AGB 121: Principles of Animal Genetics and Population Genetics (2+1)

PHYSICAL MUTAGENS

Radiation induced mutation

• That portion of the electromagnetic spectrum containing wavelengths that are shorter and of higher energy than visible light (wavelengths below about 100 nm) can be subdivided into
  • Ionizing radiations (X-rays, protons, neutrons, and alpha, beta and gamma rays) and
  • Non-ionizing radiations (UV light).

Ionizing radiation

• Ionizing radiations induce point mutations as well as various kinds of gross changes in chromosome structure such as deletions, duplications, inversions and translocations.
• The ionizing radiations such as X-rays (about 0.1 to 1 nm) are of high energy and can penetrate living tissues.
• In the process of penetrating matter, these high energy rays collide with atoms and cause release of electrons, leaving positively charged ions or radicals.
• These ions, in turn, collide with other molecules, causing the release of further electrons.
• The net result is that a “core” of ions is formed along the track of each high energy ray as it passes through matter or living tissues.
• This process of ionization is produced by X-rays, protons, neutrons, and alpha, beta and gamma rays released by radio active isotopes of the elements such as ³²P, ³⁵S or Cobalt-90 etc.

Non - ionizing radiation

• Ultra violet rays, having lower energy, penetrate only the surface layer of cells in higher animals and do not induce ionization.
• UV lights dissipate their energy to atoms that they encounter, raising the electrons in the outer orbital to higher energy levels, a state referred to as excitation.
• Molecules containing atoms in either ionic forms or excited states are chemically more reactive than those containing atoms in their normal stable states.
• The increased reactivity of atoms present in DNA molecules is the basis of mutagenic effects of UV and ionizing radiations.
• Ultra violet rays are readily absorbed by purines and pyrimidines in DNA, which then enter a more reactive or excited state.
• The maximum absorption of UV by DNA is at wavelength of 260 nm.
• Maximum mutagenicity also occurs at 260 nm, suggesting that the UV induced mutation process is mediated directly by the absorption of UV by purines and pyrimidines.
• UV absorption by pyrimidines results in pyrimidine hydrates ( example : Cytosine hydrate ) and pyrimidine dimers ( example: thymine dimer ) which cause mispairing of bases and blockage of DNA replication, respectively.