 When radiation interacts with target atoms, energy is deposited, resulting in ionization or excitation.  The absorption of energy from ionizing radiation produces damage to molecules by direct and indirect actions.  For direct action, damage occurs as a result of ionization of atoms on key molecules in the biologic system. This causes inactivation or functional alteration of the molecule.  Indirect action involves the production of reactive free radiacals whose toxic damage on the key molecule results in a biologic effect.

 Ionization of atoms in molecules is a result of absorption of energy by photoelectric and Compton interactions. Ionization occurs at all radiation qualities but is the predominant cause of damage in reactions involving high LET radiations. Absorption of energy sufficient to remove an electron can result in bond breaks.  Ionizing radiation+RH R - + H +

 These are effects mediated by free radicals.  A free radical is an electrically neutral atom with an unshared electron in the orbital position. The radical is electrophilic and highly reactive. Since the predominant molecule in biological systems is water, it is usually the intermediary of the radical formation and propagation.

Free radicals readily recombine to electronic and orbital neutrality. However, when many exist, as in high radiation fluence, orbital neutrality can be achieved by: 1.Hydrogen radical dimerization (H 2 ) 2.The formation of toxic hydrogen peroxide (H 2 O 2 ). 3.The radical can also be transferred to an organic molecule in the cell. H-O-H  H OH + + e (ionization) H OH + + e  H 0 +OH 0 (free radicals)

 H 0 + OH 0  HOH (recombination)  H 0 + H 0  H 2 (dimer)  OH 0 + OH 0  H 2 O 2 (peroxide dimer)  OH 0 + RH  R 0 + HOH (Radical transfer)  The presence of dissolved oxygen can modify the reaction by enabling the creation of other free radical species with greater stability and lifetimes  H 0 +O 2  HO 2 0 (hydroperoxy free radical)  R 0 +O 2  RO 2 0 (organic peroxy free radical)

 The lifetimes of simple free radicals (H 0 or OH 0 ) are very short, on the order of sec. While generally highly reactive they do not exist long enough to migrate from the site of formation to the cell nucleus. However, the oxygen derived species such as hydroperoxy free radical does not readily recombine into neutral forms. These more stable forms have a lifetime long enough to migrate to the nucleus where serious damage can occur.

 The transfer of the free radical to a biologic molecule can be sufficiently damaging to cause bond breakage or inactivation of key functions  The organic peroxy free radical can transfer the radical form molecule to molecule causing damage at each encounter. Thus a cumulative effect can occur, greater than a single ionization or broken bond.

 DNA is the most important material making up the chromosomes and serves as the master blueprint for the cell. It determines what types of RNA are produced which, in turn, determine the types of protein that are produced.  The DNA molecule takes the form of a twisted ladder or double helix. The sides of the ladder are strands of alternating sugar and phosphate groups. Branching off from each sugar group is one of four nitrogenous bases: cytosine, thymine, adenine and guanine. I S-AT-S I P I S-CG-S I P I S-GC-S I P I S-TA-S I

 There is considerable evidence suggesting that DNA is the primary target for cell damage from ionizing radiation.  Toxic effects at low to moderate doses (cell killing, mutagenesis, and malignant transformation) appear to result from damage to cellular DNA. Thus, ionizing radiation is a classical genotoxic agent.

 The lethal and mutagenic effects of moderate doses of radiation result primarily from damage to cellular DNA.  Although radiation can induce a variety of DNA lesions including specific base damage, it has long been assumed that unrejoined DNA double strand breaks are of primary importance in its cytotoxic effects in mammalian cells.

 Residual unrejoined double strand breaks are lethal to the cell, whereas incorrectly rejoined breaks may produce important mutagenic lesions. In many cases, this DNA misrepair apparently leads to DNA deletions and rearrangements. Such large-scale changes in DNA structure are characteristic of most radiation induced mutations.

 Chromosomes are composed of deoxyribonucleic acid (DNA), a macromolecule containing genetic information. This large, tightly coiled, double stranded molecule is sensitive to radiation damage. Radiation effects range from complete breaks of the nucleotide chains of DNA, to point mutations which are essentially radiation-induced chemical changes in the nucleotides which may not affect the integrity of the basic structure.

 After irradiation, chromosomes may appear to be "sticky" with formation of temporary or permanent interchromosomal bridges preventing normal chromosome separation during mitosis and transcription of genetic information. In addition, radiation can cause structural aberrations with pieces of the chromosomes break and form aberrant shapes. Unequal division of nuclear chromatin material between daughter cells may result in production of nonviable or abnormal nuclei.

 Biological membranes serve as highly specific mediators between the cell (or its organelles) and the environment. Alterations in the proteins that form part of a membrane’s structure can cause changes in its permeability to various molecules, i.e., electrolytes. In the case of nerve cells, this would affect their ability to conduct electrical impulses. In the case of lysosomes, the unregulated release of its catabolic enzymes into the cell could be disastrous. Ionizing radiation has been suggested as playing a role in plasma membrane damage, which may be an important factor in cell death ( interphase death )

 DNA- mutation:Unusual permanent change in the primary structure of DNA ( gene mutation).  Chromosomal mutation: change in the amount of DNA in chromosomes (aberrations) leads to abnormalities in cell division.  Genotoxic:damging the genetic information

 MARK [T] FOR TRUE AND [F] FOR FALSE STATEMENTS FROM THE FOLLOWING.(correct the false statement):  Drugs in aqueous media are generally more stable to radiation than in dry state?  Free radicals, regardless of how they are formed, are chemically less active than neutral atoms.?

 Ionization occurs at all radiation qualities but is the predominant cause of damage in reactions involving low LET radiations.?  The lifetimes of simple free radicals (H 0 or OH 0 ) are very short, on the order of sec. While generally highly reactive they do not exist long enough to migrate from the site of formation to the cell nucleus? 

 The oxygen derived species such as hydroperoxy free radical does not readily recombine into neutral forms. These less stable forms have a lifetime long enough to migrate to the nucleus where serious damage can occur.?  There is a considerable evidence suggesting that RNA is the primary target for cell damage from ionizing radiation.? 

 Ionizing radiation has been suggested as playing a role in plasma membrane damage, which may be an important factor in cell death or mutagenic effect ?  Although radiation can induce a variety of DNA lesions including specific base damage, it has long been assumed that unrejoined DNA double strand breaks are of primary importance in its cytotoxic effects in mammalian cells.? 

 What are the temporal stages of radiation action?  What are the products of water radiolysis?  What is the difference between direct and indirect effects of radiation?