Replicating and trapping DNA

In living organisms, the complex directional capabilities of cell membranes ensure the concentration and confinement of DNA and the ingredients needed for its replication. But how was that accomplished at the dawn of life on Earth, when there were no such membranes to constrain indiscriminate diffusion and entropy growth? At the University of Munich, Dieter Braun’s biophysics group has addressed those questions with a suggestive demonstration of efficient DNA replication and accumulation driven only by a quasistatic thermal gradient in a fluid-filled glass capillary stocked with nucleotides, a polymerizing enzyme, and a small initial template charge of a DNA with 143 base pairs. The 0.1-mm-wide capillary is meant to mimic pores at hydrothermal vents in early seas. The thermal gradient is provided by an IR laser. The gradient drives DNA molecules away from hot spots and sustains a convective flow that subjects the DNA to thermal cycles that create new DNA and concentrate it at the sealed ends of the capillary. The convective cycles promote unraveling of the double helix at 86 °C and its subsequent replication at 60 °C. The DNA population doubles every minute or so, until the closed system’s exhausted nucleotide reserve eventually puts an end to that exponential growth. The figure shows the growing DNA concentration at one of the capillary ends over a span of 14 minutes. With continual nucleotide replenishment, such a system might replicate DNA at the rate of 1700 doublings per day. So aside from its possible relevance to paleobiology, the result is of biotechnological interest. (C. B. Mast, D. Braun, Phys. Rev. Lett. 104 , 188102, 2010 http://dx.doi.org/10.1103/PhysRevLett.104.188102 .)