Home > RNA world hypothesis
The RNA world hypothesis proposes that RNA was, before the emergence of the first cell, the dominant, and probably the only, form of life. The phrase "The RNA World" was first used by Walter Gilbert in 1986.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.
1 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 peptidesPeptides are the family of molecules formed from the linking, in a defined order, of various amino acids. The link between one amino acid residue and the next is an amide bond, and is sometimes referred to as a peptide bond. An amide bond is somewhat shor, 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 moleculesIn science, a molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. A molecule consists of multiple atoms joined by shared pairs of electrons in a covalent bond''. It may consist of atom are especially fragile, subject to hydrolysisHydrolysis is a chemical process in which a molecule is cleaved into two parts by the addition of a molecule of water. This is distinct from a hydration reaction, in which water molecules are added to a substance, but no cleavage occurs. Types Hydrolysis that degrades these long biopolymersA biopolymer is a polymer found in nature. Starch, proteins and peptides, and DNA and RNA are all examples of biopolymers, in which the monomer units, respectively, are sugars, amino acids, and nucleic acids. The exact chemical composition and the sequenc into their constituent monomericIn chemistry, a monomer (from Greek mono "one" and meros "part") is a small molecule that may become chemically bonded to other monomers to form a polymer. Examples of monomers are hydrocarbons such as the alkane, alkene, and alene ( homologous) series. 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 ultravioletNote: Ultraviolet is also the name of a 1998 UK television miniseries about vampires. Ultraviolet UV radiation is electromagnetic radiation of a wavelength shorter than that of visible light, but longer than that of soft X-rays. It can be subdivided into 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.
