Reading: Chapter 5 Lecture 15. Prebiotic Chemistry, Pyrite, Clays, RNA World, Transition from Abiotic to Biotic World.

reading: Chapter 5

Lecture 15. Prebiotic Chemistry, Pyrite, Clays, RNA World, Transition from Abiotic to Biotic

World

Prebiotic Chemistry

1. Taking natural geochemicals, reacting them together, eventuallyproducing an early cell.

2. Darwinian Evolution.3. Last common ancestor that gives rise to all known Earth life.

Prebiotic chemistry likely produced amino acids, nucleicacids, and lipids.

Assembled to make early cells.

Concept of the Prebiotic Soup

Organic compounds dissolved in the oceans.A dilute ‘soup’.Organic compounds react when they encounter each other.

Earliest types of organisms would have consumed the organiccompounds in the soup as C and energy source.

Heterotrophs - use organic carbon as a carbon source.

Eventually soup would have been exhausted, leading to a crisis.

Heterotrophs would have had to evolve into autotrophs - can useCO2 as a carbon source.

Problems with the Prebiotic Soup

Major Problem with the Soup:Soup is dilute.Dilute solutions favor molecules breaking apart, not forming.

One Possible Solution:Evaporation could increase the concentration.Then they could react together to form more complex molecules.

To make sugar ribose: formose reactionNeed to polymerize formaldehyde in strong base

-high concentration of formaldehyde-formaldehyde with react with other stuff-will generate hundreds of sugars, only one is ribose

Problems with the Prebiotic Soup, cont.

To make bases found in nucleic acids (RNA, DNA):Oró-reaction using HCN-need high concentration of HCN-yields are low-products degrade under polymerization conditions

To form a nucleotide:have to hook up D-ribose, base, and phosphate.

To form a strand of DNA:have to polymerize nucleotides.

No way nucleotides would form spontaneously!

Amino acids would have formed more easily.

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Adenine

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Surfaces

Act as catalysts and help facilitate chemical reactions.

1. bind reactants - concentrate them so that they can reactmany surfaces will have a net charge

2. orient the reactants - can react fastersurfaces act as catalysts

3. chemistry of the surface different than in bulk solution4. surfaces can contain mineral catalysts

“Good” Surfaces:Would have been present and abundant on the early Earth.Those that bind and facilitate reactions with organic compounds.

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Pyrite

-FeS2

-“fool’s gold”-common mineral in

magmatic rocks-also common in sedimentary rocks

Gunter Wachtershauser patent lawyerin 1988 suggested that pyrite could have acted as a

a catalystproposed an early “Iron Sulfur World”organic compounds bound to (+)-charged pyritereacted together to create more sophisticated moleculeseventually encased the surface with lipidsthen evolved into cellular life

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Why Pyrite?

Cells use FeS clusters in metabolic proteins.Many organisms make pyrite as a product of metabolism.Pyrite can be readily oxidized - is reactive.

The formation of pyrite generates energy that can be coupled tothe formation of organic compounds.

Catalytic Cycles

Reactions occur on the surface start forming catalytic cycles.Cycles become more complicated with time.

Catalytic Cycles, cont.

Catalytic cycles eventually become similar to the chemicalreactions seen in cells.

Generate amino acids and nucleic acids.Lipids form to create a boundary with the environment.Eventually formed a cell.

Power of this approach:Can have an autotrophic origin of life (ancient form of metabolism)

Weakness:Don’t know if pyrite would catalyze the host of prebiotic reactions.

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Clays

Are also abundant, but in sedimentaryrocks.

Clays formed when igneous rocks reactwith water.

Layered structures.Layers can bind a number of substances.

- ions (K, Na)- water- organic compounds

Wide variety of clay minerals.

Clays are Natural Catalysts

Add amino acids, subject to heating and/or drying cycles.

Amino acids will polymerize into peptides.

Will also catalyze polymerization ofmodified RNA monomers.

Binding can solve problem of chirality.

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Lipid World

Organic solvent extracts of Murchison meteoriteProduces lipid compoundsMolecules are not as large as lipids found in cells.

Could have formed early lipid barriers forprimitive organisms.

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RNA World

First Proposed by Walter Gilbert in 1988.(Nobel Prize winner 1980 - method for sequencing DNA)

Prebiotic chemistry produced the building blocks of RNA.RNA molecules reacted together and replicated each other.

RNA world an intermediate stage in the origin of life.

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Why RNA?

DNA:Bases adenosine (A), guanine (G), thymine (T),

cytosine (C).Is an information carrier - genetic material of the cell.One strand is a template for the other strand.Very stable, linear molecule.

RNA:DNA ---> RNA ----> Protein (The Central Dogma)More complex roles:

-makes temporary copies of genes (mRNA)-adaptor molecules in protein synthesis (tRNA)-catalytic role in protein synthesis (rRNA)

Most roles are in protein synthesis.

ATP is energy currency of the cell.Many protein enzymes use RNA-like catalysts.

Very unstable, reactive molecule.Simpler (single-stranded).Not a linear molecule - highly folded.

Reactive molecule - ribozymes.Nobel Prize - 1989.Can act as a template.

RNA viruses have genomes madeof RNA instead of DNA. RNA genomesused to both carry genetic information andto synthesize proteins.

Why RNA?, cont.

RNA World

1. catalytic - react with one another2. templating - copy each other

RNA viruses have RNA genomeshave very high mutation ratesrapid rates of change

use nucleotides from the “soup” (??)

Problems with The RNA World

1. RNA is not very stable-breaks down with heat (>50˚C)-breaks down in the presence of Mg2+, Ca2+, Mn2+, Fe2+

2. The reactions RNA can catalyze are limited.3. Has to be some external source of nucleotides (the soup??)

RNA world would have eventually given rise to The DNA/Protein world.

DNA is much stabler.-less sensitive to heat-stable in the presence of Mg2+, Ca2+, Mn2+, Fe2+

Proteins with 20 amino acids can catalyze a much larger varietyof chemical reactions.

The RNA world hypothesis calls for a temporary phase in the origin of life.

reading: Chapters 3, 5

Lecture 16. Prokaryotes, Eukaryotes, and the Tree of Life, rRNA, Constructing Trees.

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Cassini Spacecraft found older terrainsand major fractures on moon Enceladus

Course crystalline ice which will degrade overtime.

Must be < 1000 years old!Organic compounds found in the fractures.Must be heated - required T > 100K (-173˚C)Erupting jets of water observed.Cause of eruptions not known….

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Mystery of Enceladus

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