Lecture 1 Physical and chemical interactions DNA strand breaks DNA

Wayne State University Karmanos Cancer Institute Detroit Medical Center

Applied Radiobiology in Radiological Science

Lecture 1Physical and chemical interactions

DNA strand breaksChromosome aberrations

DNA as the principle target


Physical Interactions ofRadiation with Matter: 1

Types of radiationw ionizing and non-ionizingw directly ionizing

• charged particles such as electrons, protons, α particles

w indirectly ionizing

• x rays, γ rays, neutrons

w electromagnetic radiationsw particulate radiations



Ionizing and non-ionizing radiationsw excitationw ionization

w average energy dissipated per ionizingevent •34 eV

w typical energy required to break a molecularbond = 2 6 5 eV



Electromagnetic radiationw photons

• wavelength λ• frequency ν: λν = c (velocity of light)

• energy = hν = hc/λ (h = Planck’s constant)

• energy (keV) = 1.24/λ (nm)

w the electromagnetic spectrum• radiowaves, radar, microwaves, infrared, visible,

ultraviolet, x and γ rays



Photon interactionsw photoelectric absorption

• absorption % Z3 and E-3

w Compton attenuation (independent of Z)• absorption

• scatter

w pair production• > 1.02 MeV, absorption % Z

w photonuclear interactions• important for energies above ~6 MeV



Particulate radiationselectrons

negative

positive (positrons)

protonsneutrons

α particlesheavy ions



Direct and indirect actions of radiationw Direct action: charged particle “directly” interacts with

the target molecule, e.g. breaks bond in DNA moleculew Indirect action: charged particle interacts with a water

molecule producing “free radicals” which then interactwith the target molecule

w For x and γ radiations, indirect interactions cause about80% of the biological damage


Free Radicals

wHave unpaired electrons• e.g. H •, OH •

wAre highly chemically reactivewHave lifetimes of the order of

10-6 seconds


Direct and indirect actions of radiation


Direct Action

w Charged particle (electron, proton,…) causesionization of target moleculew Passage of a single charged particle can

cause single- or double-strand breaksw Action completed in about 10-15 seconds


Indirect Action

w Charged particles ionize molecules (mostcommonly H20) close to the DNA moleculew Free radicals formed from radiolysis of H20

“attack” DNA molecules and cause single-strand breaksw Reactions usually completed in 10-12 to 10-9 s


Chemical interactions with H2O Ionization: H2O 6 H2O+ + e-

H2O+ is an ion free radical (free radicals are moleculeswith unpaired electrons)

Free radicals are highly chemically reactiveH2O+ + H2O 6 H3O+ + OH·

OH·, the hydroxyl free radical, is highly reactive anddiffuses in tissue within a cylinder about 4 nm indiameter (about double the diameter of a DNA doublehelix) and causes about 2/3 of all biological damage


Radiolysis of H20

Other reaction sequences: H20 6 H20

+ + e-

H20+ 6 H+ + OH ·

H20 + e- 6 H20-

H20- 6 H · + OH-

OH · + OH · 6 H2O2

where H2O2 is relatively stable, has time to diffuse,and is a powerful oxidizing agent


Single- and double-strand breaks

w Double-strand breaks considered “lethal”w If produced by the passage of a single charged

particle, often called α-type damagew Two interacting single strand breaks may combine to

form a “lethal” double-strand break, and this is oftencalled β-type damagew Single-strand breaks may be repairedw Repair half-time of the order of hours


Chain of events leading tobiological effects

Incident photon 9

fast electron 9

free radical 9

chemical changes and breakage of bonds 9

biological effects


What Biological Effects?

wCell killingwAcute (early) tissue and organ damagewLate (delayed) tissue and organ damagewCarcinogenesiswGenetic (hereditary) effects



Absorption of neutronsw Elastic scattering

• neutron collides with proton (e.g. hydrogen nucleus)and shares its kinetic energy

• dominant process with fast neutrons of energy < 6MeV in tissue

w Inelastic scattering• fast neutron (~ 6 MeV and above) interacts with

nucleus and causes disintegration


Elastic Scattering


Inelastic Scattering with Carbon Nucleus


Inelastic Scattering with Oxygen Nucleus


Direct Action Dominates forHigh LET Radiations


Physical, chemical, and biological interactions:summary of pertinent conclusions

w X and γ radiations are indirectly ionizing• the first step in their absorption is the

production of fast recoil electrons

w Neutrons are also indirectly ionizing• the first step in their absorption is the

production of fast recoil protons, α particles,and heavier nuclear fragments


Physical, chemical, and biological interactions:summary of pertinent conclusions (cont’d)

w Biological effects of x rays may be due to directaction• the recoil electrons directly ionize the target molecule

w or to indirect action• the recoil electron interacts with water to produce an

hydroxyl free radical, which diffuses to and ionizes thetarget molecule

w About 2/3 of all biological damage with x rays isdue to indirect action


Physical, chemical, and biological interactions:summary of pertinent conclusions (cont’d)

w High-LET radiations produce most biological damage bydirect actionw Physical interactions are completed in about 10-15 sw Chemical effects take longer since the lifetime of a free

radical is about 10-6 sw Biological effects take even longer

• days to months for cell killing• months to years for late tissue damage• years to decades for carcinogenesis• generations for genetic effects


DNA Strand Breaks

A. Normal double helix

B. Single-strand break

(readily repaired)

C. Double-strand break

(repairable)

D. Directly-opposed double-

strand break (irreparable)


Folding and packing of DNAinto a chomosome


Chromosome Aberrations

wChromosome breaks may recombinenormallywThe breaks may recombine abnormally to

give rise to chromosome aberrations suchas dicentric and ring aberrations,translocations and deletions


Translocations and Deletions


Biological Dosimetry


DNA as the principle target

w Microbeam experiments with a particles frompolonium show that the cell nucleus is thesensitive site


DNA as the principle target (cont’d)

w Cells are killed by radioactive tritiated thymidineincorporated during synthesis into the cellw Halogenated pyrimidines incorporated into DNA

in place of thymidine increase radiosensitivityw Factors that modify cell lethality such as type of

radiation, oxygen status, and dose rate alsoaffect chromosome damage in a fashionqualitatively and quantitatively similar


DNA as the principle target (cont’d)

wRadiation sensitivity correlateswell with chromosome volumewA direct correlation has been

reported between chromosomeaberrations and cell lethality


DNA and Chromosome Aberrations:summary of pertinent conclusions

wMany single-strand breaks occur in DNAbut are readily repaired using the oppositeDNA strand as a template (“enzymaticrepair”)wBreaks in both strands that are opposite, or

separated by a few base pairs, may lead todouble-strand breaks


DNA and Chromosome Aberrations:summary of pertinent conclusions (cont’d)

w Radiation-induced DNA breaks in G1

phase cells and incorrect rejoining maylead to chromosome aberrationsw Radiation-induced DNA breaks in S or

G2 phase cells and incorrect rejoiningmay lead to chromatid aberrationsw Principal aberrations include dicentrics,

rings , translocations and deletions



w The incidence of most chromosome aberrations isa linear-quadratic function of dosew Scoring of aberrations in human lymphocytes

from peripheral blood may be used as a whole-body biological dosimeter for doses above about25 cGyw Aberrations can be detected in people even 40

years after radiation exposure



wThere is good evidence that the nucleus,specifically DNA, is the principal target forradiation-induced cell lethalitywThere is a correlation between cell lethality

and the number of chromosome aberrationsper cell

Documents

Lecture 1 Physical and chemical interactions DNA strand breaks DNA