DNA damage induced by ionizing radiation can lead to mutation and cancer transformation of the cell and cause the death of the cell or the whole organism. Among the most abundant types of radiation-induced structural damages in DNA are modifications and destruction of nitrogenous bases, and also local breakages of hydrogen bonds (partial denaturation) mainly in the lesion sites of the macromolecule [1].

One of the most sensitive method for revealing alterations in DNA primary and secondary structure is DNA melting [2]. It is a transition of double-stranded molecule (helix) in single strands with coil conformation. This process can be induced by different denaturants or by heating, and can be investigated with different methods: absorbance, optical activity, microcalorimetry. The most common method is measuring of UV-absorption DNA solution at 260 nm. This method is based on hyperchromic effect – increasing of DNA absorption when hydrogen bonds between complimentary nitrogenous bases disappears [2].

In the presence work we study aqueous-salt DNA solutions exposed by gamma-radiation with the doses of 0-100 Gy at the ionic strengths 0.15 M and 0.005 M NaCl. DNA melting curves were measured in these solutions and DNA melting temperatures which characterize a middle point of the "helix-coil" transition were obtained. The increase in electrolyte concentration in a solution augments the stability of native DNA helix and the melting temperature enlarges. Gamma-irradiation causes decrease of DNA melting temperature both in 0.15 M and 0.005 M NaCl solutions. We also obtainedthe concentration of unaltered nucleobases and total hyperchromic effect in irradiated solutions. It is found that bases destruction is reduced in the solutions with larger NaCl concentration. Differential melting curves of irradiated DNA display more then one maximum as opposed to ones of native DNA. We suppose that it can indicate that in irradiated macromolecule there are regions with essentially different thermostability. The additional maxima often lie at the temperatures larger then melting temperature of native DNA. It permit us to propose that the observed effect is due to inter- or intramolecular crosslinks in DNA.