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Biogenesis - Formation of Life
One of the most argued about topics of science and philosophy is the question of the origin of life on planet Earth. There are several competing hypotheses such as the theory of spontaneous generation and the cosmozoic theory which states that life has an extra-terrestrial origin. And there are those who believe that DNA - the molecule of life - was responsible for getting the whole show on the road. However, there is another candidate, the cellular messenger boy that is RNA.
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Origin of Life
DNA is transcribed to RNA which is then translated to protein, and proteins build and maintain living organisms. However, if you consider DNA to be the molecule that originated life then there is a problem. Namely that DNA requires proteins to be actively replicated and this leads us to the conundrum of which came first - DNA or protein? It is a classic chicken and egg scenario.
For a molecule to take on the weighty responsibility of the one that started life, it has to be able to do two things. One is to replicate and the other is to catalyze reactions to enable this to happen.
When RNA was first proposed as the originator molecule there really wasn't much evidence to back the idea. Proponents of RNA argued that it could be formed more easily than DNA, that it could withstand harsher environments than DNA (such that would be found in the early Earth) and finally, that it could evolve into DNA.
However, this idea did not gain much traction until the 1980s when Sidney Altman & Thomas Cech discovered a reaction where RNA could cleave itself or act on itself without the participation of enzymes (proteins). Prior to this, these biochemical reactions were thought to be a function of protein enzymes only. These RNA enzymes were given the name ribozymes and earned their discoverers the Nobel Prize.
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RNA as the founding molecule for Life
The term "RNA World" was proposed by Walter Gilbert in 1986. He promoted the idea that the initial molecule that gave life could have been RNA because it can catalyze its own reactions without the need for other entities (proteins). He also argued that it could give rise to proteins and have the ability to store genetic information - an important trait for the molecule that promoted life. During the course of evolutionary time these proteins would help to synthesize DNA from RNA by a process known as reverse transcription.
RNA and DNA are very similar molecules - they are both nucleic acids - and for this reason RNA cannot easily be ruled out as the one that gave rise to life. As RNA can effectively be replicated, store information and transmit the information when required, it is a worthy competitor to DNA. Furthermore, the ability to catalyze its own reactions without the requirement of proteins makes the case of the RNA World a strong one.
However, the idea of RNA World has not enjoyed full scientific support. There are several compelling reasons that argue against it. First of all, storing complex information in RNA molecules is not an easy task. And secondly, the larger the RNA molecule the more fragile it becomes. Given the harsh environmental conditions billions of years ago, it may not have been able to promote life.
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Present State of RNA World
Although many scientists have provided great insights into the possibility of RNA as the origin of life, there is still no unified opinion about RNA World theory. Some argue that it was an intermediate molecule, between the first molecule and DNA. Research continues of course, as it is a tantalizing dilemma, a challenge for scientists to solve, keen to find out once and for all where we came from.
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Proceedings of the National Academy of Science, http://www.pnas.org/content/95/14/7933.abstract