RNA world hypothesis

The RNA world hypothesis is a theory that says that a world with life based on ribonucleic acid (RNA) as a universal building block for information storage and catalysis of chemical reactions preceded our present forms of life. It is a link between the hypotheses of chemical evolution, which explain the formation of organic molecules from inorganic compounds, and the advent of the first cellular life forms. As part of the RNA world hypothesis it is believed that free or cell-associated RNA in the context of the evolution of the information storage medium chemically stable deoxyribonucleic acid ( DNA) and detached from the functionally more flexible proteins was. As an indication of the existence of the RNA world ribozymes, such as the catalytically active ribosomal RNA (rRNA ) is considered, the evolutionary remnant of this period to constitute.

Concept

The concept of a postulated RNA world based on two fundamental properties of the RNA. On the one hand they like the DNA serves as a medium for the storage of genetic information. On the other hand the RNA as well as proteins capable of catalyzing chemical reactions.

Formation

Can be the basis for the development of the RNA world hypothesis be attributed to the results of the Miller - Urey experiment. In this experiment it was shown that under conditions that existed on Earth more than three billion years, could arise organic compounds from inorganic substances. Although RNA is a very complex molecule, which consists of ribonucleotide units, which in turn are condensation products of the ribose, a nucleobase and phosphate, their abiotic origin applicable in the soup as possible. Although a direct condensation is catalytically very expensive and therefore unlikely for an abiotic origin, however, it was shown experimentally that from the simple molecules were cyanamide, cyanoacetylene, glycolaldehyde, glyceraldehyde and phosphate, which are potential components of the primordial soup, can be formed in a few reaction steps ribonucleotides can.

Interestingly, phosphate in this reaction not only as the starting material (starting material ) in the esterification of the nucleoside to the nucleotide of importance. Rather, the presence of the phosphate appears to selectively control several sub-steps of the reaction by, for example, as a pH buffer and a catalyst, it functions with nucleophilic properties. Thus, the combinatorial diversity is suppressed, so that hardly any undesired byproducts.

Information storage

Due to their chemical and physical properties of the RNA is maintained for the older storage medium for genetic information as DNA. For example, can easily be formed by an aldol condensation of the RNA component ribose as opposed to deoxyribose of DNA. Like DNA, RNA is also able to join to double-stranded macromolecules. For long-term storage of information, the RNA is, however, the DNA inferior because this is error prone and thus energy intensive.

Catalysis

With the discovery that the ribosomal RNA of Tetrahymena eyelash animalcule can splice itself, evidence of catalytic properties of ribonucleic acids ( ribozymes) has been provided in 1982 for the first time. In addition, RNA molecules could be detected, which produce important components of cellular metabolism, including their own devices, and even those for identical reproduction ( replication ) are capable.

As part of the RNA world hypothesis is assumed that ribozymes were able to translate their genetic information through the synthesis of proteins (translation ), the triumph of the proteins as a function of carrier ushered. Proteins are superior in their catalytic properties and the catalytic rate of the RNA. This assumption was supported by the discovery that the key catalytic sites of ribosomes from RNA and not, as assumed previously found, of proteins.

Cellular RNA

For the development of the RNA world under the terms of Darwinian evolution, compartmentalisation is considered as an important prerequisite. It is believed that this first surrounded by a cell membrane hypothetical RNA-based life forms that Ribozyten rapidly advanced and basic cell functions, such as mass transport could control through the cell membrane and the cell shape.

Alternative hypotheses

Since the RNA is already a very complex molecule, alternative and complementary hypotheses for the development of life on Earth developed. These include, in particular, a possible role of peptide nucleic acids, nucleic acids and glycerol, threose nucleic acids. They are considered as a possible precursor of RNA, since they are built chemically simpler. For peptide nucleic acids could be demonstrated that they replicate themselves and can serve as a template of RNA. A formation of peptide nucleic acids in the primordial soup is considered possible. The suspected in the primordial soup polycyclic aromatic hydrocarbons are considered as a possible predecessor of the RNA.

The alternative hypothesis of panspermia is another possible explanation of the origin of complex shapes our lives dar. This largely regarded as purely speculative theory is that simple life forms over large distances moved by the universe or were brought and so the beginnings of life on the earth marked. However, since the question of the origin of life is thereby simply just moved from the earth to any other place in the universe, this hypothesis has ultimately to no basic explanation.

History

The Origins of the RNA world hypothesis can be applied to the published in 1953, the observations from the Miller - Urey experiment lead back, which proves the possibility of the formation of organic molecules in the primordial atmosphere. In his book " The Genetic Code" presented in 1967 Carl Woese before the hypothesis of a simple RNA -based life. Also Leslie Orgel described in 1968, the RNA as an important stage in the evolution. The term " RNA world " was used in 1986 by Walter Gilbert and has established itself since then.