Radiation can damage the cellular DNA and its ability to replicate correctly. Ionizing radiation such as that created by cosmic rays can damage the DNA in different ways. One way is to damage the nucleotides or the sugar moieties. Electrons from the high energy rays are transferred to the oxygen or nitrogen atoms in the DNA nucleotides for example, causing the formation of a mutation—or to be more exact, the wrong base pair to be formed during replication. Another possibility is that the high energy electrons actually cause breaks in the DNA strands. Usually, in cells with DNA damage, the cell would either repair the DNA or execute cell death (apoptosis). If the mutations are not eliminated through the repair system, the cells begin to divide and produce more copies of the damaged DNA (Figures 11 and 12, above).

A third way that ionizing radiation causes DNA damage is indirectly by increasing the amount of reactive oxygen species in a cell. Reactive oxygen species are chemically reactive, charged molecules that damage many parts of the cell, including the DNA. Although reactive oxygen species are formed naturally in the body, our body has repair mechanisms (such as p53) that can account for these errors. However, when reactive oxygen species increase and there are mutations in p53 genes, the combination is highly likely to lead to uncontrollable cell growth and cancer.