Biology - A2.1 HL Origin of Cells

What were the conditions of early earth?

• Higher temperatures (75°-95°)

• Only 1 ocean with no solid masses

• Many bombardments of comets and asteroids

• Higher concentration of carbon dioxide and methane

• No ozone layer because there was no free oxygen available

Definition of cells

Cells are the smallest unit of self-sustaining life.

• viruses are not considered to be living as they can’t reproduce independently which is one of the 8 life processes

Spontaneous origin of cells - process

1) Simple, carbon-based compounds were formed

2) The reducing gases in the atmosphere could donate electrons so more complex compounds were formed (e.g. amino acids and hydrocarbons). This is catalysis.

3) Larger organic molecules such as phospholipids and RNA were constructed

4) These molecules could self-replicate

5) There was formation of a cell-surface membrane so there was a membrane bound compartment that had it’s own microenvironment.

Difficulty with explanation of uncertainty of cells

• The first protocells did not fossilize

• Evidence became broken or destroyed because it was so long ago, is also thought to have been in aquatic environment (the sea) which is difficult to reach

• The exact conditions of early / pre-biotic Earth are still unsure

Evidence for the spontaneous creation of cells

Miller-Urey Experiment

How did the Miller-Urey experiment work?

They simulated earth’s pre-biotic atmosphere with water, gas in-let with reducing gases and electrical sparks. The water was also cooled to condense.

After 1 week, there were basic organic monomers.

Proved that in these conditions organic molecules were created but it didn’t prove that the carbon molecules did definitely originate in this manner.

Formation of vesicles

Fatty acids spontaneously coalesced (merged) to form a spherical bilayer of lipids and an enclosed space

A microenvironment was created with specific conditions (e.g. concentration of solute and pH)

Why is RNA presumed to be the first genetic material?

It is both a genetic material and a catalyst.

1) RNA formed from inorganic sources, self-assembly from monomers to polymer

2) It was able to be self-replicated through the use of ribozymes

3) It could catalyse protein synthesis

4) Once the plasma membrane was formed (compartmentalisation), DNA and proteins were made from RNA

What is LUCA?

Last Universal Common Ancestor

A single-celled autotrophic (can make ATP by itself) microorganism that lived aorund 3.5 to 4.5 billion years ago

Evidence of LUCA

• the universal genetic code that can be seen in all organisms

• shared genes across all organisms, which can be seen in the molecular phylogenetic/genomic analysis and 2/3 genes were present and transferred from LUCA.

• it is believed that LUCA outcompeted other life forms

When was LUCA alive and how do we know?

3.5 - 4.5 billion years ago (Gya)

• Stromatolites (layered rock from from Australia) were analysed and resulted in originating from around 3.42 Gya so LUCA was before as stromatolites aren’t the simplest life form

• Isotope ratios of rocks can be used as the ratio of C12 or C13.

• A molecular clock, as gene mutations happen at relative constant rate so extrapolation back to ancestral gene and LCUA

LUCA evolution in the vicinity of hydrothermal vents

Hydrothermal vents: hot water, high conc of hydrogen, and other reducing gases

Evidence:

• Hydrothermal vents are the oldest fossils found to date, dating back to 3.7 Gya

• Means LUCA did anaerobic respiration/metabolism which was one of the genes found that LUCA had