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How radiation affects cells |
Radiation
Ionizing radiation is energy transmitted via X rays, gamma
rays, beta particles (high-speed electrons), alpha particles
(the nucleus of the helium atom), neutrons, protons, and other
heavy ions such as the nuclei of argon, nitrogen, carbon,
and other elements. X rays and gamma rays are electromagnetic
waves like light, but their energy is much higher than that
of light (their wavelengths are much shorter). Ultraviolet
(UV) light is a radiation of intermediate energy that can
damage cells (the well known sunburn), but UV light differs
from the forms of electromagnetic radiation mentioned above
in that it does not cause ionization (loss of an electron)
in atoms or molecules, but rather excitation (change
in energy level of an electron). The other forms of radiation--particles--are
either negatively charged (electrons), positively charged
(protons, alpha rays, and other heavy ions), or electrically
neutral (neutrons). |
Ionization
As an example of ionization, beta rays are fast electrons
that lose energy as they pass through cells and interact with
molecules. The transferred energy is high enough to disrupt
chemical bonds, which results in radical formation
(or ionization). Ionization differs from the ion formation
that occurs in ordinary chemical reactions. The process that
takes place when salt (sodium chloride, NaCl) is dissolved
in water is a good example of an ordinary reaction. Sodium
and chloride bind together because, separately, each atom
is unstable. The sodium (Na) atom has only one electron in
its outermost orbit, and loss of that electron makes it more
stable. In contrast, the chloride (Cl) atom has seven electrons
in its outermost orbit and gaining one electron to have a
full complement of eight outer electrons makes it more stable.
When the two atoms bind to form NaCl, sodium shares its single
outer electron with chloride, and so, both are stable. In
ordinary chemical reactions, such as the binding of Na to
Cl, electrons that are lost or gained are always those on
the outermost orbit. When NaCl is dissolved in water, the
two atoms separate, with chloride keeping the extra outer
electron; thus, the sodium has a net positive charge (hence
Na+) and the chloride has a net negative charge
(hence Cl-), but the net charge (balance between
positive and negative) remains neutral. These charged atoms
are called ions, and they are stable in water despite their
electrical charges.
In contrast, when an electron passes through a cell, it releases
its energy along its path (called a track) by interacting
with the electrons of nearby molecules. The released energy
is absorbed by atoms near the track, resulting in either excitation
(a shift in the orbit of an electron to a higher energy level)
or ionization (release of an electron from the atom).
What differs from an ordinary chemical reaction is that when
radiation donates energy to atoms or molecules, electrons
other than those on the most outer orbit can be released,
which makes the atoms very unstable. Such unstable atoms are
called radicals and are chemically very reactive. Some
radicals are so reactive that they exist only for as short
a time as a microsecond.
X and gamma rays differ from beta particles in that they release
high-speed electrons from atoms first. Positively charged
particles transfer energy to molecules in cells by essentially
the same mechanisms. Neutrons are somewhat different since
they are electrically uncharged, and their main effect is
to impact the nuclei of hydrogen atoms, namely protons. Since
the masses of a neutron and a proton are similar, the impact
results in an elastic scattering process like in billiards.
The ejected protons behave as charged particles. |
How ionizations affect cells
Radiation-induced ionizations may act directly on the cellular
component molecules or indirectly on water molecules, causing
water-derived radicals. Radicals react with nearby molecules
in a very short time, resulting in breakage of chemical bonds
or oxidation (addition of oxygen atoms) of the affected
molecules. The major effect in cells is DNA breaks. Since
DNA consists of a pair of complementary double strands, breaks
of either a single strand or both strands can occur. However,
the latter is believed to be much more important biologically.
Most single-strand breaks can be repaired normally thanks
to the double-stranded nature of the DNA molecule (the two
strands complement each other, so that an intact strand can
serve as a template for repair of its damaged, opposite strand).
In the case of double-strand breaks, however, repair is more
difficult and erroneous rejoining of broken ends may occur.
These so-called misrepairs result in induction of mutations,
chromosome aberrations, or cell death. |
Characteristics of DNA damage
by radiation exposure
Deletion of DNA segments is the predominant form of radiation
damage in cells that survive irradiation. It may be caused
by (1) misrepair of two separate double-strand breaks in a
DNA molecule with joining of the two outer ends and loss of
the fragment between the breaks or (2) the process of cleaning
(enzyme digestion of nucleotides--the component molecules
of DNA) of the broken ends before rejoining to repair one
double-strand break.
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Biological effects differ by type
of radiation
Radiations differ not only by their constituents (electrons,
protons, neutrons, etc.) but also by their energy. Radiations
that cause dense ionization along their track (such as neutrons)
are called high-linear-energy-transfer (high-LET) radiation,
a physical parameter to describe average energy released per
unit length of the track. (See the accompanying figure.) Low-LET
radiations produce ionizations only sparsely along their track
and, hence, almost homogeneously within a cell. Radiation
dose is the amount of energy per unit of biological material
(e.g., number of ionizations per cell). Thus, high-LET radiations
are more destructive to biological material than low-LET radiations--such
as X and gamma rays--because at the same dose, the low-LET
radiations induce the same number of radicals more sparsely
within a cell, whereas the high-LET radiations--such as neutrons
and alpha particles--transfer most of their energy to a small
region of the cell. The localized DNA damage caused by dense
ionizations from high-LET radiations is more difficult to
repair than the diffuse DNA damage caused by the sparse ionizations
from low-LET radiations. |
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Figure.
Both examples produce the same total number of ionizations,
thus represent the same dose. |
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