Mona Lisa is a work of art by Leonardo Di Vinci known for its enigmatic smile and now the scientists have recreated the Mona Lisa on a microscopic level too.
Experts built the world’s smallest version of Leonardo’s masterpiece, measuring 0.5 square micrometers. Which is about the size of an e-Coli bacterium. They used a process called DNA origami, where they manipulated genetic code gene strands to fold and assemble into the right shape.
While DNA is best known for encoding the genetic information of living things, the molecule is also an excellent chemical building block. The Caltech team developed a software that can take an image, divide it up into small square sections and determine the DNA sequences needed to make up those squares. The next challenge was to get those sections to self-assemble into a superstructure that recreates the final image.
The key to doing this task was to assemble the tiles in stages like assembling small regions of a puzzle. They then joined those regions together to make larger areas, finally putting them together to make the complete puzzle.
Grigory Tikhomirov, senior postdoctoral scholar and lead author of the new study said, “We could make each tile with unique edge staples so that they could only bind to certain other tiles and self-assemble into a unique position in the superstructure. But then we would have to have hundreds of unique edges which would be not only very difficult to design but also extremely expensive to synthesize. We wanted to only use a small number of different edge staples but still get all the tiles in the right places.”
To make a single square of DNA origami, the team requires a long single strand of DNA and many short strands called staples. These are designed to bind the multiple designated places on the long strand. When the short staples and the long strand are combined in a test tube, the staples pull regions of the long strand together, causing it to fold over itself into the desired shape.
A large DNA canvas is assembled out of many smaller square origami tiles. Molecules can be selectively attached to the staples in order to create a raised pattern that can be seen using atomic force microscopy. The edges of the pieces are the same but because they are assembled in stages, there is no risk of tile being put in the wrong place. So the corner file will not attach to the wrong corner.
The team named the method “fractal assembly” because as with abstract mathematical objects called fractals, the same set of assembly rules is applied at different scales. This implies that the same rules govern the construction of a puzzle no matter what the size is.
Their final structure was 64 times larger than the original DNA origami designed by Caltech’s Paul Rothemund in 2006. Another software was also created that enables the scientists to create DNA nanostructure using fractal assembly. The four molecules which form DNA are called nucleotides. The scientists are very particular about which nucleotides they will bond with and this level of specificity allows the team to build very precise patterns.
Cytosine (C) and Guanine (B) will not bond with Adenine (A) and Thymine (T) bond together with two hydrogen bonds. This predictability let the researchers forge whatever patterns and pictures they want.
To demonstrate the precise capabilities of the approach the scientists designed a number of unique patterns – including the Mona Lisa, a rooster, and a bacterium. The researchers are hoping that their breakthrough will allow for bigger structures to be made using DNA such as building electronics circuits with tiny features or fabricating advanced materials.