As mentioned at the start of this section, the first mutagenic agent to be demonstrated were X-rays; along with ultraviolet light, they are the most commonly used physical mutagen. X-rays, like other forms of ionising radiation, cause the formation of highly reactive free radicals. These can bring about changes in base structure as well as gross chromosomal alterations. DNA strands can be broken and reannealed incorrectly, to produce errors in the sequence.
In recent years, we have all become much more aware of the possible perils of excessive exposure to sunlight. Ultraviolet light (UV) from the sun damages DNA in skin cells, which can lead to them becoming cancerous. The specific action is on adjacent pyrimidine bases (usually thymines) on the same strand, which are cross-linked to form a dimer (Figure 11.21). This results in a distortion of the helix and interferes with replication. UV light is most effective at wavelengths around 260 nm, as this is the wavelength most strongly absorbed by the DNA bases.
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