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Evolution

Physics at the dawn of life

Munich, 04/30/2013

After accretion of the Earth, chemical evolution in the oceans led to the production of molecules that could store and transmit genetic information. LMU physicists now show how modest temperature gradients could have started the process.

All living things start off small. Indeed, life itself got off to an inconspicuous start on our planet, as atoms reacted to form covalent bonds giving rise to simple molecules, and these interacted further to generate larger structures that gradually became more complex. The most important type of complex molecule for the future evolution of life was probably the polymer ribonucleic acid (RNA), for RNA strands can fold into specific shapes that have catalytic properties. Such RNA enzymes (“ribozymes”) can facilitate biochemical transformations of other compounds and even catalyze their own synthesis. Moreover, like the DNA polymers that emerged later, RNA can store genetic information.

How the very first RNA polymers originated remains unclear. The minimal length of RNA capable of catalyzing further RNA synthesis is thought to be on the order of 200 monomer subunits (“nucleotides”). Hence molecules of this length must have arisen without the benefit of catalysts. However, the largest molecules so far produced under prebiotic conditions in the test-tube are only around 20 nucleotides long.

Rock pores as reaction vessels
LMU physicists led by Professor Dieter Braun and Professor Ulrich Gerland, both of whom are members of the Excellence Cluster “Nanosystems Initiative Munich” (NIM), have now invoked simple physical principles to show how prebiotic evolution could have got around this size limitation – and in doing so, they have brought us closer to a solution of the riddle of the origin of life.

The team first developed and analyzed a theoretical model, which indicated that a modest temperature gradient should be capable of concentrating monomers sufficiently to selectively facilitate the synthesis of long polymers. The model essentially simulates a typical primeval microenvironment - a porous rock on the seafloor in the vicinity of a source of heat, such as a deep-sea vent. In this scenario, one side of the pore is significantly warmer than the other. The resulting temperature gradient causes the fluid to circulate between the hot and cold sides of the pore. In addition, the gradient concentrates the biomolecules present in solution at the cold end as a consequence of the so-called thermophoretic effect.

“The circulation of the solvent, combined with the thermophoresis of the solutes dissolved in it, creates a thermal trap, which may concentrate long polymers more efficiently than short ones and therefore perturbs the chemical equilibrium of the system,” explains Christof Mast, first author of the paper. “Since the rate of polymerization of the strands depends on their local concentration, the trapping effect increases the likelihood that already long strands will be preferentially extended. These two factors together exert a synergistic effect that is more than exponential.”

Inspired by the primeval seas
The Munich physicists have now been able to corroborate this model experimentally. Working with tiny glass capillary tubes as substitutes for rock pores, they set up a temperature gradient over a range of 10 degrees Kelvin. In place of seawater, they used a simple saline solution, and they added deoxyribonucleotides (rather than ribonucleotides) as building blocks for the reversible synthesis of (DNA) polymers. The choice of DNA instead of RNA was dictated by expediency: Given the reaction rates expected in the primeval ocean, the formation of sufficiently long polymers of RNA would take hundreds of years under – even optimal – laboratory conditions. In principle, polymerization of RNA and the DNA used here involves the same steps, so the experimental approach chosen is an equally valid test of the predictions of the theoretical model.

Dieter Braun sums up the results of the test as follows: “The physics associated with a simple temperature gradient in a pore is sufficient to permit the polymerization of very long RNA strands. This work provides an experimental demonstration of an important intermediate step in the development of living systems.” (PNAS online 30. April 2013) göd

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