Skip to main content
Biology LibreTexts

7.3: Ionizing Radiation

High energy radiation, such as X-rays, \(\gamma\)-rays, and \(\beta\) particles (or electrons) are powerful mutagens. Since they can change the number of electrons on an atom, converting a compound to an ionized form, they are referred to as ionizing radiation. They can cause a number of chemical changes in DNA, including directly break phosphodiester backbone of DNA, leading to deletions. Ionizing radiation can also break open the imidazole ring of purines. Subsequent removal of the damaged purine from DNA by a glycosylase generates an apurinic site. 

Figure \(\PageIndex{1}\): Formation of thymine dimer lesion in DNA. The photon causes two consecutive bases on one strand to bind together, destroying the normal base-pairing double-strand structure in that area. Ultraviolet (UV) photons harm the DNA molecules of living organisms in different ways. In one common damage event, adjacent bases bond with each other, instead of across the “ladder.” This makes a bulge, and the distorted DNA molecule does not function properly. Image used with permission (Public Domain; Master Uegly).

Ultraviolet Radiation

Ultraviolet radiation with a wavelength of 260 nm will form pyrimidine dimers between adjacent pyrimidines in the DNA. The dimers can be one of two types (Fig. 7.11). The major product is a cytobutane-containing thymine dimer (between C5 and C6 of adjacent T's). The other product has a covalent bond between position 6 on one pyrimidine and position 4 on the adjacent pyrimidine, hence it is called the "6-4" photoproduct.

Figure \(\PageIndex{2}\): Pyrimidine dimers formed by UV radiation, illustrated for adjacent thymidylates on one strand of the DNA. (A) Formation of a covalent bond between the C atoms at position 5 of each pyrimidine and between the C atoms at position 6 of each pyrimidine makes a cyclobutane ring connecting the two pyrimidines. The bases are stacked over each other, held in place by the cyclobutane ring. The C-C bonds between the pyrimidines are exaggerated in this drawing so that the pyrimidine ring is visible. (B) Another photoproduct is made by forming a bond between the C atom at position 6 of one pyrimidine and position 4 of the adjacent pyrimidine, with loss of the O previously attached at position 4. Image used with permission (Public Domain; Master Uegly).

The pyrimidine dimers cause a distortion in the DNA double helix. This distortion blocks replication and transcription.

Exercise \(\PageIndex{1}\)

What is the physical basis for this distortion in the DNA double helix?