Contrary to proteins, which are also affected by ionizing radiation but are present in numerous amount, only one copy of DNA is present in the cell nucleus. Because the DNA contains all the genetic information, any damage to the structure could be harmful and potentially lethal for the cell. A cell still harboring too much DNA damage when entering mitosis is likely to die.
Single strand breaks occur when the phosphate backbone is broken on one side of the DNA helix, whereas double strand breaks involve the breakage of the two opposite sides of the DNA helix. While single strand breaks are easy to handle, because a template for repair is available on the other side of the helix, double strand breaks are more harmful and repair often introduce mutations. Other lesions, such as base damages, abasic sites, damaged sugars or DNA-protein crosslinks are also produced. A difference arises when switching from uncharged (e.g. X-ray) or charged (e.g. proton) particle as radiation source. Most of the time, X-rays generate simple lesions, such as single strand breaks or base damage, because ionization they produce are less dense, less gathered together. Hence, damage produced by X-rays are less severe and easier to repair. However, when charged particles such as protons are used, locally multiply damage sites arise. These clustered lesions, concentrated in a localized region of the DNA, are even more difficult to repair and often lead to cell death. This is due to the high ionization density occurring on the proton path.
Galactic cosmic rays (a variety of densely ionizing radiations no limited to protons) are encountered in space and impair space exploration.