Good points raised by researchers in some of the many flaws of the RNA World Hypothesis: Adenine a key component of the RNA world hypothesis has many hypothesis flaws in the origin of life:
1. Adenine synthesis requires HCN concentrations of at least 0.01 M. It is completely unreasonable to expect these concentrations on the prebiotic earth.
2. Adenine is susceptible to hydrolysis (the half-life for deamination at 37°C, pH 7, is about 80 years). Therefore, no adenine would ever be expected to accumulate in any kind of "prebiotic soup."
3. The adenine-uracil interaction is weak and nonspecific, and, therefore, would never be expected to function in any specific recognition scheme under the chaotic conditions of a "prebiotic soup."2 Similar problems apply to the abiotic synthesis of cytosine: Problems with Cytosine as a key molecule in the RNA World or Origin of Life hypotheses:
1. Cytosine has never been found in any meteorites.
2. Cytosine is not produced in electric spark discharge experiments using simulated "early earth atmosphere."
3. Synthesis based upon cyanoacetylene requires the presence of large amounts of methane and nitrogen, however, it is unlikely that significant amounts of methane were present at the time life originated.
4. Synthesis based upon cyanate is problematical, since it requires concentrations in excess of 1 M (molar). When concentrations of 0.1 M (still unrealistically high) are used, no cytosine is produced.
5. Synthesis based upon cyanoacetaldehyde and urea suffers from the problem of deamination of the cytosine in the presence of high concenrations of urea (low concentrations produce no cytosine). In addition, cyanoacetaldehyde is reactive with a number of prebiotic chemicals, so would never attain reasonable concentrations for the reaction to occur. Even without the presence of other chemicals, cyanoacetaldehyde has a half-life of only 31 years in water.
6. Cytosine deaminates with an estimated half-life of 340 years, so would not be expected to accumulate over time.
7. Ultraviolet light on the early earth would quickly convert cytosine to its photohydrate and cyclobutane photodimers (which rapidly deaminate).3 According to Robert Shapiro, a prominent origin of life researcher, the spontaneous formation of a nucleic acid replicator is a "very improbable event." This is because the mixture of amino acids the Murchison meteorite show that there are many classes of prebiotic substances that would disrupt the necessary structural regularity of any replicator.4
8. Cytosine has a half-life in isolation of 19 days at 100°C and 17,000 years in freezing water, which is still very short on the geologic time scale (5) With the Nucleotide problems aside, RNA itself has a huge problem in the origin of life RNA World Hypothesis:
1) The chemical properties of RNA make large RNA molecules inherently fragile and they can easily be broken down into their constituent nucleotides through hydrolysis. The aromatic bases also absorb strongly in the ultraviolet region, and would have been susceptible to damage and breakdown by background radiation (6,7)
2) For example, the ester linkage of ribose and phosphoric acid in RNA is known to be prone to hydrolysis.(8) Additionally, ribose must all be the same enantiomer, because any nucleotides of the wrong chirality act as chain terminators (9).
1.Orgel, L. 1994. The origin of life on earth. Scientific American. 271 (4) p. 82.
2.Shapiro, R. 1995. The prebiotic role of adenine: a critical analysis. Orig. Life Evol. Biosph. 25: 83-98 25: 83-98.
3.Shapiro, R. 1999. Prebiotic cytosine synthesis: A critical analysis and implications for the origin of life. Proc. Natl.Acad. Sci. USA 96: 4396-4401.
4.Shapiro, R. 2000. A replicator was not involved in the origin of life. IUBMB Life 49: 173-176.
5.Matthew Levy and Stanley L. Miller, The stability of the RNA bases: Implications for the origin of life, Proceedings of the National Academy of Science USA 95, 7933–7938 (1998)
6.Lindahl, T (Apr 1993). "Instability and decay of the primary structure of DNA". Nature 362 (6422): 709-15. PMID 8469282.
7.Pääbo, S (Nov 1993). "Ancient DNA". Scientific American 269 (5): 60-66.
8.Lindahl T. Nature (London). 1993;362:709–715.
9.Joyce GF; Visser GM, van Boeckel CA, van Boom JH, Orgel LE, van Westrenen J. (Aug 1984). "Chiral selection in poly(C)-directed synthesis of oligo(G)". Nature 310 (5978): 602-4. Doi:10.1038/310602a0. PMID 6462250.