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Mills [4] has developed a simple model of neural network which is capable to transport signal using DNA oligonucleotides. The states of activity of the neural model are coded by the concentrations of the neuron DNA oligonucleotides. The input and output neurons are coded by single stranded DNA sequences. Partially double-stranded DNA sequences denote the formed axons.

The DNA oligonucleotides that represent the input and output signals are used to encode the axon of the neural model. These oligonucleotides are then temporarily attached to each other by a linker oligonucleotide. The one half of the linker sequence is complementary to the half of output neuron oligonucleotide and other half is complementary to the prefix of input neuron oligonucleotide. Then, T4 DNA ligase joins these two oligonucleotides permanently. The polymerase extension of the linker sequence along with the out sequence is used to protect the output end of linked oligonucleotide. The formation of the axon sequence is explained in Figure 2.9. The rate at which input neuron DNA concentrations lead to the generation of corresponding concentration of output neuron oligonucleotides is governed by the concentration of the axon DNA oligonucleotides. Axon of each neuron can be prepared in the form of partially double-stranded DNA sequences in ideal wet lab environment.

Figure 2.9 Formation of axon. (a) Input and output neuron oligonucleotides and the linker sequence. (b) Input and output neurons get hybridized to each other using the linker sequence. (c) Input and output neurons are joined by T4 DNA ligase; DNA polymerase starts the extension of the linker molecule along the output oligonucleotide. (d) Partially double-stranded axon is formed after completion of polymerase extension.

For development of the model of single-layer neural network, a set of input neuron oligonucleotide, complementary to the input neurons, is used. This set and designed axon sequence (Figure 2.9) are mixed together. This mixed solution is treated with DNA polymerase in presence of appropriate reaction buffer. The input neuron hybridizes to the single stranded part of partially double-stranded axon. This hybridization procedure releases the output oligonucleotide from the axon molecules that were already attached on their output end. This mechanism is illustrated in Figure 2.10.

Figure 2.10 Formation of output molecule using DNA neural network. (a) Axon molecule and input oligonucleotide. (b) Input molecule hybridizes to the corresponding axon; extension of the strands gets started using DNA polymerase. (c) The extension of the primer leads to the release of output DNA oligonucleotide.

Generalization of the above discussed single-layer neural model to multi-layer networks is possible. To achieve this, each layer should be designed individually. The output of one layer is the input of the following layer in the multi-layer model.