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DNA Damage, Chromosome Aberrations and the Cell Cycle
A double-strand break is the lesion most likely to kill or mutate a cell. Cells are most sensitive in mitosis and least in late S phase.
DNA lesions
DNA is a double helix: two side rails of sugar and phosphate joined by rungs of paired bases. Radiation damages it in several ways, mostly through free radicals. A single-strand break cuts one rail. The other rail holds the pieces in place, so enzymes usually repair it quickly and correctly. A double-strand break cuts both rails at or near the same rung. It is much harder to repair, and misrepair can cause a mutation or kill the cell. A base change, or point lesion, alters or removes one base and changes the genetic code. Cross-linking joins parts of the same DNA molecule, or DNA to a protein. In other large molecules, main-chain scission breaks the long backbone into pieces.