Principles of Genetics and Breeding

6.1.4.1 Mutagens

A mutagen, by definition, is an agent which increases the frequency of mutation. Mutation which occur at a frequency above the base (spontaneous rate) are described as induced.

Physical mutations

• short-wavelength ultraviolet irradiation (254 nm) affects DNA in a number of ways, but a well-established action is the formation of thymine dimers, means that adjacent Ts will bond together, instead of with As in the complementary chain. This weakens the double helix and leads to short breakage.
• Chromosome aberrations are therefore typically associated with UV mutagenesis. UV (265mm), g , and X rays are known as physical mutagens.
• Ionizing radiation includes X-rays, g , ß-rays, which act by causing ionization of the medium through which they pass.
• These rays are usually used for mutagenesis only if other mutagens cannot be used (e.g. for cell material impermeable to UV rays).
• Single- and double-strand breaks occur with a significantly higher probability than with all other mutagens.
• Double-strand breaks result in major structural changes, such as translocation, inversion or similar chromosome mutations.
• Muller found that the frequency of induced mutation is directly proportional to the intensity of the mutagen treatment.
• The effect of dosage is cumulative, so exposure to 1rad/year for ten year is equivalent to 10 rads/year.