This interactive model illustrates the complimentary nature of nucleotides. The bases which are parts of nucleotides can connect with each other via hydrogen bonds only in a strictly defined pattern: adenine with thymine, guanine with cytosine. Thus, the complementary nature of nucleotides is manifest.
On the sides of the model there are two groups of buttons that will help you choose the needed bases (A for adenine, T for thymine, G is for guanine and C is for cytosine.) In the center of the model you con see a symbolical representation of each base; the atoms are represented as balls of different colors (O is oxygen, C is carbon, N is nitrogen and H is hydrogen.)
Press "Run". If the bases are complimentary to each other, then after a while they will become connected by yellow dotted lines - a schematic representation of hydrogen bonds. In the other case, this will not happen.
Press "Stop" to bring it to a halt, and press "Reset" to return the model to its initial state.