Building a phenotype - cell theory

What key features comprise a cell

Genetic systems allowing hereditary, replication & metabolism, compartmentatlisation

What is cell theory

All organisms are composed of one or more cells

Cells are the basic unit of life

All cells arrive from pre-existing cells —> leading back to LUCA

Theory of how life evolved in the molecular soup

Hostile volcanic environment, frequent storms including UV light and lightning that would have been catalysing reactions of atmospheric gases

Key biological precursor molecules like amino acids, sugars, nucleobases form and fall into oceans

Condensation by chance into polymers

Possibility of a polymer catalysing own replication

Self-replicating polymer would dominance molecular soup and begin evolving

Problem with DNA as the first self-replicating molecule

DNA cannot replicate itself as it requires protein catalysts

Could RNA be the first self-replicating molecule

Is not strictly self-replicating as it requires another RNA molecule to act as a template, but one RNA can make a copy of another

Catalytic RNA (ribozymes) can catalyse synthesis of a new RNA molecule

2 problems with making RNA in a molecular soup

Dilution very weak, chance of two nucleotides meeting to form a chain eventually forming RNA is very small

RNA is unstable in water. Ocean favours hydrolysis and condensation is needed to convert nucleotides to RNA

Theory of where RNA could have originated

Clay mineral layers

Why would clay mineral layers be the perfect site for prebiotic RNA

All the molecules needed to make RNA (nucleotides, nucleobases, phosphates, sugars) bind to surface of clay-silicate minerals and become concentrated together in the layers - greater chance of molecules meeting

Repeated cycles of wetting and drying, condensation becomes favoured

Problems with RNA world theory

Dependence on the wet-dry cycle, really really slow

Lots of molecules, especially lots of sugars, would be present in primordial soup —> why is RNA made out of ribose specifically

Only ticks the box of hereditary, unclear how RNA would lead to cell membrane, metabolism, and cells 

What are membranes made out of and how

Amphiphile molecules - molecule with both polar and non-polar properties

They are self-assembling, spontaneously form in the presence of water (which is polar) 

Types of membrane structures

Micelles, vesicles, bilayers

Could phospholipids have formed prebiotically

No too complex, but simpler amphiphiles may have been present and self-assembled into vesicles and micelles

Evidence of membranes forming

Experiment where nucleotides and lipids heated and concentrated until total dehydration micelles formed and then fused with one another to create multilamellar films with nucleotides inside

Then after dehydration and rehydration cycles RNA molecules were formed, ending up with RNA inside vesicles

What conditions are needed for spontaneous metabolism

High temperature

High pressure

Redox active metals to act as catalysts (e.g. iron or nickel)

Why are alkaline smokers thought to be potential sources of life

They form due to exposure of the mantle to seawater, reaction called serpentinisation occurs forming new minerals - with minerals like iron, nickel

The reactions generates heat, hydrogen gas, hydroxide ions

There is high pressure at the bottom of the ocean

Sets up all the conditions for spontaneous metabolism - between hydrogen gas and carbon dioxide dissolved in the water

How are hydrogen gas and carbon dioxide made to react

CO2 reduced to organic hydrocarbons by the proton gradient, capturing electrons from H2. FeNiS and FeS act as catalysts

Proton gradient needs to be established so enzymes can capture electrons, done using an inorganic Fe/Ni wall with acidic seawater on the outside and alkaline hydrothermal fluid coming up

Problems with the alkaline smokers origin theory

Self-assembling molecules eventually form, wouldn’t they stop molecules interacting with the metals?

How did this basic chemistry evolve into non-enzymatic metabolism and complex molecules?

Possible process of cells forming from primitive membranes with amino acids and FeS crystals

Amino acids bind to FeS crystals

Some amino acids would be hydrophobic, could embed in cell membrane

Then act as catalyst to produce the proton gradient across the cell membrane, driving the reduction of CO2 into hydrocarbons and more amino acids and fatty acids produced

Fatty acids produced would increase the cell membrane, at one point would divide into two due to the size —> cell replication

Cells with the best catalysts (fastest growth) ‘selected’ for