2.1 Design principles of logic gates based on DNA strand displacement reaction
In this paper, as shown in Fig. 1, different color line segments are used to represent different DNA sequences, complementary sequences are represented by line segments of the same color, and the names of two complementary sequences are represented by upper and lower case letters respectively. For example, in Fig. 1, two red parallel lines represent two complementary sequences, which can be hybridized to form a double helix. The upper case F and lower case f are used to represent the names of the two complementary sequences respectively. In this paper, as shown in Fig. 1, BHQ1 quencher is represented by small black circles, fluorescent groups are represented by small triangles, quenched fluorescent groups are represented by small gray triangles, and non quenched fluorescent groups are represented by small green triangles.
Figure 1 shows the principle of the three-input OR logic gate based on DNA strand displacement reaction. In the initial state, that is, without any input, the 3-terminal modified BHQ1 single DNA strand EFHG can hybridize with the 5-terminal modified FAM single DNA strand fh to form a double helix. The fluorescence signal of FAM is quenched due to the close distance between BHQ1 and FAM fluorescent group. As shown in Fig. 1, I designed three kinds of DNA input strands respectively: INPUT1、INPUT2、INPUT3. INPUT1 is efh, INPUT2 is fhg, INPUT3 is efhg. According to the principle of DNA strand displacement reaction, three kinds of input strands can replace fh and hybridize with EFHG to release fh labeled by fluorescent group. After the fh is released, its fluorescent group is far away from the quencher, so its fluorescent signal will increase. As shown in Fig. 1, when one or two or three of the three input strands are added, the fluorescence signal will rise. Therefore, a three-input OR logic gate is formed with three input strands efh, fhg and efhg as input and fluorescence signal as output.
Figure 2 shows the principle of the three-input AND logic gate based on DNA strand displacement reaction. In the initial state, that is, without any input, the 3-terminal modified BHQ1 single DNA strand ABCD can hybridize with the 5-terminal modified FAM single DNA strand abcd to form a double helix. The fluorescence signal of FAM is quenched due to the close distance between BHQ1 and FAM fluorescent group. As shown in Fig. 2, I designed three kinds of DNA input strands respectively: INPUT1、INPUT2、INPUT3. INPUT1 is ab, INPUT2 is cd, INPUT3 is bc. According to the principle of DNA strand displacement reaction, when any one or two of the three input strands are added, the input strands can not displace the abcd strand, so the ABCD strand and the abcd strand are still hybridized, and the fluorescence signal of FAM is still quenched. Only when three input strands are added at the same time, ab, bc and cd could displace abcd, hybridize with ABCD to form double helix and release abcd. After the release of abcd, the fluorescent group is far away from the quencher, so the fluorescence signal will increase. As shown in Fig. 2, a three-input AND logic gate is formed with three input strands ab, bc and cd as input and fluorescent signal as output.
Figure 3 shows the principle of the three-input MAJORITY logic gate based on DNA strand displacement reaction. In the initial state, that is, without any input, the 3-terminal modified BHQ1 single DNA strand EFHG can hybridize with the 5-terminal modified FAM single DNA strand fh to form a double helix. The fluorescence signal of FAM is quenched due to the close distance between BHQ1 and FAM fluorescent group. As shown in Fig. 3, I designed three kinds of DNA input strands respectively: INPUT1、INPUT2、INPUT3. INPUT1 is ef, INPUT2 is hg, INPUT3 is fh. According to the principle of DNA strand displacement reaction, when any one of the three input strands is added alone, the input strand can not displace the fh strand, so the EFHG strand and fh strand are still hybridized, and the fluorescence signal of FAM is still quenched. Only when any two or three of the three input strands are added at the same time, the input strands can displace fh and hybridize with EFHG to release fluorescent group labeled fh strand. After the fh strand is released, its fluorescent group is far away from the quencher, so its fluorescent signal will increase. As shown in Fig. 3, a three-input MAJORITY logic gate is formed with three input strands ef, hg and fh as input and fluorescence signal as output.
2.2 Experimental methods
The logic gates designed in this paper were verified by corresponding biochemical experiments. The specific experimental methods include the following three steps:
The first step, annealing: the DNA strands in the initial state was hybridized in 0.5×TBE and 50 mM NaCl buffer, and the concentration of each DNA strand was 2 µ m, the annealing temperature was room temperature.
The second step, adding Input strands for the strand displacement reaction: the annealed product was divided into equal samples, and the corresponding Input strands were added into the annealed product respectively. The concentration ratio of Input strands to DNA at the annealing step was 1:1.
The third step, measure the fluorescence value: take out solutions with a DNA molar mass of 20 pmol from the solutions after strand displacement reaction respectively, then add the mixture of 0.5×TBE and 50mm NaCl into the solutions to the volume of 200 ul, and detect the fluorescence signals at the excitation wavelength of 495nm and the emission wavelength of 520nm with a fluorescence spectrophotometer.