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The Origin of Life
An RNA World first?
It is now widely agreed that at the origin of life there was not the current DNA/(RNA)/protein system for gene information on one hand and catalysis, regulation, and structural function on the other. It would beg the question, what came first, protein or DNA? Protein catalysis without gene information, which allows it to be maintained and propagated, is not sufficient in the long term, and DNA gene information without catalysis, necessary for the function of life, would be useless as well.
Instead, it is assumed that RNA acted as a precursor of both protein and DNA, in the sense that it can serve both as catalyst (like protein enzymes) and as carrier of genetic information (like DNA, RNA is a polynucleotide). Even in the modern cell ribozymes (catalytic RNAs) still play a vital, albeit limited, role. In the ribosome, the synthesis of the peptide chains of proteins from RNA code is accomplished by ribozymes. They also catalyze splicing of RNA.
Could this so-called RNA World have offered a good basis for the origin of life? Although this is a commonly held view, it appears to have been made obsolete by ongoing research.
Leslie Orgel is one of the leading figures in origin-of-life research since many years, and he is one of several researchers who independently from each other proposed in the 1960s the RNA world as a precursor of the current DNA/protein world. Gerald Joyce is also a top scientist in the field. The authors argue in a joint article published in The RNA World, 2nd edition (2000), p. 68, on solid chemical grounds that, because of the complex and stereospecific chemistry required, "the de novo appearance of oligonucleotides on the primitive earth would have been a near miracle" (1). After describing a chemically more plausible scenario, PNA (peptide nucleic acid) as a precursor to RNA, the authors point to the enormous difficulties of a transition from PNA to RNA, and to the fact that it yet has to be established that PNA could result in a replicating system. They go on to say that although the presumed RNA World should be considered a milestone and a plateau in the early history of the earth, the concept "does not explain how life originated" (p.74). They conclude (p. 74): "One can sketch out a logical order of events, beginning with prebiotic chemistry and ending with DNA/protein based life. However, it must be said that the details of these events remain obscure and are not likely to be known in the near future."
I refer the reader to Orgel’s authoritative 2004 review article "Prebiotic Chemistry and the Origin of the RNA World" (Orgel 2004; free full text, you may need to register on the link, which is free). It is a must-read for scientists interested in origin-of-life research, and for those who are otherwise interested in this field and have some basic understanding of chemistry. Orgel reviews a lot of impressive chemistry yielding building blocks for life (with a considerable amount of the experiments over the last few decades having come from his own lab, published in leading journals), but he concludes that even just prebiotic synthesis of nucleotides is "unlikely" (nucleotides are the monomer precursors of oligonucleotides and polynucleotides).
Indeed, even though it is commonly accepted that the RNA World presumably had played an important role in the development of life, it now seems clear that it must have been preceded by other steps, if life were to have arisen spontaneously.
Difficulties remain with a precursor genetic system. In his 2004 review, where he describes the already mentioned PNA system and also a TNA (threose nucleic acid) system – the latter being more promising, and simpler to synthesize than RNA (2) –, Orgel states that "the idea that RNA was ‘invented’ by a simpler genetic system is now a popular one, but no convincing precursor system has been described". However, it is not implausible that such a system existed.
A highly interesting and chemically appealing, yet still untested idea is the PAH World (PAH = polycyclic aromatic hydrocarbons) which is described extensively in Gen-e-sis: 222 ff and also at Wikipedia and at pahworld.com.
Other difficulties are that a ribozyme (catalytic RNA) that can copy itself completely has not yet been found, but this rather seems just a matter of time – so far, a 200 base ribozyme can copy 14 bases of its sequence. Also, "the formidable problem of separating the double-stranded product of the copying reaction so as to permit a second round of copying would remain to be solved" (Orgel 2004 review). Solutions for these problems of copying a long ribozyme sequence and of double strand separation have been proposed in the article "Synthesizing Life" (Szostak et al. 2001); experiments will have to show if they are feasible.
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