FIGURE 1 (a) A three-dimensional DNA octahedral structure as an example of DNA nanotechnology. The structure has 12 struts—the octahedron edges (double cylinders; each cylinder is a double-stranded DNA)—connected by six flexible joints (labeled I–VI). The joints are four-way junctions that connect the core-layer double helices of each strut. Colors correspond to the colored segments in (b), which shows the secondary structure of the branched-tree folding intermediate. This consists of a single 1,669-nucleotide DNA chain, or heavy chain, and five unique 40-nucleotide light chains (orange). (c) A close-up of the base-pairing scheme for each strut. Black and gray indicate two separate parts of the DNA strand that interact to form a strut. The cross-over base pairing gives the strut its structure and strength.