This led to the first prokaryotic cells, and eventually to life as we know it.

This led to the first prokaryotic cells, and eventually to life as we know it.

This led to the first prokaryotic cells, and eventually to life as we know it.

These and other problems forced Swedish scientists in the 1960s: chemist L. Sillen and geologist M. Rutten – to reject the concept of “primary broth” as chemically unlikely.

Later, it was suggested that the primary atmosphere contained carbon dioxide in a relatively high concentration. Recent experiments using Miller’s installation, which contained a mixture of CO2 and H2O, and only trace amounts of other gases, gave the same results that Miller obtained.

Oparin’s theory has been widely accepted, but it does not answer the question of how the transition from complex organic matter to simple living organisms took place. In this aspect, the theory of biochemical evolution represents a general scheme acceptable to most biologists.

Oparin believed that the decisive role in the transformation of the inanimate into the living belonged to proteins. Due to the amphotericity of proteins, they can form colloidal hydrophilic complexes – to attract water molecules that form a shell around them. These complexes can be separated from the aqueous phase in which they are in suspension, and form an emulsion. The fusion of such complexes with each other leads to the separation of colloids from the environment – a process called coacervation.

Colloid-rich coacervates may have been able to metabolize with the environment and selectively accumulate various compounds, especially crystalloids. The colloidal composition of this coacervate obviously depended on the composition of the medium. The diversity of the composition of the “broth” in different places led to differences in the composition of coacervates and thus supplied raw materials for “biochemical natural selection”.

It is assumed that in the coacervates themselves, the substances entered into the following chemical reactions. At the same time there was an absorption by coacervates of metal ions and formation of enzymes. Lipid molecules “lined up” at the boundary between the coacervates and the medium, which led to the formation of a primitive cell membrane that provided the coacervates with stability.

Due to the inclusion in the coacervate of the primary molecule capable of self-reproduction, and the internal rearrangement of the lipid-coated coacervate, could form a primary cell. The increase in the size of the coacervates and their fragmentation, led to the formation of identical coacervates, which could absorb more components of the environment.

Such a sequence of events would lead to the emergence of a primitive self-reproducing heterotrophic organism, which would feed on the organic matter of the primary broth. This hypothesis of the origin of life is recognized by scientists, but for some it is questionable due to the large number of assumptions.

Astronomer Fred Hoyle has argued that the hypothesis is “as absurd and implausible as the claim that a hurricane that swept over a landfill could lead to the construction of a Boeing 747.” The most difficult point of this theory is the explanation of the emergence of the ability of living systems to self-reproduction. Hypotheses on this issue in the framework of this theory are unconvincing.

RNA world hypothesis

This hypothesis of the origin of living organisms has recently gained more and more supporters. Its essence is the assumption that the founders of living cells were RNA molecules, not proteins. Important for the development of this theory is the discovery of the phenomenon of self-replication (self-reproduction) of RNA molecules. It is believed that by self-replication, RNA molecules evolved into more complex cell formations.

The main problem of the hypothesis is the complexity of spontaneous synthesis of single RNA molecules, as well as their sequences.

Comparison of RNA (left) with DNA (right)

The RNA world hypothesis believes that RNA was actually the first form of life on Earth, which later developed a cell membrane around itself and became the first cell of prokaryotes.

Idea support

The phrase “RNA World” was first used by Walter Gilbert in 1986. However, the idea of ​​independent life based on RNA is much older and can be found in Karl Voez’s book “Genetic Code” [1]. Five years earlier, molecular biologist Alexander Rich of the Massachusetts Institute of Technology, in a publication on Nobel laureate physiologist Albert St. George, set out almost the same idea in his article. The RNA world hypothesis is based on the ability of RNA to remember, transmit, and duplicate genetic information, just as DNA does. In addition, RNA can also act as a ribozyme (an enzyme made from ribonucleic acid). Because RNA can reproduce on its own, performing the tasks of both DNA and proteins (enzymes), it is thought to have once been capable of independent living. Moreover, while nucleotides were not found in Miller-Urey’s experiments to reproduce conditions that existed on Earth in the Archean era, they were found in other experiments with such modeling, such as the experiments of Joan Oro. Experiments with ribozymes similar to viral RNA Q-beta have shown that simple self-replicating RNA structures can withstand even strong selection pressures (eg, terminators of chains of opposite chirality) [2].

The idea of ​​the world of RNA

 

Ribozyme structure, PDB 2GOZ.

RNA and DNA are made from multiple repeats of certain nucleotides, which are also called “bases” associated with the phospho-sugar “skeleton”. The RNA world hypothesis believes that free-floating nucleotides existed in the original soup. These nucleotides constantly formed bonds with each other, but the chains often broke because the binding energy was low.

However certain, sequences of base pairs have catalytic properties that actually reduce the energy of the chain, causing the nucleotides to stay together for a long time. When many long strands are created, more relevant nucleotides will be attracted, so the strands will form faster than they will break.

These nucleotide chains have been proposed as the first, primitive life forms. In the world of RNA, different forms of RNA competed with each other for free nucleotides and thus participated in natural selection. The most efficient RNA molecules, which were able to effectively catalyze their own reproduction, withstood this selection and evolved into modern RNA.

Competition between RNA could lead to cooperation between different RNA strands, paving the way for the formation of the first proto-cell. Eventually, RNA chains spontaneously evolved with catalytic properties that help bind together amino acids (peptide bonds). These amino acids could then aid in RNA synthesis by giving those RNA strands that serve as ribozymes an advantage in selection. Eventually DNA, lipids, carbohydrates, and all other organic chemicals came into play. This led to the first prokaryotic cells, and eventually to life as we know it.

The fragility of nucleic acids

At first glance, the hypothesis of the world of RNA may seem implausible, because in today’s world, large RNA molecules are inherently unstable and can be easily broken into nucleotides by hydrolysis. Even without hydrolysis, RNA eventually decomposes under the action of ultraviolet radiation, which significantly limits the life cycle of the “organisms” of the RNA world (Pääbo 1993, Lindahl 1993).

An alternative to the world of RNA – peptide nucleic acids, PNA (English PNA). PNA is much more stable than RNA and seems to be able to be synthesized in pre-biotic conditions faster than RNA, especially if the synthesis of ribose and the addition of phosphate groups are problematic. As other RNA alternatives, threo nucleic acid (TNA) and glycolic nucleic acid (GNA) have been proposed.

A fundamentally different alternative to RNA collection processes is proposed in the PAH world hypothesis, according to which polycyclic aromatic hydrocarbons (PAHs) could be catalysts for the formation of RNA structures.

This RNA molecule in the past could live longer than today. Ultraviolet radiation can also cause RNA to polymerize, while breaking down other types of organic molecules that could have the potential to catalyze the hydrolysis of RNA (ribonuclease), suggesting that RNA could be a much more common substance on Earth. However, this theory has not yet been experimentally confirmed, as it is based on a constant concentration of sugar-phosphate molecules.

Value

The RNA world hypothesis, if true, is important for determining life. Now the concept of life is largely defined in terms of DNA and proteins, in today’s world, DNA and proteins are the dominant macromolecules in a living cell, while RNA serves only as an auxiliary molecule in the creation of proteins according to the information encoded in DNA. But the RNA world hypothesis places RNA at the center of the origin of life, requiring the definition of life in terms of RNA and the strategies that RNA uses for its reproduction.

In 2001, the RNA world hypothesis received considerable support with deciphering the structure of the ribozyme, which showed that the key catalytic sites of the ribozyme are composed of RNA that has a 3-dimensional structure, and proteins play only a structural role in keeping the ribozyme together. In particular, the formation of a peptide bond that binds together amino acids to form proteins can be catalyzed by RNA. This discovery shows that RNA molecules may be capable of synthesizing the first proteins.

Difficulty

There is no plausible pre-biotic method for the synthesis of cytosine, one of the bases in modern RNA, because it is easily hydrolyzed.

Pre-biotic models in which nucleotides are created are incompatible with the conditions necessary for the formation of sugars (due to the high concentration of formaldehyde). Therefore, they must be synthesized in different places and then transferred to some one place. However, they do not react in water.

Anhydrous reactions readily bind purines to sugars, but only 8% of them combine the correct carbon atom on the sugar with the correct nitrogen atom on the base. Pyrimidines, however, will not react with ribose, even in anhydrous conditions.

In addition, the phosphates required for synthesis in nature are extremely rare because they precipitate easily. When phosphate is introduced, the latter must rapidly combine with the correct hydroxyl group of the nucleotide.

In order for nucleotides to form RNA, they must be activated themselves. Activated purine nucleotides form small strands on an existing pyrimidine RNA template, but this process is not the opposite because pyrimidine nucleotides do not polymerize so easily.

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