|Fig. 1: Two ringed base pairs fuse together. (Source: C. Carlton)|
Radiation damage to cells can either occur directly or indirectly.  Most of the notable damage occurs to the cell DNA. Cells have repair mechanisms to fix this damage, but these mechanisms are not perfect and occassionally damage persists. This damage can have consequences such as cell death or development of cancer.
In direct action, the DNA is hit by radiation directly resulting in damage. When DNA molecules are directly exposed to UV radiation, the energy absorbed by the light makes the molecules more reactive. This energy is often diffused with no consequences but this energy can occassionally cause the rings of two base pairs to fuse in a pericyclic reaction (Fig. 1). This fusion can result in mutations or interfere with the ability of proteins to correctly read DNA. Radiation can also cause double strand breaks, where the backbone structure of the DNA is broken. 
In indirect damage, radiation hits water and other organic molecules, generating free radicals. These free radicals can react with DNA molecules and cause structural damage. This damage is very common. One study has found that up to 50,000 DNA lesions occur daily via these free radicals. In comparison, a therapeutic dose of 2 Gy/fraction of ionising radiation causes about 3000 lesions per cell. 
Luckily, cells also have the ability to repair DNA damage. Certain repair proteins can detect these damaged sites, and repair the damage. However, not all damaged sites are detected or repaired, which is why radiation can cause damage that persists as a mutation. This mutation is passed on to daughter cells as the DNA replicates with the mutation conserved during mitosis. 
© Cody Carlton. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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