Quantum dots owing to their unique properties have been the subject of extensive investigations in different areas of science and technology in the past two decades. In this study, preparation of the electrochemical sensor for the selective and sensitive determination of guanine (G) and adenine (A), two of the most important components of DNA and RNA, were investigated by modifying a glassy carbon electrode (GCE) with ZnS@CdS quantum dots and graphene oxide (GO). This sensor shows separated and well-defined peaks for A and G, by which one can determine these biological bases individually or simultaneously. The novel electrode showed linear responses towards G and A over the concentration range of 0.01 mu M to 50 mu M. Detection limit was determined as 1.45 and 1.81 nM according to signal to noise ratio (S/N = 3). Furthermore, the measurements of thermally denatured single-stranded DNA were carried out and the value of (G + C) / (A + T) of DNA was calculated as about 0.77 for various DNA samples. The proposed electrochemical sensor exhibited some advantages in terms of simplicity, rapidity, high sensitivity, good reproducibility and long-term stability. The relative standard deviation of the proposed system, obtained using A and G standards, were 2.7% and 1.5% for intra-day and 4.3% and 4.1% for inter-day precisions, respectively. The results demonstrated that the proposed electrochemical sensor not only provides a novel and sensitive approach to detecting purine bases, but also ascertained that the proposed electrochemical sensor can be profitable to evaluate DNA bases damage.