Origins of Life – Can RNA Spontaneously Assemble?

We know that it is almost trivial for inorganic compounds to spontaneously react to form organic compounds.  We can see this in hundreds, if not thousands of similar experiments since Miller-Urey.  We can also observe these organic products in places that no organic life form could survive; comets and nebula.  So that part is easy.  We can get the precursor materials needed for life to form without invoking anything bu basic chemistry and physics.

So, now the question is, can we get from amino acids, simple sugars, basic nucleic acids and the like to something complicated like RNA?  Can this happen spontaneously, with nothing more than basic physics and chemistry?

The answer, as shown by Matthew W. Powner, Beatrice Gerland & John D. Sutherland, is simply… yes.

First of all, everyone should remember that science will probably never know exactly how life on Earth began.  However, as long as it can be shown that the materials and processes can arise by purely natural means, then there is no need to incorporate a designer or deity to do the job.

Now, on with the research.  First, what’s the big deal?  Why is it so hard to generate X provided that you have V and W?

I’ll let the authors explain:

In particular, although there has been some success demonstrating that ‘activated’ ribonucleotides can polymerize to form RNA [6,7], it is far fromobvious howsuch ribonucleotides could have formed from their constituent parts (ribose and nucleobases). Ribose is difficult to form selectively [8,9], and the addition of nucleobases to ribose is inefficient in the case of purines [10] and does not occur at all in the case of the canonical pyrimidines [11].

In other words, there is evidence that activated ribonucleotides (A, U, C, and G) can form RNA, but the problem is in the sugar and phosphate backbone.  The sugar (ribose) is hard to form and it’s even harder to attach it to some of the nucleobases and it won’t attach to other nucleobases at all.

As my dad would say; “That there is some trouble.”

So, the researchers took a different route.  Instead of starting with the base materials (ribose, the nucleotides, and phosphates), they started to explore alternative chemical paths to get the same result (that is, RNA).

What they found, was that by starting from the exact same precursors that has been shown to be chemically possible, they found another path.  A series of chemical reactions that avoids all the problems of the ‘traditional’ pathway.

Their chemical path simply bypasses ribose and the free pyrimidine nucleobase.  What they also found was that by avoiding trying to incorporate phosphate into the structure directly and, instead, using inorganic phosphate as a catalyst, they were able to increase the yield of the materials they needed at the conditions (pH mainly) that those materials had to have to form.  In fact, having the phosphate present helped increase yields of all intermediate materials.

Finally, in the last step, inorganic phosphate acted as a buffer to prevent an increase in pH that would reduce yields.

I freely admit that I’m not a biochemist (thank Cthulu), but I can understand conclusions very well and considering that this paper appears to be highly cited and generally well received (I do know of one or two hold outs), I think this paper shows what it needs to.  That there is a chemically plausible way to get from inorganic materials all the way to RNAs.

I’ll let the authors’ conclusion speak for itself:

Our findings suggest that the prebiotic synthesis of activated pyrimidine nucleotides should be viewed as predisposed. This predisposition would have allowed the synthesis to operate on the early Earth under geochemical conditions suitable for the assembly sequence.

_________________________________________________

Powner MW, Gerland B, & Sutherland JD (2009). Synthesis of activated pyrimidine ribonucleotides in prebiotically plausible conditions. Nature, 459 (7244), 239-42 PMID: 19444213

references (from reviewed paper)

8. Kofoed, J., Reymond, J.-L. & Darbre, T. Prebiotic carbohydrate synthesis: zincproline
catalyzes direct aqueous aldol reactions of a-hydroxy aldehydes and
ketones. Org. Biomol. Chem. 3, 1850–1855 (2005).
9. Ricardo, A., Carrigan, M. A., Olcott, A. N. & Benner, S. A. Borate minerals stabilize
ribose. Science 303, 196 (2004).
10. Fuller, W. D., Sanchez, R. A. & Orgel, L. E. Studies in prebiotic synthesis VI.
Synthesis of purine nucleosides. J. Mol. Biol. 67, 25–33 (1972).
11. Orgel, L. E. Prebiotic chemistry and the origin of the RNA world. Crit. Rev. Biochem.
Mol. Biol. 39, 99–123 (2004).