Physics And Chemistry Basis Of Biotechnology - De Cuyper - tiera.ru

Radiation-induced bioradicals: physical, chemical and biological aspects
5. I. I. Radiation damage to DNA
The primary target for radiation-induced cell damage is the DNA molecule (Kiefer
1990; von Sonntag 1994). This is supported by many sources of evidence, including the
following (McMillan 1993):
microirradiation studies show that to kill cells by irradiation of cytoplasm
requires far higher doses than irradiation of the nucleus
isotopes with a short range emission, incorporated into cellular DNA, produce
very effectively radiation cell killing and DNA damage
the incidence of chromosomal aberrations following irradiation is closely
linked to cell killing
thymidine analogues, as IUdR and BrUdR (iodo- and bromo-deoxyuridine)
incorporated in chromatin modify radiosensitivity.
DNA is a complex molecule, a long chain polymer composed of nucleotides. Each
nucleotide contains a nitrogenous base (adenine, guanine, thymine and cytosine), linked
through a sugar (deoxyribose) to a phosphoryl group. The backbone of the molecule
consists of alternating sugar-phosphate groups.
LET and track structure play an important role in the production of DNA damage
(Frankenburgschwager 1994; Hill 1999; McMillan 1993). The exposure of mammalian
cells to 1 Gy of low-LET ionising radiation leads to the production of around 1000
tracks with 2 x 105 ion pairs per cell nucleus, roughly 2000 of which may be produced
directly in the DNA itself. The same dose of high-LET radiation produces only about 4
tracks per cell nucleus, but the intense ionisation within each track leads to more severe
damage where the track intersects the DNA. In addition to this direct effect, damage
may result indirectly from free radicals produced in water close to DNA. Free radicals
produced in a radius of 2 nm around a DNA molecule are believed to contribute to the
radiation damage.
DNA may be affected by one or more of the following types of damage (Alberts
1994; Box 1995; Hildenbrand 1990):
strand breaks, single or multiple
modification of bases and sugars
cross-linking and dimerisation
The amount of DNA damage that can be detected immediately after irradiation is
substantial. Often quoted are the estimates for a clinical dose of 1 Gy of > 1000 base
damages, ~ 1000 single-strand and ~ 40 double-strand breaks, together with cross-links
between DNA strands and with nuclear proteins (Ward 1990). During the first
millisecond after radiation exposure, free radicals take part in a variety of competitive
reactions, some of which lead to the fixation of damage, others to the scavenging and
inactivation of radicals. Besides these chemical repair processes, enzymatic repair and
rejoining of DNA breaks, further reduce the damage during the subsequent few hours.
Irradiation at clinically used doses induces a vast amount of DNA damage, most of
which is successfully repaired by the cell. The frequency of lethal lesions is typically 1
per Gy (Steel 1996). It is clear that cells generally have a remarkably high ability to
repair radiation-induced DNA damage.
263