RADIATION-INDUCED DAMAGES IN SINGLE- AND DOUBLE-STRANDED DNA

In the present study, we searched for possible effects of DNA stranded ness (single and double), on two types of damages, frank strand breaks (FSB, observed at neutral pH) and alkali labile sites (ALS, leading t o breaks at alkaline pH) induced by irradiation with gamma-rays (Co-60 ) Or fast neutrons (p34,Be). Sequencing gel electrophoresis allowed us to follow the occurrence of these damages at each nucleotide site in single (ss-ss), double (ds-ds), and half single-half double (ss-ds and ds-ss) stranded oligonucleotides. Globally, in DNA with random sequen ces of bases, no differences in FSB and ALS yield between the single a nd the double-stranded conformations were observed. One observes, howe ver, an increased alkaline liability at some guanine sites belonging t o single-stranded region of ss-ds or ds-ss. Nevertheless, strandedness influences the radiosensitivity of some particular sequences, i.e. th e 5'-AATT sequences. This region is less radiosensitive than the rest of DNA in the double helical, but not in the single-stranded conformat ion. The results are discussed in terms of DNA conformation.