Fragment distributions of DNA molecules after X-ray and C ion irradiation

Investigation of DNA damages induced by heavy ions is of great interest for a better understanding of the processes which appear in clinical radiotherapy and are responsible for the destruction of the cancer cells. The most serious damages of DNA molecules are doublestrand breaks (DSB), i.e. the rupture of both DNA strands in the range of a few base pairs and single-strand breaks (SSB), when one of the DNA strands is broken [1]. In particular, DSBs lower effectively the cell repair rate and leads often to cell death. Thus, heavy ion irradiation of tumour cells has been used very successful in cancer therapy. However, a detailed understanding of the processes which occur on the molecular level, leading to this effective cell death, is still lacking. Conventional methods, e.g. gel-electrophoresis measurements, are suitable to analyse the percentage of supercoiled and relaxed molecules and also linear DNA fragments, but it can not provide detailed information about the fragment distribution [2]. The latter can be achieved by scanning force microscopy (SFM) which permits direct visualisation and measurement of individual DNA fragments with an accuracy of a few nanometers. In this contribution we present a SFM analysis of radiation-induced damages in ФX174 plasmids in 20 mM Hepes solution after X-ray and C ion irradiation at a dose of 500 Gy. For sample preparation, 3μl plasmid solution in 10 mM MgCl2 [3] has been deposited on mica substrates and rested in air for 3 minutes. Subsequently, the substrates were washed with 400 μl double distilled water and dried with nitrogen. Nine samples have been prepared for each irradiated solution and around 10 images were recorded for each sample. The samples have been imaged in air with tapping mode using a CP-R microscope from Veeco Instruments. All images were recorded with a scan area of 3 μm x 3 μm and a resolution of 512 pixels x 512 pixels. The length of the fragments has been analysed using ImageJ software.