3. Tissue Response to Radiation Flashcards
what are the 3 natures of radiation induced DNA damage
hydrogen bond break
molecular degradation
cross linking
what is DNA base damage
one is damaged and doesnt bind to the neighbouring molecule
what are the 7 forms of DNA damage
base damage base deletion base substitution Hydrogen bond disruption single strand break double strand break complex damage
what are the 5 ways DNA repair can occur after radiation induced damage
base excision repair
nucleotide exicsion repair
homologous recombination repair
non-homogolous end joining
apoptosis
what are the 4 steps of base excision repair
damaged base identified and removed
cleaves sugar phosphate backbone
replace missing base
join strand segments
after base excision repair is the DNA still the same form and info as it was beofre
yes
can base excision repair be used to repair DNA if complex damage has occured (eg multiple base pairs, strands damage)
no other mechanisms needed
what is the difference between homologous recombination repair and non-homologous end joining
homo recomb uses homologous DNA strands for repair
non homo end joining uses end to end joining of broken ends
HRR involving exchanges with homologous DNA strands (from sister chromatids after replication or from homologous chromosomes).
NHEJ involves end-to-end joining of broken strands
what does HRR do in terms of genetic integrity
preserves it
which is more common HRR or NHEJ and which is faster and which is more erroneous
NHEJ more common and faster and leads to more errors
what is the process of NHEJ and HRR
identify where ends are and process them to make sure they are able to be recombined and synthesise info along the strands
join damaged ends (HRR replace with same info, NHEJ not replaced with same info as before)
which is more error prone HRR or NHEJ
which is more complex
NHEJ = more errors
HRR = more complex process
what are the 2 types of lethal chromosomal aberrations
dicentric and ring aberrations
why are some chromosomal aberrations lethal
disrupts mitosis and can lead to cell death
mitosis disrupted owing to chromosome aberration
what is dicentric aberrations
2 chromosomes cleaved at similar places and recombine with 2 contromere on chromosome or one without a centromere (acentric fragment) so cant go through mitosis normally
what is ring aberrations
acentric fragment left but dont have structure that can be passed on to go through effective mitosis and cell replication
why are some chromosomal aberrations nonlethal
despite not being lethal what is a major long term problem
single intact centromere allows mitosis to proceed.
but errors can be passed down through generations of cells as genetic info is different in each set of chromosomes but still suitable for mitosis to proceed
what are the 4 checkpoints for a DNA chromosome
G0
G1/S
G2/M
M
what is are the 4 DNA chromosome checkpoints for
checks integrity of DNA chromosome so it can progress to the next stage of cell formation or initiate apoptosis if integrity has been compromised
where is the G1 check point and what does it do
at the end of G1 stage pre DNA synthesis/before S stage of interphase
pre DNA synthesis and check integrity of DNA
where is the G2 check point and what does it do
in the G2 phase of interphase post DNA synthesis and pre mitosis
checks DNA synthesis has gone normally and is completed before mitosis is triggered
what are the possibilities for the cell to go through if there is no DNA damage
what if there is damage what are the 2 routes
no damage = cell continues to function normally
damage = lethal or potentially repairable
what are the possibilities for the cell to go through if there is lethal DNA damage
reproductive death so cell dies
what are the 2 possibilities for the cell to go through if there is potentially repairable DNA damage
repair error or correct repair
what are the possibilities for the cell to go through if there is potentially repairable DNA damage and it goes through the repair error route
cell dies or there is a mutation
mutation could be lethal so cell dies or viable so cell passes mutation to future generations
what are the possibilities for the cell to go through if there is potentially repairable DNA damage and it goes through the correct repair route
cell continues to function normally
what are the 4 steps of calculating cell survival
perform a survival assay on a cell culture
repeat survival assay on genetically identical cell cultures
repeat for various radiation doses
plot the SF at each dose on a graph
what are the 3 steps for performing a survival assay when calculating cell survival
seed a number of cell colonies
incubate for desired time with no radiation present
count surviving colonies
what is the plating effciency equation
PE = # of colonies counted/# of colonies seeded
what is the surviving fraction equation
SF = # of colonies counted/(# of colonies seeded x PE)
what does it mean when you calculate the cell survival
investigating if a group of cells are exposed to radiation how does it affect the overall survival of cells
want to find the cell survival at different doses
what is the purpose of calculating the plating efficiency when calculating the cell survival
seed a number of colonies and incubate for a period of time without excess radiation and find the survival of this population
count how many cell colonies remain (PE) to see what the expected survival time is in that cell population
what is the purpose of calculating the surviving fraction when calculating the cell survival
incubate and now exposed to radiation
count surviving colonies to find the SF
multiply with PE as a measure of how many would have survived at that point anyways
in a cell survival fraction graphs as the dose increases what happens to the number of cells surviving
decreases
for a cell survival fraction graph what is the scale
logarithmic
going up by factors of 10
what are the 3 models for the cell survival curve
exponential response model
linear-quadratic
multi target model
what is the survival fraction equation for the exponential cell survival curve
SF = N/No = e^(-D/Do)
N = total number of cells measured along that curve No = number of cells started with Do = dose that reduces the number of cells to 37% of the initial number
in the exponential cell survival curve what is the Do a measure of and what does a high Do show on the graph
measure of slope
higher Do = much sharper decline
what is the relationship between Do and the slope in the multi target model of cell survival
what is Do a measure of
Do = 1/slope
measure of how radiosensitive a cell population is
what is ‘n’ in the multi target model of cell survival
what does it measure
extrapolation of the linear portion
measures the size of the shoulder
what is the Dq in the multi target model of cell survival
what does it measure
width of the shoulder region
measures sublethal damage
in the curved portion and the shoulder width of the multi target model of cell survival what does it represent in terms of cell recovert
cell is likely to recover
not all the cell pop is going to die, some will recover
shoulder width indicates more than one hit of the cell may be recoverable
which aspects of the graph line in the multitarget model of cell survival indicates the sublethal damage
Survival fraction of 1 the intersection with other line the width
in the linear quadratic model of cell survival what does alpha represent
in terms of which portion of the graph line, what it actually represents
is it independent or dependent on the dose rate
linear portion of graph line
individual particle tracks
independent of dose rate
in the linear quadratic model of cell survival what does the beta stand for
in terms of which portion of the graph line, what it actually represents
is it independent or dependent on the dose rate
quadratic portion of the graphline
2 or more particle tracks
depends on the dose rate
in the linear quadratic model of cell survival is the alpha portion or the beta portion of the graphline dependent on the dose rate
beta
what does alpha/beta represent in the linear quadratic model of cell survival
what is it a measure of
dose where alpha = beta
where the amount of dose causing cell death is about the same as recoverable, ratio lower in pop that does more lethal radiation
measure of radio sensitivity
what is the equation for survival fraction for the linear quadratic model of cell survival
SF(D) = e^(-aD + BD^2)
which portion in the linear quadratic model of cell survival represents cell death and recovery? in terms of alpha and beta
linear alpha portion is high LET radiation so more likely to deposit energy and more cell death
Beta Quadratic portion is low LET radiation, more recoverable, may damage certain parts of DNA that can be repaired reasonably we
what does a long shoulder portion mean in the linear quadratic model of cell survival
indicates it takes higher and higher doses to get significant cell death
what is the LET
linear energy transfer (LET) is the amount of energy that an ionizing particle transfers to the material traversed per unit distance
in the linear quadratic model of cell survival is there a shoulder portion for alpha particles
why
no shoulder at all as all of the damage is lethal
what is the effect of dose rate on the cell survival curve
a low dose rate and high dose rate results in what for the curve
low dose means not bombarding with radiation the whole time so has a chance to recover (shoulder broad as damage is recoverable until you get to much higher doses)
High doses = more lethal damage much earlier on so less shoulder
what is the effect of Presence of O2 (environment where cells exist) on the cell survival curve
why
higher O2 presence decreases shoulder width
(hypoxic environment has wider shoulder on curve)
When there’s free radicals O2 has to stabilize them which increases probability that the free radicals will combine with nearby substances
O2 stabilizes free radicals and allows more damage in tissues so lethal damage
More free radicals = more damage, O2 stabilizes free radicals and can potentially do more damage as free radicals hang around longer
what is the effect of fractionation on the cell survival curve
why
fractionation is how cells react to radiation
One big dose of rad therapy to tissue, lots of cell death if it’s a high dose, fractionation is where you apply small doses at certain intervals
Total curve a lot shallower so gives healthy tissue time to recover where if you applied it at once they wont have time to recover
