*1.3.1 Alpha particles*

*DNA - Damages and Repair Mechanisms*

radiation safety [1–3].

nuclear weapon development etc., has increased the risk of exposure. Inadvertent accidents from nuclear power plant installations, nuclear weapon testing and illegal use of radioactive material in dirty bomb have raised an international concern for

Radiation therapy is the most common and deliberate exposure of high energy rays to living organisms. This exposure is mainly therapeutic for treatment of cancer but since there is no clear demarcation to protect the adjacent noncancerous cells leads to disastrous effect. The immediate exposure of high energy beam of radiation leads to destruction of cancerous cells. Whereas the adjacent normal cells are however exposed to these rays suffer adverse effects. It indirectly generates reactive oxygen species (ROS) inside the cellular system through hydrolysis of water. ROS directly targets cellular DNA and affect the cell survival by damaging macromolecules like lipid, proteins and carbohydrate. The damage induction in DNA molecules could be of various types like double strand break, single strand break, dimer formation, alteration of bases etc. Mammalian cells are equipped with very efficient DNA repair mechanism to handle these different damages [4]. Moreover among all, double strand breaks are known to be lethal damage for the cells. There are mainly two repair mechanisms that operate for repair of double strand breaks. These are 1) Homologous repair pathway (HR) 2) Non homologous end joining pathway (NHEJ). The basis on which cell decides to choose one of the two available pathways is simply on cell cycle phase, its type and damage threshold [5]. There are also exists third repair pathway i.e. Alternative non homologous end joining pathway (A-NHEJ) it is much slower than the above mentioned pathway. It comes into play when above mentioned pathway fail to repair the damage thus acting as a backup pathway. The presence of this alternative pathway has not been studied extensively but it has been speculated for its role in combinational cancer therapeutics. In this chapter, we have briefly described the various kind of DNA damage generated by radiation and role of DNA repair pathways specially NHEJ and A-NHEJ in handling the

repair and their applications in progression of disease [6].

and non-ionizing radiation [6, 7].

examples of non-ionizing radiation [5].

**1.1 Non-ionizing radiation**

**1.2 Ionizing radiation**

Radiation, which has particles with enough energy to rip electron from atoms or molecules is known as ionizing radiation. Radiation is the emission and propagation of energy in the form of rays or waves. The term radiation comes after the discovery of X-ray in 1895 by Wilhelm Conrad Roentgen. Henri Becquerel and Marie Curie have made significant contributions in studying the effect and application of radiation in various fields. Excitation and ionization properties are common responsible factor for radiation emitted by any radioisotopes. It has two major types: ionizing

It does not carry enough energy to remove electrons from an atom or molecule. Because of their low energy, non-ionizing radiation poses a lower risk than ionizing radiation. Visible light, near ultra violet, infrared, microwave and radio waves are

Ionizing radiation (IR), as the name indicates carry sufficient energy to remove

electrons from atoms or molecules. It can be in particulate or electromagnetic form. The particulate forms consist electrons, protons, neutrons, α-particles etc. and the electromagnetic form includes as cosmic rays, X-rays, gamma rays etc. [5]. Ionizing radiation exposure may cause tissue injuries to the biological system

**100**

An alpha ray consists of two protons and two neutrons. These rays have a strong nuclear force and have the ability to bind to the nucleus of any atom. Due to their charge and mass, alpha particles interact strongly with matter and only travel a few centimeters in air. Alpha particles are unable to penetrate the outer layer of dead skin cells but are capable of causing serious cell damage if an alpha emitting substance is ingested in food or air [5].
