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Among the physical mutagens radiation is the most important. The energy content of a radiation depends upon its wavelength. In general, the shorter the wavelength the greater the energy value of the radiation. High-energy radiations can change the atomic structure of a substance by causing the loss of an electron and the formation of an ion. In this excited state the atom is highly reactive and is called a free radical. Radiation which brings about such a state is called ionizing radiation. Alterations are nucleic acids caused by radiation are of great genetic importance. High-energy ionizing radiation and ultraviolet (UV) light are important mutagenic agents.

Both DNA and RNA preferentially absorb UV-light, causing their nitrogen-containing bases to become highly reactive free radicals. Moreover, if the mutation happens to take place in germ cells the mutated DNA strands could be passed on to succeeding generation.

The primary mutagenic effect of UV-light appears to be due to the production of the thymine dimmers. DNA yields three possible type of pyramiding dimmers in DNA.
Thymine-thymine – 50%
Thymine-cytosine – 40%
Cytosine-cytosine – 10%

Pyramidine dimmer can also be formed between adjacent strands. In RNA pyrimidine dimmers are formed between adjacent uracil and cytosine rings. Distortions DNA caused by thymine dimmers can be corrected by a repair mechanism. An exonuclease recognizes the distorted region and excises it. A second enzyme, DNA polymerase inserts the correct bases in the gap. A third enzymes, ligase, joins the inserted bases. The DNA is thus restored to its original conditions.
UV-radiation also causes addition of water molecules to pyrimidines in both DNA and RNA, resulting in the formation of photo hydrates. The water molecule is added across the C5-C6 double bond.

X-rays bring about mutations by breaking the phosphate ester linkages in DNA. The breakage may take place at one or more points. As a result a large number of bases are lost (deletion) or rearranged.

thymine dimer

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