Effect of Radiation on Nucleotide Sequence
Radiation is the emission of energy in the form of waves, rays or particles. These include ionizing and non-ionizing radiations. Ionizing radiation is radiation with sufficient energy to remove an electron from an atomic orbital, forming an ion. Ionizing radiation includes x-ray, gamma rays, alpha particles, and beta particles.
Non-ionizing radiation is the release of energy from the lower-energy region of the electromagnetic spectrum. Sources of non-ionizing radiation include light, infrared (heat), and UV light.
The radiations can have direct effect on chromosomes. They may directly break chromosomes or alter one of the DNA bases or indirectly may initiate a chain of chemical reactions. The biological effect also depends on the kind of cell and stage of nuclear cycle. For instance, chromosomes are extremely sensitive to breakage in meiotic prophase. The frequency of mutants per viable organisms often increases linearly with the dose.
Timofeeff-Ressovsky et al (1935) target theory:
Timofeeff-Ressovsky interprets this relationship in terms of a target theory. The theory states that single hit of the particle on the target (i.e., the genetic material) inactivates or mutates it. There is an assumption that each event occurs at random in an irradiated system, so that any target may be hit. Therefore, for small doses, the number of targets hit and the number of targets affected both will have linear relationship with amount of radiation. When the doses increase further, some of the hits may go waste, since they may hit a target already affected. Therefore, at higher doses the affected targets will be less than expected on the basis of linear relationship.
It is possible that radiations act through production of a chemical. The meaning of the word target then depends on the nature of the chemical produced or metabolic lifetime of such a chemical i.e., whether it can get to any other place in the cell or is limited to the closest surroundings. Single hit may then mean single chemical event or a single ionization or even a single cut by a densely ionizing particle.
The frequency of simple chromosomal aberrations e.g., terminal deletions, is proportional to the dose, since they arise by single hits. More complicated aberrations needing two breaks (i.e., interstitial deletions and exchanges), increase approximately with the increase of square of the dose.
The frequency of two hit abberations is more if radiations is given continuously than if given in fractions, since in the latter case healing of the breaks may take place.
Intensity of radiation
Intensity of radiation may also influence the frequency of aberrations. The frequency will be higher at higher intensity and will increase with increase of square of the dose. Chloramphenicol keeps the breaks open longer, showing a role of protein in healing.
The indirect effect of chemical was obvious when certain chemicals were found to protect the organism from radiation and some other chemicals could reinforce the effect. For instance low oxygen concentration reduced the frequency of chromosome breaks induced by radiations. This oxygen effect also termed as anoxia effect has been extensively studied and it is probable that radiations in the presence of oxygen form some peroxide radicals which may then influence the frequency of breaks and mutation. The peroxides may cause breaks or may prevent rejoining. Free radicals and peroxides form along the path of radiations in cells and cause chromosome breaks or mutations.
Ionization of water in the cells may give free radicals and hydrogen peroxide.
Non ionizing radiations
UV has more specific chemical effect than ionizing radiations. UV absorption limits to molecules carrying conjugated double bonds and each molecule has a special absorption spectrum with maxima at certain specific wavelengths. For nucleic acids these wavelengths are in the range 260 to 280 nm. UV of this range of wavelength exerts strong direct effect on nucleic acids.
In bacteria there is a possibility of an indirect induction of mutations by UV. UV irradiation of culture media produces mutagens which induces mutations in unirradiated organisms present in this media. The radiation product is apparently unstable and organic peroxides or radicals are the cause of mutation. Organic peroxides are mutagenic while H2O2 in saline is not. But since UV also acts on the pyrimidine bases and on the corresponding nucleotides within the bacterium it’s not clear, how the mutagenic effect comes about. That most mutations due to UV involve extrachromosomal material is obvious from the fact that their frequency reduces if post irradiation protein synthesis is blocked . The frequency increases by addition of nucleic acid bases.
The relationship of mutation frequency and UV dose is also not always linear or exponential. For instance, in Aspergillus, at higher doses the frequency falls down. A relationship between UV and DNA is shown by in vitro studies. Here, addition of H2O molecule hydrates thymine and cytosine.
Similarly two molecules of thymine connect to give rise to a thymine dimer.
According to Kelner, there is a reversal of UV effect i.e., by exposing the cells to visible light containing wavelength in the blue region of the spectrum. This phenomenon is termed as photoreactivation ( in bacteria and bacteriophages). An enzyme causes photoreactivation, whose specific activity lies in its ability to split thymine dimers and repair DNA molecule.
Other enzymes completely remove these dimers and repolymerise missing nucleotides in E coli. In human beings also, the sunlight (due to UV light component) causes DNA damage, producing thymine dimers.