DNA nanotechnology: The world's smallest assembly line.

A t the dawn of the twentieth century, humankind began to make mechanical parts in large quantities, and manufacturers assembled them into early automobiles. The development of the assembly line by Olds and by Ford 1 led to mass production on a scale that allowed the automobile to shift from a boutique product to a consumer good. Today's nano-scientists find themselves in a similar position to that of Olds and Ford. New methods allow for the synthesis of a diverse array of nano-sized and colloidal 'parts' , which would in principle only need blueprints and an assembly line for their large-scale use to construct the next generation of materials and products. What might such an assembly line look like on the nanoscale? In two recent papers, Nadrian Seeman and co-workers provide a first glimpse into how this might work, through two systems based on DNA. DNA strands consist of sequences of four complementary bases (A, C, G and T), which preferentially pair up as CG and AT through hydrogen bonds. A single strand of DNA with a particular arrangement of letters will therefore bind strongly to a strand with the complementary arrangement of letters. DNA tiles or materials bound to DNA (for example, gold nanoparticles) can thus be endowed with highly specific interactions, a characteristic that has been exploited in the field of DNA nanotechnology to construct a variety of assemblies and dynamic systems 2. Writing in Nature, Seeman and colleagues have now described 3 a system that uses DNA in two different ways to create a prototype assembly that can self-replicate. Several DNA double helices were first arranged into previously designed 4 molecular tiles ('bent triple crossover' motifs). The tiles were given specific edge-to-edge as well as face-to-face interactions, through the complementarity of protruding DNA strands called 'sticky ends'. This led to four types of tiles — A, B and complementary A' and B'. A and B' tiles were also tagged with either a biotin– streptavidin label or hairpin loops so that they can be easily observed through atomic force microscopy imaging. A string with a prototype pattern (ABBABAB) was first prepared, and replication then occurred through a two-step process (Fig. 1). In the first step, the prototypes were copied by immersion in a bath of complementary tiles, somewhat reminiscent of impression moulding. The complementary tiles arrange in an A'B'B'A'B'A'B' fashion through face-to-face interactions with the prototype, and are …