  DNA double-strand breaks (DSBs) represent a serious source of damage for all
living things and thus there have been many quantitative studies of DSBs both
in vivo and in vitro. Despite this fact, the processes that lead to their
production have not yet been clearly understood, and there is no established
theory that can account for the statistics of their production, in particular,
the number of DSBs per base pair per unit Gy, here denoted by P1, which is the
most important parameter for evaluating the degree of risk posed by DSBs. Here,
using the single-molecule observation method with giant DNA molecules (166
kbp), we evaluate the number of DSBs caused by gamma-ray irradiation. We find
that P1 is nearly inversely proportional to the DNA concentration above a
certain threshold DNA concentration. A simple model that accounts for the
marked decrease of P1 shows that it is necessary to consider the
characteristics of giant DNA molecules as semiflexible polymers to interpret
the intrinsic mechanism of DSBs.
