Effects of Radiation at th

Cellular LevelCellular Level

10/25/2010
Table of Contents

Topic Slid

Cellular Components Chromosomes DNA and RNA Apoptosis Cells Life Cycle Interaction of Radiation with Cellular Components Interaction of Radiation with Cellular Components Direct and Indirect Effects DNA Damage and Repair Chromosomal Aberrations Cytogenetic Dosimetry Consequences of Damage to DNA Cell Survival Curves Factors Affecting Radiosensitivity References 
Cellular Components
General

The cell is the basic building block of biological systems.Th 1014There are ca. 10cells in the human body.A typical cell is 10 to 20 um in size.
Cell Membrane (plasma membrane)

A thin bilayer of phospholipids surrounding It serves as a passive barrier impenetrable polar molecules (water is an exception). Proteins distributed throughout the membraProteins distributed throughout the membra

actively control the movement of many ions molecules into and out of the cell
Proteins

Organic molecules consisting of many aminComplex three-dimensional shapes.Serve a structural role in the cell (e.g., filamEnzymes, a particular type of protein, serve metabolic role. They control the rate of biocreactions. 
Nucleus

Usually the most visible cell component.Dense and granular in appearance. 

Surrounded by a double membrane (envelope)membrane (envelope).

Contains the chromosomes.Site of RNA synthesis (transcription).

Image copyrighted by Boston University. All rights reserved. Reprinlicense from The American Society for Cell Biology. 
Cytoplasm

The cell contents (excepting the nucleus)Components: organelles

cytosolmisc. inclusions (e.g., pigme

CytosolThe cell fluidComponents: water (makes up the bulk o

saltsorganic molecules (e.g., pro
Ribosomes

Small particles found in the cytoplasmSome are attached to the surface of endoplreticulum (rough ER)They consist of two subunitsThey consist of two subunitsEach subunit is made up of ribosomal RNAThey organize the transfer and messenger the proper geometry for the production of p
Endoplasmic Reticulum (ER)

Convoluted system of membranous passagcytoplasmTwo types: rough (ribosomes on surface)

smooth (no ribosomes on surfac

Involved in:protein and lipid production

calcium sequestrationmoving proteins and carbohydrathe plasma membrane, lysosomGolgi apparatus
Endoplasmic Reticulum (ER)

Rough ER

Copyright Boston University. All rights reserved. Reprinted under liceThe American Society for Cell Biology. 

Smooth ER
Golgi Complex

Organelle consisting of stacks of flattened mbound sacs (cisternae)Vesicles bud off the cisternaeInvolved in the processing andInvolved in the processing andpackaging of organic molecules, e.g., carbohydratesPackage organic molecules in membranes use or exocytosis (excretion)

Image copyright ULB. Louis de Vos. All rights reserved. Reprinted license from The American Society for Cell Biology. 
Lysosomes

Small vesicles in cytoplasm bounded by a single mNo discernible internal structure Contain a variety of digestive enzymesL f ith d di t th t t Lysosomes can fuse with and digest the contents o

endosomes (vesicles formed by endocytosis that cmaterial from outside the cell).Lysosomes can carry undigested material to cell mexpulsion from the cell.Sometimes referred to as "suicide bags", a rupturinmembrane can lead to cell death.
Mitochondria

Organelle bounded by double membrane (e

All rights reReprinted from The ASociety for 
Mitochondria

High degree of internal organizationContain their own complement of DNA and Involved in respiration, an oxygen fueled prwhich energy is obtained from the breakdowwhich energy is obtained from the breakdow

organic molecules into water and carbon diPlay an important role in apoptosis
Cytoskeleton

Cytoplasmic component maintaining the cestructure and shapeThe cellular “skeleton”Components: filaments

microtubules 
Chromosomes, DNA and R
Chromosomes

Usually found inside the nucleusEach human somatic cell has 46 chromosofrom each parent) A chromosome consists of a single moleculA chromosome consists of a single molecul

(or two identical molecules of DNA) and histype of protein
Chromosomes

Chromosomes are usually amorphous, i.e., have no recognizable structureWhen they do have structure (during mitosilook like either of the followingNo matter which form they take, they have a centromere (a region of repetitive DNA that appears as a constriction)
DNA

Deoxyribonucleic acidIn 3-D, a “double helix” In 2-D, a ladderThe sides of the ladder are made of an altesequence of sugar and phosphate groupsThe ladder’s rungs join the sugar groupsEach rung consists of two nitrogenous base
DNA

Four types of nitrogenous bases:

Adenine

Thymine

CytosineGuanine

Adenine and thymine always pair upCytosine and guanine always pair up
DNA

“Controls” the cell by controlling protein proThe sequence of the nitrogenous bases aloside of the DNA molecule determines which RNA are produced The type of RNA that is produced determineof protein that is producedThe sequence of nitrogenous bases that coparticular protein is called a “gene”The DNA is a “master blueprint.” Only one cexists.
RNA

Ribonucleic acidProduced (transcribed) from DNASimilar to DNA but single strandedThree types: transfer RNA

messenger RNAribosomal RNA

The three types come together in the cytopproduce proteins (transcription)
DNA

RNA

Nuclear 

Membrane

Cell 

Membrane

RNA

Protein
Apoptosis
Purpose of Apoptosis

Apoptosis (aka programmed cell death) is a by which an organism induces cells that miga threat to commit suicide. By destroying specific cell types it also servBy destroying specific cell types, it also serv

selective tool that guides development of the embryo.Apoptosis can be induced by radiation. This generally a good thing because dead cells dbecome cancerous.
Failure of Apoptosis

If radiation damages the genes that induce apoptosis, there is an increased possibility thmalfunctioning cancerous cells will proliferatAs a “survival mechanism ” some cancer celAs a survival mechanism, some cancer cel

produce chemicals that suppress apoptosis. the ability to suppress apoptosis seems to be characteristic of all cancers.
Role of mitochondria in apoptosis

When the cell is stressed by radiation, proapoptosis bcl-2 proteins (e.g., Bad) in the cycan move to the surface of the mitochondriaHere they interfere with anti

-apoptosis bcl

-

2

Here they interfere with anti apoptosis bcl

2

proteins. The result is that pores are formed mitochondrial envelope.Cytochrome C and other pro-apoptosis moleleave the mitochondria and go into the cytos
Role of mitochondria in apoptosis

In the cytosol, the cytochrome C interacts wiproteins Apaf-1 and caspase 9 to form a comknown as the apoptosome.This complex activates the capase enzymes This complex activates the capase enzymes

“executioner” enzymes responsible for breakdown proteins.
Role of mitochondria in apoptosis

The cells undergoing apoptosis then:

shrink

degrade and fragmentdegrade and fragmentare engulfed (eaten) by phagocytes
Cell’s Life Cycle
Cell Life Cycle

There are four phases to the cell’s life cycle

• G1 when each chromosome contains 

molecule of DNA

• S (synthesis) when the DNA molecule 

chromosome is perfectly (almost) repchromosome is perfectly (almost) rep

• G2 when each chromosome has two n

identical molecules of DNA joined at tcentromere. The pair of molecules forcalled a pair of sister chromatids.

• M (mitosis) when the cell divides
Chromosomes during G

1 and G

2 don’t lo

they are shown here. They do look like tduring mitosis (M).
Nucleus

Cell 

Membrane

Cell Division

(mitosis)

Chromosomes

Cell 
Nuclear 

membrane 

breaks

Cell Division

(mitosis)

C

downs
Chr

“co

an

a

e

Cell Division

(mitosis)
c

m

so

a

ch

c

Cell Division

(mitosis)

C

m
c

s

thtw

ch

Cell Division

(mitosis)

O

p
Cell Division

(mitosis)

Copyright ULB. Louis de Vos. All rights reserved. Reprinted under from The American Society for Cell Biology. 
Cell Division

(mitosis)
Cell Division

(mitosis)

Whe

chromoso

separat

two side

cell, the 

reform

chromoso

their d

structure

cell pinch
Cell Division

(mitosis)

Two “daughter” cells, each with a complcomplement of genetic material.
Interaction of Radiation with Ce

Constituents 

Free Radical Production
Interactions of Radiation with Cellular Cons

Gamma rays or x-rays interact with atoms iby the photoelectric effect, Compton scatterpair production.In human tissue, Compton scattering is the likely interaction. In these interactions the gamma-ray or x-ratransfers energy to an electron which then tthrough the cell. This electron then interacts same way that other charged particles inter
Interactions of Radiation with Cellular Cons

Charged particles (such as alpha or beta patransfer energy to the molecules they pass The result is that these molecules are exciteionized, or broken.In some cases free radicals are produced.
Radiolysis of Water

The splitting apart of water by radiation Water is the major constituent of the cell. Solike 80% of the radiation energy absorbed bbody is absorbed by water. The following reactions are brought about btransfer of energy from betas (electrons) or the water molecule. Excitation of the water is of little importance and will not be conside
Radiolysis of Water

Ionization 

Radiation + H2O ÷ H2O+ + eRadiation + H2O ÷ H+ + OH

Formation of Free Radicals Radiation + H2O ÷ H + OH Free radicals are uncharged and have an un

electron. Sometimes symbolized as H0 or OH
Free Radicals

The unpaired electron makes the free radicextremely reactive.Within a nanosecond of its formation, a free will reacts with some nearby molecule (the the cell and damages itthe cell and damages it.

The free radical might react with this molecand produce a new radical species that prodamage other molecules.

RH + OH ÷ R + H2O
Free Radicals

Two hydroxyl free radicals can combine to phydrogen peroxide. Since the latter, a poweoxidizer, is relatively long lived, it has a subrange of damage.

OH + OH ÷ H OOH + OH ÷ H2O2

Two hydrogen free radicals can combine toproduce hydrogen. 

H + H ÷ H2
Free Radicals

A hydrogen free radical might combine with oxygen molecule to produce a hydroperoxyThe latter might then combine with another free radical to produce hydrogen peroxide.

H+ O ÷ HOH + O2 ÷ HO2

HO2 + H ÷ H2O2 

Other types of free radicals can combine wto produce organic peroxy free radicals:

R + O2 ÷ RO2
Free Radicals

Much of the damage brought about by radiabelieved to be due to oxygen containing radions and peroxides collectively known as reoxygen species (ROS).The hydroxyl free radicals are believed to bThe hydroxyl free radicals are believed to b

particularly important. 
Direct and Indirect Effects
Two general categories of radiation effects:

direct effectsindirect effects

Direct EffectsDirect effects of radiation involve a transfer from the radiation (e.g., alpha or beta) directarget molecule (typically DNA).Direct effects are believed to be the most imtype of effect for high LET radiation (e.g., neand alpha particles).
Indirect Effects

Indirect effects occur when radiation producradicals which react with the target molecul(typically DNA). The free radicals must be produced very clotarget (e g within a few nm) since they havtarget (e.g., within a few nm) since they hav

short lifetimes.Indirect effects seem to be most important fLET radiation (e.g., gammas, x-rays and beAbout 65% of low LET damage has an indircomponent whereas 35% is exclusively dire(UNSCEAR 2000).
Subcellular Targets

Everything in the cell is subject to direct and damage, but some targets are considered to important than others:

DNA

membranes (especially the nuclear m

The DNA molecule is considered to be the mimportant site of damage because it serves master blue print for the cell.
DNA Damage and Repair
DNA Damage

With low LET radiation, the damage tends tspatially isolated along the DNA molecule.With high LET radiation, clusters of damage a given chromosome. Such clustered damage is more complex anto repair than the isolated damage caused LET radiation. Repair may require enzymes multiple repair pathways.Clustered damage rarely occurs in the abseradiation, i.e., it is somewhat “unique” to rad
DNA Damage

The following table (UNSCEAR 2000) indicestimated DNA damage per gray (100 rads

Base damage 500 pe

S 1 000

ingle strand breaks 1,000 pe

Double strand breaks 40 peDNA-protein crosslinks 150 pe
1. DNA Base Damage

This involves alterations to individual nitrogebases.These alterations are primarily due to the fofree radicals in the water that is closely bounchromosomes.As an example of such damage, the free radmight induce the deamination of the nitrogecytosine and convert it to uracil. It is even pothat the base might be completely removed. 
1. DNA Base Damage and Repair

DNA base damage is often (although not alwwhat is meant by the term mutation. It is somreferred to as a genetic (vs. chromosomal) mLarge stretches of the chromosome consist “ ”“nonsense

” repetitive DNA. Base damage to 

regions is inconsequential.
1. DNA Base Damage and Repair

Base damage is usually repaired. The classiexcision repair mechanism operates as follow

• First, the damaged section is excised by a 

glycosylase enzyme. Various glycosylases

recognize different types of base damagerecognize different types of base damage

• Then a DNA endonuclease cuts the DNA 

“backbone”

• Next, a phosphodiesterase removes the s

phosphate remnants. 
1. DNA Base Damage and Repair

• After this, the excised segment is resynth

a polymerase using the undamaged strantemplate. 

• Finally, ligases attach the newly synthesi

segment in place.

Sometimes the damage necessitates the remlarge segment of one side of the DNA molecujust one base and the associated sugar and pgroups. 
1. DNA Base Damage and Repair

A TA T

C GG*

C

GC

A T

C G

A

T

A T

C G

C

Damaged base removed

Damaged base

Sugar and phosphate r

Resynthesis

C

G

A T

C

GC

A

T

A T

C GGC

GC

A TA T

C GGC

GC

A

T

C

G
2. DNA Single Strand Breaks (SSBs)

One side of the DNA molecule is broken. This is the most common type of DNA damage, but perhaps the least important. As a rule, an SSB also involves damage to the sugar groupRepair of Single Strand Breaks (SSBs)Single strand breaks are the easiest to repairepair mechanism is similar to that for base dsugar group.
3. DNA Double Strand Breaks (DSBs) and R

Both sides of the DNA molecule are broken. The least common type of DNA damage, but perhaps the most important. One concern is that the broken two pieces of DNA (and hence the chromosome) might not rejoin. If this happens, the chromosome has broken into two separate pieces. Another concern is the possibility of misrepair. 
3. DNA Double Strand Breaks (DSBs) and R

Single event DSB Double event 

With a double event DSB, opposite strands of the Dmolecule are broken separately, but close together and time.
3. DNA Double Strand Breaks (DSBs) and R

Since the hydrogen bonds joining the nitrogenous base pairs are weak, the breaks in the two strands don’t have to be immediately opposite each other. They just need to within 10 to 20 base pairs of each 

other.

Fragmented 

DNA molecule 

(chromosome)
3. DNA Double Strand Breaks (DSBs) and R

Two types of repair mechanisms for DSBs: 

non-homologous end joining (NHEJ) homologous recombination (HRR) 

NHEJ predominate in mammals.

“Quick and 

NHEJ predominate in mammals. Quick and 

Simply involves attaching the broken ends (wmight be missing material). More error prone than HRR, but this is not so of a concern since mammalian chromosomecontain a great deal of repetitive DNA. 
3. DNA Double Strand Breaks (DSBs) and R

Homologous recombination is more compleless prone to error. HRR is only possible in S or G

2.

In HRR, the broken end of a single strand oDNA “invades” the appropriate unbroken sean homologous DNA molecule (e.g., that of chromatid). The homologous DNA is used as a template produce any missing DNA on the broken se
3. Double Strand Breaks (DSBs) and Repair

DSBs are more likely to result in the death othan base damage. Even so, only a small peof DSBs are lethal.Most DSBs are repaired and do not result in chromosomal aberrationschromosomal aberrations.

The number of DSBs is 40 to 100 times the lethal events. Presumably, these lethal evento the failure of the repair mechanisms to rep2.5% of the DSBs.
Chromosomal Aberrations
Chromosomal Aberrations

Unrepaired DSBs can lead to chromosomal aberrations (gross structural abnormalities vunder a microscope during mitosis).Some of these aberrations are due to the facbroken ends of the chromosome at the site obroken ends of the chromosome at the site o

are "sticky." That is, they tend to attach to tof other broken, or unbroken, chromosomes
Chromosomal Aberrations

The nature of the aberration depends on the the cell cycle at which the exposure occurredeach chromosome consists of one DNA moleG

2 each chromosome consists of two DNA m

(one per chromatids). 

The aberrations that result from DSBs in G

1 

chromosomal aberrations. Those that result from DSBs in G

2 are some

called chromatid aberrations.
Chromosomal Aberrations

These figures show common chromosomal achromatid aberrations, e.g., rings, fragmentsdicentrics
Chromosomal Aberrations

Centric fragment

Acentric fragment

Acentric fra

Ring
Chromosomal Aberrations

Centric fragment

Acentric fragment

Unbroken chromosome 

Dice

Chrom
Likely Results of Chromosomal Aberrations

If the cell attempts to divide, it is likely that sochromosomal material will not be properly dibetween the two daughter cells.For example, the spindle fibers cannot attac

f

separate the acentric ragments. In late mito

the two daughter cell nuclei are being formeof remainder genetic material might end up bincorporated into a “micronucleus.”Dicentric chromosomes might even be pulledirections!
Chromosomal Aberrations

Dicentric Bridge

Photo of a cell at a later stage of mitosis.

Acentric 

fragments
Likely Results of Chromosomal Aberrations

Due to the chromosomal aberrations, the ce

• Survive but never attempt to divide.• Die at mitosis without dividing.

• Divide successfully, but the daughter c

because they lack a complete complemDNA.

This is referred to as “mitotically-related deasimply “mitotic death.”Radiation can also induce cell death via apo
Cytogenetic Dosimetry
Cytogenetic Dosimetry

For low LET radiation, the relationship betweand the number of DSBs (and therefore dicechromosomes) seems to be linear-quadraticAt low doses, the number of DSBs increases due to

t

h

e in

c

reased n

u

mber 

of cases wh

e

r

e

due to t e c eased u be o cases e e results from a single event. At high doses, the number of DSBs increasequadratically due to the increased frequency caused by two discrete events. 
Cytogenetic Dosimetry

Following an acute radiation exposure, a blocan be taken and the peripheral blood lymphcultured. Since these cells are relatively longcan be done up to several weeks after the exwithout the cells entering mitosis and the abbeing "lost". Using colchicine, the lymphocytes are “arresmitosis and then, the average number of diccell is determined. The probable dose is obtaa calibration curve similar to the following:
Dicentrics/cell

1.51.0

Cytogenetic Dosimetry

High LET

Radiation

Low 

Dicentrics/cell

Dose (Gy)

0

0.5

01234

Radia
Cytogenetic Dosimetry

Cytogenetic dosimetry can be effective for edoses above 0.1 Gy (10 rads) or so.In the US, there are two groups with expertiscytogenetic dosimetry:

REACTs (part of Oak Ridge Associated 

Universities)AFFRI (Armed Forces Radiobiology ReseaInstitute).
Consequences of DNA Dama
DNA damage has no effect if:

• The damage is completely repaired• The damage is unrepaired (or misrepai

it is limited to a non-functional (meaninsegment of DNA 

• The damage is unrepaired (or misrepai

and the damaged cells die or becomenon-functional. There is no significant eif only a small number of cells die or benon-functional
Deterministic effects (e.g., cataracts) occur if

• The damage is unrepaired (or misrepa

and many cells die or became non-funcThis can involve mitotically-related deaand/or apoptosis

Stochastic effects (cancer) might occur if:

• The damage goes unrepaired (or 

misrepaired). The cells survive and ultproduce many malfunctioning cells.
Even unexposed cells might be affected

Cells “interact” with each other via the transfvarious chemicalsIn the bystander effect, a radiation exposure cell affects adjacent cells. 

e.g., DNA repair mechanisms in adjacent c

be stimulated thus providing a protective e

Alternatively, the metabolism of the adjace

might be damaged, possibly leading to cell 
The effects of radiation are not unique – the 

effects can result from other causes, e.g., eto chemicals.
Cell Survival Curves
Cell Survival Curves

While it is difficult to determine the number olethal mutations induced by radiation, it is reeasy to quantify cell death. In radiation biolodeath is defined as the inability of individual reproduce.The relationship between dose and cell deata simple exponential as seen in the next slidThe mean lethal dose is the dose that kills 6original population. An optimist would say thdose at which 37% survive.
N

(number of 

surviving

N0

Cell Survival Curves

N number of surviving N0 original number of cD dose

surviving

cells)

Dose (Gy)

0

D0 mean lethal doseD0

0.37 N0
S 1.0

Cell Survival CurvesThe surviving fraction (S) plotted against doslog paper is referred to as a survival curve. 

10.0 n

Surviving

Fraction

Dose (Gy)

0.1

0.01

2 4 6 8 10 1

Dq

0.037

D0
Cell Survival Curves

The most significant feature of this type of cushoulder at low doses. At higher doses the relationship becomes exponential (the straigportion of the curve). A mathematical description of this curve not A mathematical description of this curve not 

requires Do, but also n (the extrapolation num

The latter is the value for the survival fraction when the exponential (straight line) portion ocurve is extrapolated back to D = 0. 
Cell Survival Curves

In most mammalian cell lines, Do is 1 to 2 Gy 

200 rads) and n is between 1 and 5. The quasi-threshold dose (Dq), is often spec

is the dose at which the extrapolated expone

portion of the curve crosses the 100% survivportion of the curve crosses the 100% surviv

= 1).The following slides discuss three ways in wsurvival curves can be interpreted.
Cell Survival Curves

1.Q Repair.

One explanation for the shoulder on the s

curve is that it indicates the presence of a repair mechanism, the so called Q repair p

Dq then becomes a measure of the effectiv

this repair process

Do a measure of the cell's sensitivity to rad

induced death. 

The extrapolation number (n) is given no "

biological meaning.
Cell Survival Curves

2.Target Theory (multitarget/multihit process

Survival curves are explained in terms of tnumber of targets in a cell that must be hit number of hits that an individual target in t

must receive) for cell death to occurmust receive) for cell death to occur. 

At the center of such theories are equation

attempt to describe the observed relationsbetween cell survival and dose. One goal these equations should be biologically just
Cell Survival Curves

2.Target Theory (multitarget/multihit process

As an example of such an equation:

This equation is interpreted to mean that t

more unspecified cellular targets must eaca "hit" for death to occur. The extrapolatio(n), represents the number of targets that hit (a "hit" is taken to mean a direct ionizinHowever, this model predicts a slope of zecurve at low doses--something rarely obse
Cell Survival Curves

3. Target Theory (multitarget/multihit proces

Cell death is assumed to be due to DSBs. latter can be produced by a single event wstrands are broken at once, or by two sepa

events when each strand is broken individevents when each strand is broken individ

α represents the probability of single-evenβ reflects the probability of double-event D
Factors Affecting 

Radiosensitivit

y
1. Radiation Quality

While different types of radiation produce the same magnitudes of the effects per unit dose can be quite The amount of biological damage per unit dose incrthe linear energy transfer (LET) of the radiation. The higher the LET the greater the radiation energy The higher the LET, the greater the radiation energy 

unit distance, and the greater the density of ions anradicals in the charged particle tracks. As LET increases, more energy is deposited in the icells, but fewer cells are affected per unit dose. Increasing the LET of the radiation increases the cothe damage to the DNA. This makes repair more dif
1. Radiation Quality

In radiation protection the effectiveness of different tradiation is indicated by a quality factor (Q). In radiation biology these differences are indicated brelative biological effectiveness (RBE):

the samemagnitudeof the sameeffect

doseof a different radiation needed to produ

a given magnitudeof a specified effect

doseof kVp xraysrequired to produce

RBE

250

=
1. Radiation Quality

Low LET 

(gammas, x-rays, betas)

High L

(neutrons, 

Dose Rate Effect dependent independ

inverse dep

Dose Response often linear quadratic often li

i il i di t i il 

DNA Damage primarily indirect primarily 

DNA damage simpler DNA dama

comp

DNA Repair easier hard

Effect of Oxygen sensitizes tissue non

Effect of Radical 

Scavengers reduces damage non
2. Dose Rate Effect

The dose delivery can be classified as a:

• Chronic Exposure. The dose is delivered at a 

over a long time, e.g., 0.1 rad/hr for 10,000 hou1000 rad).

• Fractionated Exposure. The dose is delivered 

quantities, e.g., 100 rad are delivered per week weeks (total dose: 1000 rad).

• Acute Exposure. The total dose is delivered at 

a very short time (total 1000 rad).
2. Dose Rate Effect

Dose

(rad)

1000

Dose

(rad)

1000 1000

Dose

(rad)

Chronic Fractionated 

Time Time 

For low LET radiation, the magnitude of the effect peis greatest following acute exposures and least with exposures. This applies to dose rates between 10 arad/hr. Lowering the dose rate below 10 rad/hr doesreduce the magnitude of the effect. Increasing the doabove 5,000 rad/hr has no additional effect.
2. Dose Rate Effect

For high LET radiation, this "dose rate effect" is less pronounced or absent. There is some evidence that chronic exposures with radiation may be more carcinogenic than acute expan inverse dose rate effect.
2. Dose Rate Effect

One possible explanation involves the cell's repair mbelow 10 rad/hr the repair mechanisms are maximafrom 10 to 5,000 rad/hr the repair rate falls behind th

damage, above 5,000 rad/hr any repair is superfluoAnother possible explanation involves the fact that t

of two event DSBs per unit dose increases with dostwo single strand breaks to produce a DSB, they muclose together on the DNA molecule and close togethe higher the dose rate, the greater the probability SSBs will be sufficiently close in time.
3. Tissue Exposed

A tissue is a group of similar cells organized to perfocommon function.The Law of Bergonie and Tribondeau (1906)The "law“ characterizes those tissues in the body thThe law characterizes those tissues in the body th

most radiosensitive. Like all laws in biology, it is a generalization and has exceptions. Although radiobiologists tend to dismiss the “law”, it its uses.
3. Tissue Exposed

The law states that the most radiosensitive tissues pcells are:

• dividing at the time of exposure (mitosis is the 

sensitive stage of the cell cycle)

• of an undifferentiated type, i.e., unspecialized 

t t d f ti

structure and function

The law describes those tissues where early ( acuteare observed.These early effects are due to large doses in short ptime and primarily involve cell death.
3. Tissue Exposed

Examples of Radiosensitive Tissues:

• germinal cells of the ovary and testis

(spermatogonia)

• hematopoietic (blood forming) tissues:

• red bone marrow

red bone marrow

• spleen• lymph nodes

• epithelium of skin• epithelium of gastrointestinal tract • lymphocytes (exception to the “law”)• oocytes (exception to the “law”)
3. Tissue Exposed

Examples of Radioresistant Tissues:

• bone• muscle• cartilage• nervous tissue 

During embryonic and fetal development, the these tissues are reproducing and much mosensitive to radiation.
4. Exposure Time During the Cell Cycle

For low LET radiation, the most sensitive stages of tcycle, with respect to cell death, are mitosis and late the G1 -S border). This might be because the chromosomes are condedurin

g mitosis and the re

pair mechanisms have 

poo

gp p

to the DNA molecule. For high LET radiation, all phases of the cell cycle aequally sensitive.
4. Exposure Time During the Cell Cycle

In radiation therapy, the cancer cells most likely to bare those in the sensitive stage of the cell cycle. Ce

stages of the cell cycle survive. This is how radiation exposures can synchronize cells in the cell cycle. Sradiation exposures to the synchronized cancer cell

particularl

y effective.

p yRadiation can slow down a cell’s passage through GOne possible effect of this is to give the cell more timrepair damage prior to division.
5. Adaptive Response

Low doses of radiation appear capable of initiating ccells that reduce the consequences of subsequent eFor example, a conditioning dose of 5 to 200 mGy (20,000 mrad) to lymphocytes can result in an adaptresponse some four to six hours later. This adaptive results in a lower than expected number of chromosaberrations following a second “challenge” dose. This adaptive response seems to involve the activatcertain genes that increase the production of enzymin DNA repair.
6. Oxygen Tension

Cells with normal concentrations of oxygen (40+ mm to be 2-3 times as sensitive to low LET radiation as in oxygen) cells. For a given effect, this difference ias the oxygen enhancement ratio (OER). Poorly vascularized tissue, i.e., tumors, tend to be hTissues well supplied with blood tend to have normatensions.The relationship between oxygen and radiosensitivipronounced below 20 mm Hg. Above this, an increaoxygen concentration does little to increase the radiof a tissue. 
6. Oxygen Tension

This effect of oxygen is probably due to several thin

• a resulting increase in the production of H2O2 

reactive oxygen species.

• increasing the stability and toxicity of free rad

B i l t ti it Because oxygen is electronegative, it can com

the free electrons produced during the radiolyThis might slow down the recombination of ceradiolytic products and increase the capacity fby extending the latter’s effective lifespan. 

• oxygen might combine with DNA damage site

interfere with repair. 
7. Chemical Protective Agents

Certain chemicals, injected in substantial quantities or so prior to an acute exposure, can significantly reeffective dose of the radiation. Post-irradiation and oadministrations are less effective. The dose reduction factor (DRF) is the ratio of the Lunprotected and protected animals. Typical DRFs foradioprotective agents range from 1.5 – 2.0.Examples of radioprotective drugs include cysteine, and glutathione. A sulphydryl group is common to magents.
7. Chemical Protective Agents

The interest in radioprotective drugs usually focuses protection afforded to large acute exposures. This is by the fact that the DRF is defined in terms of the LD

Nevertheless, some of these chemicals (e.g., Amifoprovide protection from late effects such as cancer.

The protective mechanisms vary. Radioprotectants scavenging (quenching) free radicals, enhancing cemechanisms, stabilizing membranes, enhancing hy
7. Chemical Protective Agents

While many of these agents are toxic and must be unear-lethal levels, some have been found to reduce radiosensitivity of tissues when administered in reladoses. Of these, WR-3689, is of particular interest since it awork better with healthy (well oxygenated) tissue thaneoplastic (hypoxic) tissue. Furthermore, it is slightthan WR-2721 and may be effective when administeThe aminothiol WR-2721 (better known as Amifostinbeen approved by the FDA and is used to protect thtissue of patients undergoing radiotherapy.
Radioprotector1 Toxicity LD50 (mg/kg) Protective doses (mg/kg) 

Cysteine 1700 1200 

MEA 200 150 

Cysteamine 220 150 

AET 480 400 

WR-638 1120 500 

WR-2721 950 500 

WR-3689 1120 450 

WR-77913 3574 2200 

WR-1513272 785 315 

Mercapto-propionyl glycin3 2100 20 

Glutathione 4000 4000 

1 Results from the research for LD50/30 on mice radiated with X or gam2 Achieves high protectiveness from neutron radiation3 Achieves considerable protection applied after radiation
8. Temperature

In many experiments, a dramatic increase in radiore

has been produced by lowering an animal's body teFor example, in mice the LD50 can be doubled by re

body temperature to 5 oC. The increase in radioresistance is apparently due to 

in oxygen tension that accompanies a lower body teIn some cases the lowered temperature merely servthe effect, not reduce it. If the effect is simply delayedue to a reduced mitotic rate; once the animal warmmitosis resumes, mitotic cell death can occur.
9. Sex

In some species females tend to be slightly more rathan males. That this is related to differences in horlevels is suggested by the fact that castrated males tendency. Differences in sensitivity depend on the effect being For example, human males are more susceptible to induced leukemia, while females are more susceptiradiation-induced thyroid cancer.Lymphocytes of males and females differ in their radiosensitivity, something that can introduce uncertcytogenetic dosimetry. Variability in female hormonabeen linked to variations in lymphocyte radiosensitiv
10. Age

More Radiosensitive Embryo

Fetus

Child

Less Radiosensitive Adult
11. Species

For animals, the more primitive the species the morradioresistant the organism. The more advanced the the more radiosensitive it is.With plant species, the larger the interphase chromovolume (ICV) the greater the radiosensitivity. The ICdefined as the average volume of the nuclei divided number of chromosome characteristic of the specie
11. Species

More Radiosensitive mammals

reptiles, fish, amphibian

insects

protozoa

Less Radiosensitive bacteria
11. Species

A useful way to compare the radiosensitivities of diffspecies is by comparing their LD50s. 

The LD50 is the dose from an acute exposure that w

50% of the population (with the exposed organisms untreated). 

We usually specify that the deaths must occur within time after the exposure. For example the LD50/30 is 

that will kill 50% of the population within 30 days. T

indicates that the deaths occur within 60 days.
11. Species

There is probably no single reason as to why varioushow such a range of sensitivities. Possible explanations include differences in the effethe DNA repair mechanisms, differences in mitotic rdifferences in oxygen tension, differences in the num

i f th i h t

size of their chromosomes, etc. 

The high radiosensitivity of mammals (i.e., the low L

possibly related to their complexity. The more compsystem and the wider the range of tissue types, the likelihood certain essential tissues will be radiosens
Species LD-50 (rad) Species LD-

amoeba ca. 100,000Paramecium ca. 300,000Ctenophore ca. 1,200snail (Radix) 2,000snail (Thais) 17,000“amphipod” 600amphipod (Daphnia) 3,000cockroach 10,000beetle (Tribolium) ca. 100,000fruit fly (Drosophila) ca. 100,000wasp (Bracon) ca. 300,000grain weevil ca. 5,000

guinea pig 200pig 275goat 35dog 325laboratory mouse 400cattle 5monkey 500burro 585hamster 610man 250

rabbit 750

g ,salmon 1,500swordtail 1,000goldfish 670-800“frog” 700“newt” 1,500-3,000turtle 850-1,500“snakes” 300-400lizard 1,200-2,000Eastern bluebird 2,500pigeon 2,000-3,160parakeet ca. 2,300chicken 600-1,630canary 1,015

rabbit 750 laboratory rat 600bobcat ca. raccoon ca. gray fox ca. opossum ca. wild mouse 11harvest mouse 11cotton rat 12pocket mouse 120pocket mouse ca. bat ca. 
References
Blaisdell, J.O., Harrison, L. And Wallace, S.S. Base Excision Repair P

Radiation Induced Clustered DNA Lesions Radiation Protection Dosim

25-31; 2001.Casarett, A.P. Radiation Biology Prentice-Hall, New Jersey, 1968.

Denekamp, J. (Ed.) Special Issue in Honor of Tikvah Alper - covering 

chemical, biological and medical effects of ionizing and non-ionizing ra

J. Of Rad. Biology 71 (6) June 1997.Gonzalez, A.J. Biological Effects of Low Doses of Ionizing Radiation: A 

Pi t IAEA B ll i 36 (4) 3 4 1994Pi

c

ture. IAEA

B

ulletin 36 (4):37-

45; 1994.

Jonah, C.D. A Short History of the Radiation Chemistry of Water, RadResearch 144:141-147; 1995.NIH Workshop Draft Report. Molecular and Cellular Biology of Modera10 Sv) Radiation and Potential Mechanisms of Radiation Protection, BDecember 17-18, 2001.
Pizzarello, D.J. Radiation Biology, CRC Press, Florida.

Seed, T. Radiation Protectants: Current Status and Future Prospects. Physics. 89(5):531-545; 2005.UNSCEAR Report of the United Nations Scientific Committee on the EAtomic Radiation to the General Assembly, 2000.Wald, N. Fundamentals of Radiobiology Professional Enrichment Pro

Physics Society Annual Meeting July 1988.