Leonardo da Vinci's Mona Lisa painting isn't actually that big (30 inches tall), but Caltech researchers have found a way to make that seem downright gargantuan. They've used DNA to construct the smallest known Mona Lisa. At several hundred nanometers across, they're roughly as large as a lone E. coli bacterium -- the iconic smile is just 100nm wide. The trick was an adaptation of a DNA "origami" method that got the gene strands to fold and assemble into the right shape.
The Mona Lisa is divided into squares, each of which is folded by using one long DNA strand manipulated by "staples" (short, custom-designed strands) that bind to and pull on it. After that, it's a matter of attaching the squares to a DNA canvas. You do that by isolating each square into a test tube and combining them in progressively larger squares (2x2, 4x4 and finally 8x8) until Mona Lisa shows her mysterious face. Each square has edges designed only to join in a specific way, so the wrong pieces can't attach to each other by mistake.
As you can use a combination of software and automatic liquid handling to make these mini paintings, you're really just limited by your creativity -- the team also 'drew' portraits of bacteria and a rooster to show what was possible. And that, in turn, could lead to more practical uses. DNA-based nanostructures like this could help build extremely dense circuits, exotic organic materials or just tests for chemical and molecular interactions. This might not be the smallest piece of art you've ever seen, but the technology behind it could be incredibly useful beyond recreating masterpieces.