In 1927, Herman Muller demonstrated that mutations in fruit flies could be induced by X-rays. The results of subsequent studies showed that X-rays greatly increase mutation rates in all organisms. The high energies of X-rays, gamma rays, and cosmic rays (< Figure 17.23) are all capable of penetrating tissues and damaging DNA. These forms of radiation, called ionizing radiation, dislodge electrons from the atoms that they encounter, changing stable molecules into free radicals and reactive ions, which then alter the structures of bases and break phosphodiester

17.23 In the electromagnetic spectrum, as wavelength decreases, energy increases. (Adapted from Life 6e, figure 8.5).

17.22 Intercalating agents such as proflavin and acridine orange insert themselves between adjacent bases in DNA, distorting the three-dimensional structure of the helix and causing single-nucleotide insertions and deletions in replication.

Cosmic rays/Gamma rays


Wavelength (nm)

Ultraviolet (UV)

Visible light

Infrared (IR)

Shorter wavelengths are more energetic.

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