Fig. 2. Advances in structural DNA–nanotechnology. (A) Four-arm Holliday-junctions with complementary sticky ends selfassemble into 2D DNA-arrays (reprinted with permission from ref. 6, copyright 2009, American Chemical Society). (B) DNA-origami using a 7 kb ssDNA template (black) that folds into a predesigned shape directed by ∼200 short “staple” strands. Cross-overs of the staple strands between helices provide rigidity to the structure. By computer aided design of the staple strand sequences the template strand can be folded in virtually any structure. Figure adapted from ref. 7 by permission from Macmillan Publishers Ltd: Nature, copyright (2006). (C) Predesigned DNA bricks selfassemble into a 3D cube. Each brick is formed by a 32 nt long oligonucleotide containing four domains that hybridize to neighbouring bricks. Any 3D shape can be obtained by omitting specific bricks from the cube, making this approach highly modular. Figure adapted from ref. 10. Reprinted with permission from AAAS.