(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.