BIOLOGICAL EFFECTS BIOLOGICAL EFFECTS

OF IONIZING RADIATIONOF IONIZING RADIATION

AT MOLECULAT MOLECULES ES AND AND CCELLELLSS

The stage of action The stage of action of ionizing radiationof ionizing radiation

Physical stagePhysical stage The transfer of kinetic energy from The transfer of kinetic energy from ionizing radiation to atoms or molecules ionizing radiation to atoms or molecules leads to excitation and ionization of leads to excitation and ionization of these atoms or moleculesthese atoms or molecules

10 10 –– 1616 ––

10 10 – 15– 15 secondsseconds

Physic-chemical Physic-chemical stagestage

The displace of absorbed energy of The displace of absorbed energy of ionizing radiation into molecules and ionizing radiation into molecules and between them. Formation of free between them. Formation of free radicalsradicals

10 10 – 14– 14 – –

10 10 – 11– 11 secondsseconds

Chemical stageChemical stage Reactions between free radicals, Reactions between free radicals, reactions between free radicals and reactions between free radicals and intact molecules. Formation of molecules intact molecules. Formation of molecules with abnormal structure and functionwith abnormal structure and function

10 10 – 6– 6 – –

10 10 – 3– 3 secondsseconds

Biological stageBiological stage Formation of injures on all levels – from Formation of injures on all levels – from cellular structures to organism and cellular structures to organism and population. population. Development of processes of biological Development of processes of biological damage and reparative processes damage and reparative processes

Seconds Seconds

– – yearsyears

Effect of Effect of rradiation adiation on on atom and moleculesatom and molecules

Effects of Effects of ionizing ionizing rradiation adiation at at aatomic tomic llevelevel

IonizationExcitation

Mechananisms Mechananisms of of ddamageamage at molecular levelat molecular level

Direct Direct aactionction of ionizing radiationof ionizing radiation

Ionizing radiation + RH R- + H+

OH IR – C = NHimidol (enol)

O IIR – C = NH2

amide (ketol)

Tautomeric shifts

Bond breaks

e-

X ray ray P+

OH

H OH-

H+

Ho

OHo

Indirect Indirect aactionction of ionizing radiationof ionizing radiation

Radiolysis of HRadiolysis of H22O O mmoleculeolecule

Shared electron

Shared electron

H-O-H H+ + OH- (ionization)

H-O-H H0 + OH0 (free radicals)

Reaction of Reaction of HH22O O mmoleculeolecule rradiolysisadiolysis

Н2О + h Н2О+ + е-

Н2О + h Н2О* Но + НОо

Н2О + е- е-гидр Но + НО-

Н2О + е- Н2О* Но + ОНо

Н2О Н+ + ОНо

е- + Н+ Но

Н2О+ + ОН- Н2О + ОНо

Н2О+ + Н2О Н3О

+ + ОНо

Н3О+ + е- Н2О + Но

Effects of Effects of ooxygen xygen on on ffree ree rradical adical fformationormation

Oxygen can modify the reaction by enabling creation of other free radical species with greater stability and longer lifetimes

H0 + O2 HO20 (hydroperoxy free radical)

R0 + O2 RO20 (organic peroxy free radical)

Reactions with Reactions with ffree ree rradicalsadicals

H0 + OH0 H2O

H0 + H0 H2

OH0 + OH0 H2O2

RH + OH0 R0 + H2O

RH + H0 R0 + H2

R0 + OH ROH

R0 + H RH

R0 + O2 ROO0

ROO0 + RH ROOH + R0

Lifetimes of Lifetimes of ffree ree rradicalsadicals

3nm

Because short life of simple free radicals (10-10sec), only those formed in water column of 2-3 nm around DNA are able to participate in indirect effect

Ho

OHo Ho

OHo

HO2o RO2

o

Relation Relation bbetween etween llinear inear eenergy nergy ttransfer (LET) and ransfer (LET) and ttypeype of of aactionction

Direct actionDirect action is predominant with is predominant with high LET radiation,high LET radiation, e.g.e.g. alpha alpha particles and neutronsparticles and neutrons

Indirect actionIndirect action is predominant is predominant with with low LET radiationlow LET radiation,, e.g. e.g. X X and gamma raysand gamma rays

Effect of Effect of rradiation adiation on on biological biological moleculesmolecules

Biochemical Biochemical rreactions eactions with ionizing with ionizing radiationradiation

DNADNA is is primary targetprimary target for cell for cell damagedamage from ionizing radiatio from ionizing radiationn

Radiation induced Radiation induced DNA DNA ddamageamage

The most important types of The most important types of radiation induced lesions in radiation induced lesions in DNADNA

Single-strand breaks500-1000 per 1 Gy

Double strand breaks40-50 per 1 Gy

Base damage: 1000-2000 per 1 Gy

Mechanisms of Mechanisms of base base excision and nucleotide excision and nucleotide excision excision rrepairepair

Mechanism of Mechanism of ssingle-strand breaks DNA DNA rrepairepair

1

Endonuclease

2

DNA polymerase

4

DNA ligase

5

3 Exonuclease

DNA DNA rrestoration estoration ffailureailure

Unrejoined DNAdouble strand breaks

Incorrect repair of DNA damage

Cytotoxic effect Mutations

Radiation induced Radiation induced membrane damagemembrane damage

Effect of Effect of rradiation adiation on on ccellellss

Types of Types of ccellular ellular ddamageamage

Interphase cell death

Changes ofmetabolism& function

Mitotic cell death

Norma

repair

Mutation

Cell cycleCell cycle

Radiosensitivity of Radiosensitivity of ccell ell in in ccell ell ccycleycle

Relative survivability of cells irradiated in different phases of

the cell cycle: synchronised cells in late G2 and in mitosis (M)

showed greatest sensitivity to cell killing

M G1 S G2 M

NORMAL IRRADIATED

Mitotic Mitotic ddeatheath

The most ‘radiosensitive’ cells are actively proliferating (dividing) at

the time of exposure

undifferentiated (non-specialized in structure and function)

Bergonié and Tribondeaus’ Bergonié and Tribondeaus’ ‘law’ (1906)‘law’ (1906)

Interphase deathInterphase death

Why are peripheral blood lymphocytes highly sensitive to radiation, although well differentiated?

Pyknosis: The nucleus becomes contracted, spheroidal, and filled with condensed chromatin.

Karyolysis: The nucleus swells and loses its chromatin.

Protoplasmic Coagulation: Irreversible gelatin formation occurs in both the cytoplasm and nucleus.

Karyorrhexis: The nucleus becomes fragmented and scattered throughout the cell.

Cytolysis: Cells swell until they burst and then slowly disappear.

Apoptosis: Programmed cell death, usually be fragmentation

Morphological forms Morphological forms of cell deathof cell death

Block of Mitotic Cycle: Mitosis may be delayed or inhibited following radiation exposure.

Disruptions in Cell Growth: Cell growth may also be retarded, usually after a latent period.

Permeability Changes: Irradiated cells may show both increased and decreased permeability.

Changes in Cell Motility: The motility of a

cell may be decreased following irradiation.

Changes of cell Changes of cell metabolism and functionmetabolism and function

Radiation induced Radiation induced chromosome damagechromosome damage

ChromosomesChromosomes

Radiation Radiation iinduced nduced cchromosomal hromosomal aaberrationsberrations

Vegetative Cells: these cells, comprising differentiated functional cells of a large variety of tissues, are generally the most radiosensitive.

Differentiating Cells: these cells are somewhat less sensitive to radiation; they are relatively short‑lived and include the first generation produced by division of the vegetative mitotic cells.

Totally Differentiated Cells: these cells are relatively radioresistant; they normally have relatively long lifespans and do not undergo regular or periodic division in the adult stage, except under abnormal conditions such as following damage to or destruction of a large number of their own kind.

Fixed Nonreplicating Cells: these cells are most

radioresistant; they are highly differentiated morphologically and highly specialized in function.

Relative cellular radiosensitivity

Summary of lectionSummary of lection

• Cells going through the division phase (M Cells going through the division phase (M and S) are generally the most sensitive and S) are generally the most sensitive to ionizing radiation. to ionizing radiation. Exceptions: Exceptions: lymphocytes and some bone marrow stem cells, lymphocytes and some bone marrow stem cells,

which exhibit interphase deathwhich exhibit interphase death • Bone marrow consists of progenitor and Bone marrow consists of progenitor and

stem cells, the most radiosensitive cells stem cells, the most radiosensitive cells in the human body and the most in the human body and the most important in controlling infection important in controlling infection

Lecture is endedLecture is ended

THANKS FOR ATTENTION

In lecture materials of the International Atomic Energy Agency (IAEA), kindly given by doctor Elena Buglova, were used