Concept of self-assembly of DNA molecules may be utilized to implement different algorithms of computing. In particular, this methodology is useful in artificial intelligence because it operates on symbols. As has been shown by the author  DNA computing is suitable for searching the decision tree which is encoded by DNA molecules. Thus, the algorithms of artificial intelligence can be performed by this technique. In the contribution we demonstrate that by proper encoding and manipulating with DNA molecules it is possible to implement reasoning procedure by searching the decision tree. The method uses standard genetic engineering operations like hybridization, ligation, and purification. Quantum dot technique is applied to speed up detection of the final output eliminating the slow operation of electrophoresis. The approach presented is very simple and fast because the technique used allows an enormous number of molecules to be labeled, reduces instrument tie-up and improves analysis throughout the process.