We begin with a simplified view of the structure of a DNA molecule. For our purposes, a DNA molecule may be viewed as a ladder. The two strands of the DNA molecule are the sides of the ladder.
Radiation can produce a break in a strand by destroying a P-E bond.
Think of radiation as blasting away a bond in one side of the ladder. While relatively weak, the hydrogen bonds between nucleotides cannot be permanently broken by such a radiation hit. Significant damage to a DNA molecule occurs when SSBs in opposite strands occur sufficiently near to each other. If two SSBs occur directly opposite, the molecule is immediately and cleanly broken. Thus, if SSBs in opposite strands are sufficiently far apart, the several intervening hydrogen bonds will be strong enough to hold the molecule together.