MILLER AND UREY EXPERIMENT: DESCRIPTION AND IMPORTANCE - BIOLOGY - 2025

The Miller and Urey experiment consists of the production of organic molecules using simpler inorganic molecules as starting material under certain conditions. The objective of the experiment was to recreate the ancient conditions of planet Earth.

The intention of said recreation was to verify the possible origin of the biomolecules. Indeed, the simulation achieved the production of molecules - such as amino acids and nucleic acids - essential for living organisms.

Before Miller and Urey: Historical Perspective

The explanation of the origin of life has always been an intensely debated and controversial topic. During the Renaissance it was believed that life originated suddenly and out of nowhere. This hypothesis is known as spontaneous generation.

Later, the critical thinking of the scientists began to germinate and the hypothesis was discarded. However, the question posed at the beginning remained unclear.

In the 1920s, scientists of the time used the term "primordial soup" to describe a hypothetical oceanic environment in which life probably originated.

The problem was in proposing a logical origin of the biomolecules that make life possible (carbohydrates, proteins, lipids and nucleic acids) from inorganic molecules.

Already in the 1950s, prior to the Miller and Urey experiments, a group of scientists succeeded in synthesizing formic acid from carbon dioxide. This formidable discovery was published in the prestigious journal Science.

What did it consist of?

By 1952, Stanley Miller and Harold Urey designed an experimental protocol to simulate a primitive environment in an ingenious system of glass tubes and electrodes of their own construction.

The system consisted of a flask of water, analogous to the primitive ocean. Connected to that flask was another with the components of the supposed prebiotic environment.

Miller and Urey used the following ratios to recreate it: 200 mmHg of methane (CH ₄), 100 mmHg of hydrogen (H ₂), 200 mmHg of ammonia (NH ₃), and 200 ml of water (H ₂ O).

The system also had a condenser, whose job was to cool the gases as rain normally would. Likewise, they integrated two electrodes capable of producing high voltages, with the aim of creating highly reactive molecules that would promote the formation of complex molecules.

These sparks sought to simulate possible lightning bolts and lightning from the prebiotic environment. The apparatus ended in a “U” shaped part that prevented the steam from traveling in the reverse direction.

The experiment received electric shocks for a week, at the same time that the water was heated. The heating process simulated solar energy.

Results

The first days the experiment mixture was totally clean. Over the days, the mixture began to take on a reddish color. At the end of the experiment, this liquid took on an intense red almost brown color and its viscosity increased remarkably.

The experiment achieved its main objective and complex organic molecules were generated from the hypothetical components of the early atmosphere (methane, ammonia, hydrogen and water vapor).

The researchers were able to identify traces of amino acids, such as glycine, alanine, aspartic acid and amino-n-butyric acid, which are the main components of proteins.

The success of this experiment contributed to other researchers continuing to explore the origin of organic molecules. By adding modifications to the Miller and Urey protocol, the twenty known amino acids were recreated.

Nucleotides could also be generated, which are the fundamental building blocks of genetic material: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

Importance

The experiment managed to experimentally verify the appearance of organic molecules and proposes a quite attractive scenario to explain the possible origin of life.

However, an inherent dilemma is created, as the DNA molecule is required for protein and RNA synthesis. Let us remember that the central dogma of biology proposes that DNA is transcribed into RNA and this is transcribed into proteins (there are known exceptions to this premise, such as retroviruses).

So how are these biomolecules formed from their monomers (amino acids and nucleotides) without the presence of DNA?

Fortunately, the discovery of ribozymes managed to clarify this apparent paradox. These molecules are catalytic RNAs. This solves the problem since the same molecule can catalyze and carry genetic information. This is why there is the hypothesis of the primitive RNA world.

The same RNA can replicate itself and participate in the formation of proteins. DNA can come in a secondary way and be selected as a molecule of inheritance over RNA.

This fact could occur for several reasons, mainly because DNA is less reactive and more stable than RNA.

Conclusions

The main conclusion of this experimental design can be summarized with the following statement: complex organic molecules could have their origin from simpler inorganic molecules, if they are exposed to the conditions of the supposed primitive atmosphere such as high voltages, ultraviolet radiation and low oxygen content.

Furthermore, some inorganic molecules were found that are ideal candidates for the formation of certain amino acids and nucleotides.

The experiment allows us to observe how the building blocks of living organisms could have been, assuming that the primitive environment conformed to the conclusions described.

It is very likely that the world before the appearance of life had more numerous and complex components than those used by Miller.

Although it seems implausible to propose the origin of life starting from such simple molecules, Miller was able to verify it with a subtle and ingenious experiment.

Criticism of the experiment

There are still debates and controversies about the results of this experiment and about how the first cells originated.

It is currently believed that the components that Miller used to form the primitive atmosphere did not match the reality of it. A more modern view gives volcanoes an important role and proposes that the gases that these structures produce minerals.

A key point of Miller's experiment has also been called into question. Some researchers think that the atmosphere had little impact on the creation of living organisms.

References

1. Bada, JL, & Cleaves, HJ (2015). Ab initio simulations and the Miller prebiotic synthesis experiment. Proceedings of the National Academy of Sciences, 112 (4), E342-E342.
2. Campbell, NA (2001). Biology: Concepts and relationships. Pearson Education.
3. Cooper, GJ, Surman, AJ, McIver, J., Colón ‐ Santos, SM, Gromski, PS, Buchwald, S.,… & Cronin, L. (2017). Miller – Urey Spark-Discharge Experiments in the Deuterium World. Angewandte Chemie, 129 (28), 8191-8194.
4. Parker, ET, Cleaves, JH, Burton, AS, Glavin, DP, Dworkin, JP, Zhou, M.,… & Fernández, FM (2014). Conducting Miller-Urey experiments. Journal of visualized experiments: JoVE, (83).
5. Sadava, D., & Purves, WH (2009). Life: The Science of Biology. Panamerican Medical Ed.