**2. Effects of ionizing radiation on bio-molecules**

Exposure to any types of ionizing radiations, whether man-made or natural have deleterious biological effects at any dose. Primary ionization of an atom in the biological system can induce either direct or free radicals mediated indirect damage. Radiation can damage the bio-molecules by both directly and or indirectly by generating free radicals (**Table 1**) [5, 14].

Among the bio-molecules damages, DNA damage has been shown to be most important and to contribute maximally to cell death [15, 16]. Studies made on DNA irradiated *in vitro* in solution, in the dry state or *in vivo* in the biological system have revealed that radiation causes a spectrum of damages to DNA. Among them, the important ones are an alteration of purine and pyrimidine bases, single and double strand breaks, removal of bases and crosslinking of DNA with DNA or adjacent protein molecules. When a cell is exposed to radiation reactive oxygen species (ROS) is generated which targets cellular DNA for base modification, DNA adducts, DNA single strand break and double strand breaks. All these alterations


**103**

*Recent Perspectives in Radiation-Mediated DNA Damage and Repair: Role of NHEJ…*

cause mutation and cell death (**Figure 1**). Endogenous genomic DNA damages are a relatively common event in the cellular life and if not repaired efficiently may lead

Radiation induced DNA damage can be divided into four categories [9, 10]:

The most frequent modification is formation of hydroperoxide in the presence

Alteration in deoxyribose sugar is not very well understood and the alteration is (0.2–0.3 alterations of sugar per 10 SSBs. For this modification sugar is first oxidized and then hydrolysed followed by liberation of base, with or without

of oxygen. The most important one is hydroperoxidation of thymine [5].

Intra-strand crosslinks - between two parts of a single strand.

Dimer formation - it occurs when two adjacent bases of single strands are joined by covalent bonds. It leads to the formation cyclobutane ring between them. Replication halts at the place where dimmers are formed. Thymine-thymine dimmers are most resistant and stable ones. They induce cutaneous cancers in the

There are approx 50,000 damage per day occurs inside the body due to the normal metabolic process such as maintenance of and replication of the genetic material. However, damage to DNA is native to life because its integrity is under constant attack from numerous endogenous agents such as free radicals generated during essential metabolic processes and from exogenous sources including radiation and chemicals. Endogenous damage affects the primary, rather than the secondary structure of the double helix. Four general classes of endogenous modifications can

The oxidized bases formed as a byproduct due to oxygen metabolism show miscoding eg 8-oxo-7,8-dihydroguanine (8-oxoG), thymine glycol and similar oxidized bases [17]. Among these 8-oxoG is the most abundant and most dangerous one. It mispairs with adenine [18]. Strand interruptions are also generated by reactive oxygen species [19]. The spontaneous mutation rate due to single strand break is still unknown. Activation of poly ADP ribose polymerase (PARP) exerts most

Inter-strand crosslinks - between the two strands.

*DOI: http://dx.doi.org/10.5772/intechopen.96374*

to mutation, cancer, and cell death.

2.Alteration of sugar moiety

4.Single-strand breaks

5.Double-strand breaks

**2.2 Sugar modifications**

**2.1 Base modification and damages**

breakage of phosphodiester bonds [5].

regions exposed to UV light [5].

be envisaged as follows [11].

accurate response to single strand breaks [20].

*2.3.1 Oxidation*

**2.3 Cross-links and formation of dimers**

3.Cross-links formation of dimers

1.Base damage

**Table 1.**

*Biomolecules damage by radiation exposure [9–13].*

*Recent Perspectives in Radiation-Mediated DNA Damage and Repair: Role of NHEJ… DOI: http://dx.doi.org/10.5772/intechopen.96374*

cause mutation and cell death (**Figure 1**). Endogenous genomic DNA damages are a relatively common event in the cellular life and if not repaired efficiently may lead to mutation, cancer, and cell death.

Radiation induced DNA damage can be divided into four categories [9, 10]:

1.Base damage

*DNA - Damages and Repair Mechanisms*

*1.3.3 Gamma rays*

*1.3.4 X-rays*

similar to gamma radiation [5].

generating free radicals (**Table 1**) [5, 14].

**Biomolecule Damage**

*Biomolecules damage by radiation exposure [9–13].*

are emitted by unstable nuclei rich in neutrons, they are high energy electrons. These particles are negatively charged and have intermediate penetration power [5].

Gamma radiation, unlike alpha or beta, does not consist of any particles; instead, they consist of a photon of energy being emitted from an unstable nucleus. These are produced by a change in the energy levels of the atomic nuclei. The wave length of this radiation varies from 0.0003 nm to 0.1 nm. Gamma rays do not have any mass or charge. It can travel at much higher speed in air than alpha or beta rays and loses only half of its energy for every 500 feet. Gamma rays can be stopped by dense and thick layer of material such as lead or depleted uranium. These materials

X-rays are generated from electron cloud when electron moves from higher energy level to lower energy level causing excess energy to be released. It is very

Exposure to any types of ionizing radiations, whether man-made or natural have deleterious biological effects at any dose. Primary ionization of an atom in the biological system can induce either direct or free radicals mediated indirect damage. Radiation can damage the bio-molecules by both directly and or indirectly by

Among the bio-molecules damages, DNA damage has been shown to be most important and to contribute maximally to cell death [15, 16]. Studies made on DNA irradiated *in vitro* in solution, in the dry state or *in vivo* in the biological system have revealed that radiation causes a spectrum of damages to DNA. Among them, the important ones are an alteration of purine and pyrimidine bases, single and double strand breaks, removal of bases and crosslinking of DNA with DNA or adjacent protein molecules. When a cell is exposed to radiation reactive oxygen species (ROS) is generated which targets cellular DNA for base modification, DNA adducts, DNA single strand break and double strand breaks. All these alterations

**DNA** Loss of nucleotide and base modification, deletion of hydrogen bonds, sugar-

weight modifications and change in solubility. **Lipids** Peroxidation and carbon bond rearrangement, conjugate dieneand aldehyde

**Carbohydrates** Breakage of glycosidic Bonds and monomers, alcohol oxidation to aldehydes. **Amino acids** Generation of ammonia, CO2, H2S, Hydrogen molecules, Pyruvic Acid

**Proteins** Degradation and modification of amino acids, cross linkage, denaturation, molecular

thymidyl and sugar radicals

**Thiols** Redox reactions, radical formations, cross linkage.

phosphate bonds, DNA-protein cross linking, single or double strand break, guanyl,

formation, lipid cross-linking, increased microviscosity, cell membrane rupture.

are used as an effective shielding in radiation related work [5].

**2. Effects of ionizing radiation on bio-molecules**

**102**

**Table 1.**


#### **2.1 Base modification and damages**

The most frequent modification is formation of hydroperoxide in the presence of oxygen. The most important one is hydroperoxidation of thymine [5].

#### **2.2 Sugar modifications**

Alteration in deoxyribose sugar is not very well understood and the alteration is (0.2–0.3 alterations of sugar per 10 SSBs. For this modification sugar is first oxidized and then hydrolysed followed by liberation of base, with or without breakage of phosphodiester bonds [5].

#### **2.3 Cross-links and formation of dimers**

Intra-strand crosslinks - between two parts of a single strand. Inter-strand crosslinks - between the two strands.

Dimer formation - it occurs when two adjacent bases of single strands are joined by covalent bonds. It leads to the formation cyclobutane ring between them. Replication halts at the place where dimmers are formed. Thymine-thymine dimmers are most resistant and stable ones. They induce cutaneous cancers in the regions exposed to UV light [5].

There are approx 50,000 damage per day occurs inside the body due to the normal metabolic process such as maintenance of and replication of the genetic material. However, damage to DNA is native to life because its integrity is under constant attack from numerous endogenous agents such as free radicals generated during essential metabolic processes and from exogenous sources including radiation and chemicals. Endogenous damage affects the primary, rather than the secondary structure of the double helix. Four general classes of endogenous modifications can be envisaged as follows [11].

#### *2.3.1 Oxidation*

The oxidized bases formed as a byproduct due to oxygen metabolism show miscoding eg 8-oxo-7,8-dihydroguanine (8-oxoG), thymine glycol and similar oxidized bases [17]. Among these 8-oxoG is the most abundant and most dangerous one. It mispairs with adenine [18]. Strand interruptions are also generated by reactive oxygen species [19]. The spontaneous mutation rate due to single strand break is still unknown. Activation of poly ADP ribose polymerase (PARP) exerts most accurate response to single strand breaks [20].

#### *2.3.2 Methylation*

Some small molecules such as S- adenosylmethionine can methylate bases endogenously. According to recent study from almost 4000 residues generated per day 7-methylguanine (7 meG) is most important. 7-methylguanine base is relatively harmless and doesnot show any cytotoxic properties. Whereas endogenously produced 3-methyladenine (3-meA) which are few hundred in number are building block of DNA replication and should be efficiently repaired [21].

#### *2.3.3 Hydrolysis*

The base sugar bonds in DNA are relatively labile and several thousands of bases are lost each day in human cells under physiological conditions [12]. Purines are lost more easily than pyrimidines. Base loss sites probably represent the most frequent damage in human cells.

#### *2.3.4 Mismatches*

Mismatches can occur in DNA due to the incorrect incorporation by DNA polymerases, damage to the nucleotide precursors in the cellular nucleotide pool or by damage to DNA [13].

## **3. Single strand breaks (SSBs)**

SSBs arise when diester bond between phosphate and the deoxyribose breaks. After the breakage of phosphodiester bond separation of both the strands occurs causing the water molecule to penetrate the breach. This process causes breakage of hydrogen bonds between the bases [5].

## **4. Double strand breaks (DSBs)**

When two complementary strand of double DNA breaks in a location at a point less than 3 nucleotides is known as DNA DSBs. DSBs are considered as the most deleterious type of damage because both the complementary strands are damaged and it is very difficult for the internal repair mechanism of the cell to handle this type of damage. The factors leading to the formation of DSB include endogenous factors that are associated with physiological processes occurring in the cell and the exogenous ones [22–24].

In the presence of endogenous DNA damage, a cell can survive up to some extent, however the concentrated damages accelerated by exogenous agents such as ionizing radiations, radiomimetic drugs, ultra-violet radiations, and carcinogens can induce permanent changes. These changes lead to cancer or severely impaired cellular functioning and poor repair efficiency which may eventually cause cell death by triggering apoptosis or irreversible cell growth arrest [25]. Ionizing radiations generate ROS, which cause oxidative damage to DNA. The most important ROS are O2• (superoxide radical), OH• (hydroxyl radical) and H2O2 (hydrogen peroxide). The highly reactive hydroxyl radical (OH• ) reacts with DNA and as a result, various forms of DNA damage occur. Exposure of DNA to ionizing radiations result in a number of different lesions in DNA such as base damage, single strand breaks and double strand breaks [9, 10, 26]. DNA DSBs present a major threat to the integrity of chromosomes and viability of cells. Unrepaired or incorrectly repaired DSBs

**105**

**Figure 2.**

*Double strand break repair pathway choice.*

*Recent Perspectives in Radiation-Mediated DNA Damage and Repair: Role of NHEJ…*

may lead to translocations or loss of chromosomes, which could result in cell death or uncontrolled cell growth. In addition, adjacent single-strand breaks in opposite strands may be converted to double strand breaks upon replication. DSBs are lethal unless repaired [27]. Ionizing radiations also induce clustered DNA damage in cells, which symbolize two, or more lesions formed within one or two helical turns of DNA and are in part responsible for the biological effects of ionizing radiation. The damage includes DSBs and non-DSB clustered damage such as SSB formed in close proximity to additional breaks or base lesions on both strands. An increase in the ionizing density of radiation increases the complexity of clustered DNA damage

Humans cells have two major DSBs repair mechanisms i.e. homology directed

Homologous recombination pathway (HR) generally repairs the DNA lesions in late S or G2 phase of cell cycle. HR pathway is a series of interrelated pathways that participate in the repair of different types of DNA damages like double strands breaks (DSBs), interstrand cross links and DNA gaps. Several studies have shown

repair (HDR) and non-homologous end joining (NHEJ) [23]. However, in recent years a new mechanism called as alternative non-homologous end joining (A-NHEJ) has evolved (**Figure 2**). The selection criteria for DNA repair mechanism depends upon cell type, cell cycle phase and damage threshold. The non-dividing cells do not have the option of undergoing HDR but dividing cells can use all the three repair mechanisms with some conditions. The condition is NHEJ and A-NHEJ both can act in all the phases of cell cycle, however, the HDR is only able to act at S/

*DOI: http://dx.doi.org/10.5772/intechopen.96374*

leading to decreased reparability of DSB in cells [28].

**5. DNA DSB repair**

G2 phase of the cell cycle [29].

**5.1 Homologous recombination pathway**

*Recent Perspectives in Radiation-Mediated DNA Damage and Repair: Role of NHEJ… DOI: http://dx.doi.org/10.5772/intechopen.96374*

may lead to translocations or loss of chromosomes, which could result in cell death or uncontrolled cell growth. In addition, adjacent single-strand breaks in opposite strands may be converted to double strand breaks upon replication. DSBs are lethal unless repaired [27]. Ionizing radiations also induce clustered DNA damage in cells, which symbolize two, or more lesions formed within one or two helical turns of DNA and are in part responsible for the biological effects of ionizing radiation. The damage includes DSBs and non-DSB clustered damage such as SSB formed in close proximity to additional breaks or base lesions on both strands. An increase in the ionizing density of radiation increases the complexity of clustered DNA damage leading to decreased reparability of DSB in cells [28].
