A2.1 - origin of cells

Conditions of early earth

• 3.5 - 3.9 billion years ago

• 75 - 95 degrees

• single global ocean

How was water and compounds brought to the planet

Comets and asteroids

Collisions released ammonia gases and methane, also by volcanic eruptions

What gases formed early atmosphere

• ammonia gases

• methane

also contained water vapour and CO2

What did early atmosphere lack

• lacked O2, no ozone layer

The inner core was a lot hotter and there was more UV radiation (Due to liquid core resulting in smaller protective magnetic field)

What is reducing gases in the atmosphere

• able to donate electrons to other molecules, enabling chemical reactions

  • reactions resulted in formation of carbon compounds (simple amino acids and hydrocarbons)

    • eventually formed proteins, lipids and nucleic acids

What happened with more complex carbon compounds

• came from simpler carbon compounds

developed ability to self-replicate & become packaged into membranes

• enable different environments, leading to cells

• provide structural and functional components for cells to survive

Why could there no longer be spontaneous biological molecules

changed environmental conditions

What components are necessary to carry out life processes (8 of them)

• metabolism

• response to stimuli

• homeostasis

• movement

• growth

• reproduction

• excretion

• nutrition

Why are viruses non-living

they are unable to reproduce outside of host

What are necessary steps for spontaneous origin of cells

1. simple organic molecules formed (A.A and hydrocarbons)

2. Chemical reaction accelerates - catalyst

3. larger organic molecules formed , polymers,- including RNA & phospholipids assembled after smaller molecules

4. some molecules, including RNA, able to self replicate

5. Formation of membrane bound compartment allowed internal chemistry of cell to become dif than outside

Difficulties of spontaneous origin of cells

• happened a long time ago

• first protocells didnt fossilise

• thought to have origianted deep in the ocean, harder to get samples to analyse

• uncertainty of exact prebiotic condition, hence can’t replicate them

What was the miller-urey set up

• water, stimulated the ocean

• gas inlet adding reducing gases, methane, ammonia and hydrogen

• electrical sparks stimulating electrical storms

How did miller urey experiment work

• water was vaporised and run through gases and electrical sparks, cooling jacket used to condense water

• condensed water dropped to bottom, representing primordial soup (water based sea of organic molecules)

• after a week, soup contained organic molecules

Miller urey experiment diagram

important step in first formation of cell

• formation of membrane bound compartment, from fatty acid into spherical bilayer

• allow for interior to be different to exterior, cell can control and maintain conditions needed

What did RNA serve as

• basis for cell like structure

• genetic material

• catalyst

Step of RNA becoming first genetic material

1. RNA formed from inorganic sources

2. Able to replicate using Ribozymes

3. Able to catalyse protein synthesis

4. Membrane compartmentalisation occurred

5. Inside cell, RNA produced protein and DNA

6. DNA took over, more stable

7. Protein took over as enzymes because more capable of variability

Evidence to support RNA hypothesis

• Short RNA sequences able to duplicate, replication

• RNA has catalytic activity

• Ribozymes in ribosome still used to catalase peptide bond formation

Evidence for LUCA

• genetic code is universal

• Bacteria and archea arose from LUCA, eukaryotes from endosymbiosis

Horizontal gene transfer - LUCA

• scientists focused on conserved genes, in bacteria and archea which don’t seem to have undergone horizontal gene transfer

  • found to have 355 genes presumed to have been in LUCA

Stromatolites

fossils found within rocks that thought to have been formed by layered communities of microorganisms

Features and characteristics of LUCA

• Existed between 2.5 - 3.5 mil years ago

• deep in ocean, in hydrothermal vents, rich in hydrogen and minerals, energy source

• anaerobic, fits with lack of O2

• autrophic, combining C and H