A2.1 Origin of Cells

Explain the atmosphere of prebiotic earth

Oparin and Haldane suggested that the prebiotic Earth's atmosphere was significantly different than today.

Evidence suggests that Earth's prebiotic atmosphere was primarily composed of nitrogen, carbon dioxide and water vapor with smaller amounts of methane and hydrogen.

There was no oxygen gas (O2) or ozone (O3) present in the atmosphere, as oxygen gas was first released into the atmosphere by photosynthesis.

The lack of an ozone layer resulted in high levels of ultraviolet light entering the atmosphere.

The high concentrations of carbon dioxide resulted in higher temperatures due to the greenhouse effect.

What did Oparin and Haldane propose

Oparin and Haldane proposed that the energy available from ultraviolet light and higher temperatures (as well as lightning and volcanic activity) on the prebiotic Earth allowed the spontaneous generation of organic compounds.

Over time the organic molecules became more complex, eventually leading to self-replicating structures and to the first cells.

Organic compounds

carbon-containing molecules excluding oxides and carbonates.

Inorganic compounds

all the other compounds that are not organic.

State the three parts of the cell theory.

Cells are the basic unit of life.

All organisms are composed of at least one cell.

All cells come from pre-existing cells.

All living organisms:

Have at least once cell

Are capable of carrying out metabolism

Maintain homeostasis

Respond to stimuli

Are capable of reproduction

Grow and develop

Contain genetic information (in the form of DNA)

Explain why most scientists do not consider viruses to be alive.

Viruses do have genetic material (DNA or RNA), however they do not possess most of the characteristics of life.

Viruses do not have cells which carry out metabolism and homeostasis. Viruses cannot respond to stimuli, and they do not grow.

Viruses are not capable of reproducing themselves. They are replicated by host cells.

Cells are ___

highly complex structures that can currently only be produced by division of pre-existing cells.

Four things required for the development of the first cells

Catalysis

Self-replication of molecules

Self-assembly

The emergence of compartmentalization

Limitations of the pre-biotic earth experiments

However, it is impossible to know the exact conditions which existed on prebiotic Earth.

Scientists have used evidence to identify the likely components of the prebiotic atmosphere, but they will never conclusively know if they are correct. This makes it unlikely that the exact conditions of prebiotic Earth are replicated in experiments.

Additionally the first protocells did not leave any fossils, so we do not have any evidence of their actual make-up.

What did Miller and Urey do

carried out an experiment to test Haldane and Oparin's hypothesis that the macromolecules of life could have spontaneously generated on a prebiotic Earth.

Explain the significance of Miller and Urey’s experiment.

Miller and Urey attempted to simulate the conditions on a prebiotic Earth in a laboratory setting.

The gases methane, ammonia, and hydrogen along with water vapor represented the prebiotic atmosphere.

Water was used to model the oceans.

A condenser allowed water vapor to return to the liquid water.Energy was supplied through electrical sparks, simulating lightning, and a heat source to evaporate the water.

After one week the water became a brownish black.

Analysis of the water showed that many complex organic molecules, including amino acids, had been produced.

The experiment showed that at least some of the organic molecules required for life could be spontaneously generated under certain conditions.

Limitations of Miller Urey experiment

There remains debate on the actual atmosphere of prebiotic Earth.

The experiment did not produce all of the organic molecules required for life.

The simulation could not account for all conditions on the prebiotic Earth.

Strengths of miller urey experiments

Modelled prebiotic Earth and its atmosphere.

Demonstrated that molecules such as amino acids can be generated spontaneously under certain conditions.

The design of the experiment allow it to be replicated by other scientists.

What does cell metabolism require

the separation of the cytoplasm from the external environment. Cell membranes serve this function in modern cells.

Formation of fatty acids in experiments similar to miller urey experiment

Fatty acids have formed spontaneously in experiments similar to Miller and Urey's experiment.

Fatty acids spontaneously coalesce to form spherical bilayers (vesicles) when mixed with water.

It has been hypothesized that the first genetic material was trapped within a phospholipid vesicle forming a protocell.

Outline why RNA is presumed to be the first genetic material.

RNA is hypothesized to the first genetic material because:

RNA can store genetic information

RNA is capable of self-replication

RNA can catalyze reactions

Ribozymes (RNA molecules) in the ribosome catalyze the formation of peptide bonds during protein synthesis.

Last universal common ancestor

the most recent common ancestor for all organisms on Earth.

There may have been other forms of life at the same time as LUCA, but these forms of life became extinct as LUCA and its descendents outcompeted them.

Are there fossil remains for LUCA

no

What strongly suggests that all organisms have evolved from LUCA

However the fact that all organisms share the same genetic code, strongly suggests that all organisms have evolved from LUCA.

All living things share a number of genes, which are assumed to have been inherited from LUCA.

What do cladistics do

allows scientists to estimate the age of common ancestors by analyzing changes in the genetic code.

When did LUCA appear

approximately 4 billion years ago.

Life has continued to evolve for the last 4 billion years producing the biodiversity currently found on our planet.

How old are the oldest fossils of cells

3.5 billion years old

How have minerals and rocks around the oldest fossils been dated

using radioactive dating of uranium, which decays at a steady rate.

Molecular clocks:

Comparing the number of mutations in genes between species allows scientists to estimate when the two species diverged into separate species. Scientists have used this technique to estimate that LUCA existed approximately 4 billion years ago.

Evidence for the evolution of the last universal common ancestor in the vicinity of hydrothermal vents

Evidence suggests that LUCA evolved in the vicinity of hydrothermal vents. Hydrothermal vents are rich in organic compounds, such as methane, carbon dioxide, and hydrogen.

Experiments similar to Miller and Urey's show that the building blocks of life, such as proteins and nucleotides can form from these chemicals. Fossils of cyanobacteria have been discovered from ancient seafloor hydrothermal vent precipitates.

An analysis of genes common to all the three domains of life has identified genes that are highly conserved.

Some of these genes are involved in thermophilic metabolism, suggesting that LUCA may have evolved in a high temperature environment such as hydrothermal vents.