. Direct versus indirect effects caused from free radicals (in IAEA 2010)
Free radicals are not unique to radiation, but are produced in response to many stressors:
produce damaging free radicals. Such free radical production results in humans experiencing
efficient repair mechanisms have evolved within all biological species, from yeast to humans, to
Radiation protection of the environment: providing knowledge and skills to the user
community
Tom Hinton
French Institute for Radiation Protection and Nuclear Safety Page 4 of 7
18-Mar-14
https://wiki.ceh.ac.uk/x/hI9BBw
Radiation and the free radicals produced can damage DNA by causing several different types of
lesions (e.g. single strand breaks, double strand breaks, base changes, interstrand crosslinks). The
number of DNA lesions caused by a dose of 1 to 2 Gy is some 1000 base damages, 1000 single
strand breaks (SSBs), and some 40 double strand breaks (DSBs; IAEA, 2010). DSBs are central
to radiation-induced damage and their numbers correlate with radiosensitivity and the probability
of cell survival. There are efficient DNA repair processes specific to each type of lesion. For
DSBs the two primary repair pathways are non-homologous-end-joining (NHEJ) and homologous
recombination (HR). The mechanisms of the two repair pathways are such that NHEJ is much
more prone to errors during the repair process (IAEA, 2010).
Errors in repair can result in cell death through apoptosis, chromosome aberrations or mutations.
The fate of mutations and their impacts within a population are dependent on the type of cell in
which they occur. Two general types of cells are germ and somatic. Germ cells refer to the
primordial cells from which eggs or sperm are derived. All other tissues (bone, muscle, blood,
etc.) are derived from somatic cells. A mutation within a somatic cell can lead to cell death, or if
the DNA damaged cell has undergone mis-repair such that the cell is still viable, then the
mutation in the somatic cell can lead to cancer. Mutations in reproductive germ cells can decrease
the number of gametes, increase embryo lethality, or be inherited by the offspring, resulting in
their alteration. For humans, the risk of hereditary effects in offspring of exposed individuals is
about 10% of the cancer risk to the exposed parents. The risk of non-fatal cancer for humans has
been estimated at 1 x 10
-5
per mSv. For non-human biota the risk of hereditary effects is
unknown.
Most mutations are deleterious, offer no advantage to the individual that possesses it, and are
subsequently removed from the population. Some mutations are neutral, have no apparent effect
on the individuals that possess it, and can persist over many generations within a population.
Rarely, a mutation might offer a selective advantage (e.g. increase the efficiency of water
absorption in the roots of a plant that contains the mutation). Such selective advantages would
spread in a population.
The deleterious effects of ionising radiation to biological systems have been known from the
earliest days to be primarily dose dependent. Effort has therefore been expended
over the years in
defining the effective dose to a biological system. The issue is complex because the effective dose
depends not only on the gross energy deposited, but also on the quality of the radiation and the
radiation sensitivity of the affected tissue.
In SI units, the effective dose to humans is the Sievert (Sv), which is the absorbed dose (Gy)
adjusted by two dimensionless weighting factors: the radiation weighting factor w
R
to account for
the biological effectiveness of the absorbed radiation; and the tissue weighting factor w
T
to
account for differences in the radiation sensitivities of different organs of the body. These
weighting factors have been developed solely for human radiation biology—no such factors exist
for non-human biota. Thus, dose to non-human biots is expressed in Gy, rather than Sv.
Biological effects of radiation are classified as deterministic or stochastic effects. Readers are
referred to a detailed description in IAEA (2010), and to the general information in the associated
powerpoint slides.
Radiation protection of the environment: providing knowledge and skills to the user
community
Tom Hinton
French Institute for Radiation Protection and Nuclear Safety Page 5 of 7
18-Mar-14
https://wiki.ceh.ac.uk/x/hI9BBw
Dostları ilə paylaş: