicating RNA RNA can act as enzyme RNA catalysts called: ribozymes Some can make complimentary strands of short pieces of RNA  mutations  more stable &/or successful

Ribozyme Once self-replicating RNA possible much easier for further changes to happen. Once double-stranded DNA appeared it would have been more stable so RNA left with role we see today

an index of vesicle number Figure 25.3 0.4 Precursor molecules plus montmorillonite clay an index of vesicle number Relative turbidity, 0.2 Precursor molecules only 20 40 60 Time (minutes) (a) Self-assembly 1 m Vesicle boundary Figure 25.3 Features of abiotically produced vesicles. 20 m (b) Reproduction (c) Absorption of RNA 19

Why the 2 spikes in oxygen? Most atmospheric oxygen (O2) is of biological origin 1,000 100 10 1 (percent of present-day levels; log scale) Atmospheric O2 0.1 “Oxygen revolution” 0.01 Figure 25.8 The rise of atmospheric oxygen. 0.001 0.0001 4 3 2 1 0 Time (billions of years ago) Photosynthesis and the Oxygen Revolution 20

First: ancient cyanobacteria Second: eukaryotic cells containing chloroplasts © 2011 Pearson Education, Inc. 21

How did we get beyond bacteria? © 2011 Pearson Education, Inc. 22

The First Eukaryotes The endosymbiotic theory An endosymbiont is a cell that lives within a host cell Lynn Margulis 23

Plasma membrane Cytoplasm DNA Ancestral prokaryote Nucleus Endoplasmic Figure 25.9-1 Plasma membrane Cytoplasm DNA Ancestral prokaryote Nucleus Endoplasmic reticulum Nuclear envelope Figure 25.9 A hypothesis for the origin of eukaryotes through serial endosymbiosis. 24

heterotrophic eukaryote Figure 25.9-2 Plasma membrane Cytoplasm DNA Ancestral prokaryote Nucleus Endoplasmic reticulum Nuclear envelope Aerobic heterotrophic prokaryote Figure 25.9 A hypothesis for the origin of eukaryotes through serial endosymbiosis. Mitochondrion Ancestral heterotrophic eukaryote 25

heterotrophic eukaryote Ancestral photosynthetic Figure 25.9-3 Plasma membrane Cytoplasm DNA Ancestral prokaryote Nucleus Endoplasmic reticulum Photosynthetic prokaryote Mitochondrion Nuclear envelope Aerobic heterotrophic prokaryote Figure 25.9 A hypothesis for the origin of eukaryotes through serial endosymbiosis. Mitochondrion Plastid Ancestral heterotrophic eukaryote Ancestral photosynthetic eukaryote 26

What evidence would support an endosymbiotic origin of mitochondria and plastids? Inner membranes are similar to plasma membranes of prokaryotes Division is similar in these organelles and some prokaryotes These organelles transcribe and translate their own DNA Their ribosomes are more similar to prokaryotic than eukaryotic ribosomes © 2011 Pearson Education, Inc. 27