Ionizations due to radiation may act indirectly on the water molecules forming water-derived radicals or directly on the molecules that makes up the cellular component. This formed radical reacts with adjacent molecules, resulting n breaking bonds. DNA breakage is the most adverse effect. DNA breakages are of two major criteria, single strand or double strand breaks. Most single-strand breaks can be repaired normally as the DNA molecule is double stranded in structure(the two strands complement each other, so that an intact strand can serve as a template for repair of its damaged, opposite strand). In the case of double-strand breaks, however, repair is more difficult and erroneous rejoining of broken ends may occur. These faulty repair result in induction of mutations, chromosome aberrations, or cell death.
Deletion of DNA segments is the predominant form of radiation damage in cells that survive adverse effect due to exposure to radiation. It may be caused by:
(i) Faulty repair of two separate double-strand breaks in a DNA molecule with joining of the two outer ends and loss of the fragment between the breaks or
(ii) (ii) The process of enzyme digestion of nucleotides, the component molecules of DNA, of the broken ends before rejoining to repair one double-strand break.
The video below shows sophsticated DNA repair system, but by mere chance certain mutation passes on without check and lead to the development of cancer. This certain individual with such mutation are so unlucky that even check point at cell division can't detect them. The image below shows such sophticated cellular mechanism: