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RNA world hypothesis

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The phrase "The RNA World" was first used by Walter Gilbert in 1986.

RNA world hypothesis states that RNA was, before the emergence of the first cell, the dominant, and probably the only, form of life.

This hypothesis is supported by RNA's ability to participate in the storage, transmission, and duplication of genetic information, similarly to DNA, coupled with its ability to act as a ribozyme (similar to an enzyme), catalyzing certain reactions. From the point of view of reproduction, molecules exist for two basic purposes: self-replication and catalysis assisting self-replication. DNA is capable of self-replication, but only assisted by proteins. Proteins are excellent catalysts, but fail to catalyze processes complex enough to recreate themselves, individually. RNA is capable of both catalysis and self-replication.

In the beginning was the base pair

The RNA World hypothesis holds that in the primordial soup[?] (more likely the primordial sandwich[?]), there were RNA and DNA base pairs floating around. Since it is of lower energy for base pairs to form a chain, this would happen with some regularity. However, these chains are not of much lower energy than free base pairs, so the chains would also break apart with some regularity. However, some sequences of base pairs have catalytic properties - catalytic properties which lowers the energy of that same chain being created. As more and more of these RNA chains are created, they catalyze the formation of yet more. This causes chains to form faster than they break down, creating a positive feedback loop.

These chains are proposed to be the first, primitive forms of life. In a RNA world, different forms of RNA compete with each other for free nucleotides, and are subjected to natural selection. The most efficient molecules of RNA, the ones able to catalyze their own reproduction, survived and originated the modern RNA. It's possible that competition between RNA favored the emergence of cooperation between RNA molecules opening the way to the formation of the first proto-cell. The ones with catalytic properties that affect amino acids lower the energy barrier to peptides, which would be used to assist the catalysis of RNA synthesis. Eventually, DNA, lipids, carbohydrates and all sorts of other chemicals are recruited into life. This leads to the first cells, and life as we know it.

At first glance, the RNA world hypothesis seems implausible given that in today's world large RNA molecules are especially fragile, subject to hydrolysis that degrades these long biopolymers into their constituent monomeric nucleotides. However, in today's world enzymes capable of catalyzing this hydrolysis called RNAses ("ar-en-ases") are ubiquitous, contributing to this fragility. In a pre-biotic world absent any protein, including RNAses, a given RNA molecule might have "lived" longer then than it can today. A recent study has shown that ultraviolet light can cause RNA to polymerize while at the same time breaking down other types of organic molecules, suggesting that RNA may have been a relatively common substance on early Earth.

Implications

The RNA world hypothesis, if true, has important implications for the very definition of life. We may be too ready to define life in terms of DNA and proteins. After all, in today's world, DNA and proteins seem to be the dominant macromolecules in the living cell, with RNA serving, for the most part, as a mere messenger between them. But the RNA world hypothesis places RNA at center-stage when life originated, and therefore requires that we define life primarily in terms of RNA, and only secondarily in terms of DNA and proteins. If the RNA world hypothesis is true, life can be defined as the set of strategies that RNA polynucleotides have used and continued to use to perpetuate themselves.

In 2001, the RNA world hypothesis was given a major boost with the deciphering of the 3-demensional structure of the ribosome, which revealed that at the key catalytic sites, the ribosome was composed of RNA, with proteins playing only a structural role in holding the ribosomal RNA together. Specifically, the formation of the peptide bond, the reaction that binds amino acids together into proteins, is catalyzed solely by RNA. This finding suggests that RNA molecules were responsible for (or at the very least capable of) generating the first proteins.

See also PNA, another form of nucleic acid which might have formed the original genetic material.

See also
Origin of Life
Autocatalytic set

(See "SELF-REPLICATION: Even peptides do it (http://www.santafe.edu/sfi/People/kauffman/sak-peptides) By Stuart A. Kauffman")



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