We demonstrate a very simple and generic protocol for ultrasensitive in-situlabel-free detection of DNA hybridization using third generation poly (amido amine) dendrimer(G3-PAMAM) functionalized GaN nanowires (NWs). PAMAM modified GaNNWs provides large density of docking site to immobilize Significant number of probe(p-) DNA covalently. These p-DNA/PAMAM/GaNNWs sensor probes are Employed to achieve an ultra-high detection limit down to at to molar level concentration of Complementary target (t-) DNA. Comparative in-situ studies on single/triple base-pair m is matched, Υ-irradiated and complementary t-DNA in the hybridization process reveals electivity and specificity of the p-DNA/PAMAM/GANNWs sensor probe over a wide range,10^-8 to 10^-19 M, of analyte concentration. During the hybridization process, there is a substantial change in t-DNA concentration dependent interfacial polarization resistance during electro chemical impedance measurement, which forms the basis of the present DNA bio sensor. This novel methodology for specific DNA sequence detection, as compared with the existing methods, is found to be very robust, highly sensitive, and reproducible.