Recently, DNA computing has been attracting increasing attention in the field of DNA nanotechnology. It is realized on a DNA reaction system by combining a DNA analog circuit and a DNA logic circuit. Because of this background, a number of researchers have suggested various methods of information processing using DNA computing. However, one issue has been raised: Of the four arithmetic operations, they still cannot realize the subtraction operation. Realizing a subtraction operation is necessary to expand the versatility and possibilities of DNA computing. Moreover, DNA concentration is susceptible to noise and cannot express a negative value since it is a continuous analog quantity.Therefore, DNA concentration has a considerable restriction with respect to the ability of information expression.
       In this year, we proposed a device to solve the abovementioned problems. The key device is called an A/D converter and converts analog signals into digital signals in an electronic system; we realized it with a DNA reaction system. In our project, we designed a DNA A/D converter by combining the threshold gate, AND gate, and the OR gate that we built on the basis of the seesaw gate model. In the DNA A/D converter, the input is a single-strand DNA (ssDNA) and the output is another ssDNA obtained through the DNA strand replacement reaction in each gate. The threshold gate recognizes the DNA concentration as either high or low. Then, the output ssDNA from the threshold gate is set as the input of each gate. Finally, the input ssDNA is converted into multiple output ssDNA by using a number of gates. As a result, the output ssDNA concentration is expressed as a digital bit number denoting the high or low state.
       In BIOMOD 2015, we successfully performed a numerical simulation of the abovementioned 3-bit DNA A/D converter. Moreover, we designed the DNA sequences required for the 1-bit DNA A/D converter and conducted an in silico experiment to verify its validity.