**3. Types of damages induced after radiation exposure**

Ionizing radiation can produce different types of damage to DNA, RNA, proteins and other biomolecules. DNA is the major target of radiation induced damage where as membrane is an alternative target. Because the portion of water in living matter is quite high, radiolytic product of the water, mainly hydroxyl radical is responsible for most damages to biomolecules. Hydroxyl radical causes damages to biomolecules by abstracting an H-atom from the biomolecules (from the sugar moiety of the DNA or from the peptide chain of a protein) or by addition of the double bonds of aromatic moieties (DNA bases or aromatic moieties of protein side chain) (Spothem-Maurizot et al. 2008).

Ionizing radiation causes the formation of strand breaks in cellular DNA, as well as other types of lesions in the chromatin of cells(Roots et al. 1985). The amount of DNA damage induced is determined by type of radiation as well as the presence of other molecular components in close proximity to DNA, in particular the presence of proteins because it is well known that most molecular interactions between proteins and DNA occur via amino acids. It is estimated each gray (Gy) of radiation leads to about 100,000 ionizations within a cell, damage to over 1,000 bases, about 1,000 SSBs and about 20 – 40 DSBs. Despite this, 1 Gy kills only 30% of mammalian cells due to the effectiveness of DNA repair - particularly for non-DSB lesions (Spothem-Maurizot et al. 2008).

Lipid peroxidation has been found as the main damage to membrane lipids and lipoproteins. Ionizing radiation induced lipid oxidative modifications of poly unsaturated fatty acids (PUFAs) appears as a dynamic process initiated by hydroxyl free radicals generated by water radiolysis, amplified by a propagating-chain mechanism involving alkyl and peroxyl free radicals, and leading not only to hydroperoxides but also to a lot of other lipidic oxidized end-products, lipid hydroperoxides and conjugated dienes which are early products of lipid peroxidation (Spothem-Maurizot et al. 2008).

During ionizing radiation induced damage to protein, the type of reactions and consequences are quite similar to those of DNA; abstraction of H atoms and binding to aromatic rings, leading to backbone breakage and modification of side chains. All these event leads to peptide chain fragmentation and modification of amino acid side chain (e.g.Trp/formylkynurenine, Tyr/bityrosine,Cys/disulfide) (Spotheim-Maurizot and Davidkova 2011).
