Credit: Carin Cain

Figure 1: (Top left) RNA consists of the four bases adenine (A), guanine (G), cytosine (C), and uracil (U) as building blocks, held together by a flexible backbone. (Top center) Below a critical temperature, two strands of complementary sequence form a duplex by joining A and U as well as G and C into base pairs. (Top right) Complex RNA sequences may bind very specifically to other biomolecules. They may act as ribozymes (catalysts). In this particular example, RNA holds the molecules X and Y in place so that their binding probability increases. (Bottom) Scheme of the RNA reactor, which can be imagined as a temperature-gradient-driven convection process in a porous media. The reactor possesses an influx of monomers, the RNA building blocks A,G,C,U, as well as a length-dependent outflux such that different sequences of different length are constantly present in the reactor. The reactor modifies the RNA in a three-step cyclic process: The building blocks are ligated to form random polymers (step $1$). The polymers associate (hybridize) to a duplex structure if they encounter a complementary sequence (step $2$). The RNA molecules are degraded, preferentially at the nonhybridized sites (step $3$). The process then restarts with some of the strands that result from the previous cycle.