DNA is the building material of life, but recent work has shown how the stuff can also be used to make art or even store data for long periods of time. Now, a team at Caltech has used DNA to make the world's smallest game of tic-tac-toe, and the technique could have applications in designing reconfigurable nanomachines.
The game is built on a few key properties of DNA. Essentially, the four bases – A, T, C and G – pair up in predictable ways. A goes with T, and C goes with G. This also applies in strands, so for example the sequence ATTAGCA will naturally cosy up to TAATCGT. But if there are two chains that only partially "match" those segments will still pair up, while the non-matching sections will just hang off the end.
But now let's say a new strand comes along that matches more bases. The two partially-joined strands will uncouple and the better match will pair up instead, in a process called strand displacement. The researchers use the analogy of a dating couple breaking up after one of the partners meets someone with more mutual interests.
The Caltech researchers took advantage of this phenomenon to make their playable tic-tac-toe game. First they made a blank board of nine tiles in a solution in a test tube. Each of the tiles had specific DNA strands along the seams, so they would self-assemble into the familiar 3 x 3 grid.
Then, each of the scientists playing the game took turns adding either an X or an O. The trick is that each of these "marked" pieces has a stronger bond on the edges, so when they're added they kick out the blank tile in that spot. Because it takes time for the bonding and unbonding to happen each turn, the whole game took about six days to play. And, spoiler alert, the X player won.
Technically, for the game to work each marked tile has a designated spot on the grid, and each player has nine tiles they can play – one for each spot. So for example, a player puts an O in the top left corner by playing that specific tile. It's the only way it works for the precisely-programmed DNA.
This game obviously isn't going on sale any time soon. Instead, the team says it's a fun little proof-of-concept that could have more serious and useful applications. Currently if there's a problem with a DNA nanomachine, the only option is to throw it out and make a new one. But with this technique, parts become swappable.
"When you get a flat tire, you will likely just replace it instead of buying a new car," says Grigory Tikhomirov, co-first author of the study. "Such a manual repair is not possible for nanoscale machines. But with this tile displacement process we discovered, it becomes possible to replace and upgrade multiple parts of engineered nanoscale machines to make them more efficient and sophisticated."
The research was published in the journal Nature Communications. An animated version of how the game played out can be seen in the video below.
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