UNDERSEA hydrothermal vents have generated great excitement recently as possible sites for the origin of life. But it has been difficult to explain how molecules such as DNA, RNA or proteins could accumulate in high enough concentrations near these vents to get evolution going. Now a team at Rockefeller University in New York have come up with evidence that DNA can be concentrated 1000-fold by temperature gradients and convection currents – conditions that exist around hydrothermal vents.
“This is very significant in the context of the origin of life,” says Koichiro Matsuno of Nagaoka University of Technology in Nagaoka, Japan. In 1999, Matsuno’s team simulated hydrothermal vent conditions in their lab and showed that amino acids could combine into short chains, the building blocks for proteins. A year later they showed that the nucleotides, the structural units of RNA and DNA, could also form simple chains. “But we didn’t have any mechanism to enhance their concentration,” says Matsuno.
Concentrating the newly created molecules is essential for further reactions to occur, says physicist Dieter Braun of Rockefeller University. Some theories for the origin of life assume that spherical membranes called vesicles must have formed first in oceans or ponds, providing enclosures for self-replicating chemical chains – the precursors to modern DNA. “But many people think that vesicles evolved later, because it’s hard to get molecule transfer across a membrane,” says Braun.
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Braun and his colleague Albert Libchaber have shown how DNA can be concentrated without enclosing it inside vesicles. They filled a circular glass chamber 4 millimetres in diameter and half a millimetre high with a solution of DNA fragments, each 5500 base pairs long. When they heated the centre of the chamber with a laser, the DNA molecules were repelled from the hot centre towards the cooler walls – a process known as thermophoresis. The convection currents induced inside the chamber should have redistributed the DNA throughout the solution. Instead, the DNA was unexpectedly concentrated into a ring at the bottom of the small chamber (Physical Review Letters, vol 89, p 188103).
Braun believes such conditions could have existed in the rocks around hydrothermal vents in primitive oceans. The vents would create thermal gradients and convection currents around small pores and crevasses in the rocks. It is also significant that the molecules would concentrate near the rock surfaces, says Braun, because it’s known that minerals on rocks can help nucleotides to link together into chains. “This thermophoretic trapping accumulates molecules right near the surfaces,” says Braun. “You get your material where the most interesting thing is happening.”
Not everyone is convinced that concentrating DNA gets us any closer to understanding the origin of life. Gerald Joyce of the Scripps Research Institute in La Jolla, California, points out that the real problem is how to concentrate nucleotides and amino adds.
Braun says the process should also concentrate RNA, which is thought to have originated before DNA. He’s trying to find out if it can also work for smaller molecules.