DNA compaction plays a key role in radioprotection against double-strand breaks as revealed by single-molecule observation

We performed single-DNA observation to measure double-strand breaks caused by γ-ray irradiation. To analyze the efficiency of the breakage reaction in a semi-quantitative manner, we used T4 DNA, 166 kbp. The efficiency of γ-ray-induced breakage for compact DNA in the presence of spermidine (3+) was ∼1/25 of that for elongated DNA. This finding suggests that the primary protective effect of polyamines is due to the compaction of DNA.