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. The central question here is to find the maximum number h of hydrogen bonds that is not enough to hold a molecule together. The diagram below illustrates the situation.
Vertical lines are P-E bonds and horizontal lines are hydrogen bonds.
Two curved segments at the right show repaired P-E bonds. The number h is important because it determines the stability of DNA molecules. Large values of h would mean that it is easy for radiation to destroy a DNA molecule. In a particular experimental situation, a sample of DNA was exposed to x-rays, and the following laboratory measurements were made. This number was determined by centrifuging the irradiated sample of molecules.