Our modern technological civilization exposes mankind to two general
types of genetic dangers unknown earlier: Synthetic chemicals (or
unprecedentedly high concentrations of natural ones) absent in earlier
eras, and intensities of energetic radiation equally unknown or
unprecedented.
Chemicals can interfere with the process of replication by offering
alternate pathways with which the cellular machinery is not prepared to
cope. In general, however, it is only those cells in direct contact with
the chemicals that are so affected, such as the skin, the intestinal
linings, the lungs, and the liver (which is active in altering and
getting rid of foreign chemicals). These may undergo somatic mutations,
and an increased incidence of cancer in those tissues is among the
drastic results of exposure to certain chemicals.
Such chemicals are not, however, likely to come in contact with the
gonads where the sex cells are produced. While individual persons may be
threatened by the manner in which the environment is being permeated
with novel chemicals, the next generation is not affected in advance.
Radiation is another matter. In its broadest sense, radiation is any
phenomenon spreading out from some source in all directions. Physically,
such radiation may consist of waves or of particles.[3] Of the wave
forms the two best-known are sound and electromagnetic radiations.
Sound carries very low concentrations of energy. This energy is absorbed
by living tissue and converted into heat. Heat in itself can increase
the mutation rate but the effect is a small one. The body has effective
machinery for keeping its temperature constant and the gonads are not
likely to suffer unduly from exposure to heat.
Electromagnetic radiation comes in a wide range of energies, with
visible light (the best-known example of such radiation because we can
detect it directly and with great sensitivity) about in the middle of
the range. Electromagnetic radiations less energetic than light (such as
infrared waves and microwaves) are converted into heat when absorbed by
living tissue. The heat thus formed is sufficient to cause atoms and
molecules to vibrate more rapidly, but this added vibration is not
usually sufficient to pull molecules apart and therefore does not bring
about chemical changes.
Light will bring about some chemical changes. It is energetic enough to
cause a mixture of hydrogen and chlorine to explode. It will break up
silver compounds and produce tiny black grains of metallic silver (the
chemical basis of photography). Living tissue, however, is largely
unaffected—the retina of the eye being one obvious exception.
Ultraviolet light, which is more energetic than visible light,
correspondingly can bring about chemical changes more easily. It will
redden the skin, stimulate the production of pigment, and break up
certain steroid molecules to form vitamin D. It will even interfere with
replication to some extent. At least there is evidence that persistent
exposure to sunlight brings about a heightened tendency to skin cancer.
Ultraviolet light is not very penetrating, however, and its effects are
confined to the skin.
Electromagnetic radiations more energetic than ultraviolet light, such
as X rays and gamma rays, carry sufficient concentrations of energy to
bring about changes not only in molecules but in the very structure of
the atoms making up those molecules.
Atoms consist of particles (electrons), each carrying a negative
electric charge and circling a tiny centrally located nucleus, which
carries a positive electric charge.
Ordinarily, the negative charges of the electrons just balance the
positive charge on the nucleus so that atoms and molecules tend to be
electrically neutral. An X ray or gamma ray, crashing into an atom,
will, however, jar electrons loose. What is left of the atom will carry
a positive electric charge with the charge size proportional to the
number of electrons lost.
An atom fragment carrying an electric charge is called an _ion_. X rays
and gamma rays are therefore examples of _ionizing radiation_.
Radiations may consist of flying particles, too, and if these carry
sufficient energy they are also ionizing in character. Examples are
_cosmic rays_, _alpha rays_, and _beta rays_. Cosmic rays are streams of
positively charged nuclei, predominantly those of the element hydrogen.
Alpha rays are streams of positively charged helium nuclei. Beta rays
are streams of negatively charged electrons. The individual particles
contained in these rays may be referred to as _cosmic particles_, _alpha
particles_, and _beta particles_, respectively.
[Illustration: _Cosmic ray and trapped Van Allen Belt energetic
particles produced the dark tracks in this photo of a nuclear emulsion
that had been carried aloft on an Air Force satellite. The energetic
particles cause ionization of the silver bromide molecules in the
emulsion._]
[Illustration: _Alpha particles emitted by the source at right leave
tracks in a cloud chamber. Some tracks are bent near the end as a result
of collisions with atomic nuclei. Such collisions are more likely at the
end of a track when the alpha particle has been slowed down._]
[Illustration: _Beta particles originating at left leave these tracks in
a cloud chamber. Note that the tracks are much farther apart than those
of alpha particles. As the particle slows down, its path becomes more
erratic and the ions are formed closer together. At the very end of an
electron track the proximity of the ions approximates that in an
alpha-particle track._]
Ionizing radiation is capable of imparting so much energy to molecules
as to cause them to vibrate themselves apart, producing not only ions
but also high-energy uncharged molecular fragments called _free
radicals_.
The direct effect of ionizing radiation on chromosomes can be serious.
Enough chemical bonds may be disrupted so that a chromosome struck by a
high-energy wave or particle may break into fragments. Even if the
chromosome manages to remain intact, an individual gene along its length
may be badly damaged and a mutation may be produced.
[Illustration: _Effects of ionizing radiation on chromosomes: Left, a
normal plant cell showing chromosomes divided into two groups; right,
the same type of cell after X-ray exposure, showing broken fragments and
bridges between groups, typical abnormalities induced by radiation._]
If only direct hits mattered, radiation effects would be less dangerous
than they are, since such direct hits are comparatively few. However,
near-misses may also be deadly. A streaking bit of radiation may strike
a water molecule near a gene and may break up the molecule to form a
free radical. The free radical will be sufficiently energetic to bring
about a chemical reaction with almost any molecule it strikes. If it
happens to strike the neighboring gene before it has disposed of that
energy, it will produce the mutation as surely as the original radiation
might have.
Furthermore, ionizing radiations (particularly of the electromagnetic
variety) tend to be penetrating, so that the interior of the body is as
exposed as is the surface. The gonads cannot hide from X rays, gamma
rays, or cosmic particles.
All these radiations can bring about somatic mutations—all can cause
cancer, for instance.
What is worse, all of them increase the rate of genetic mutations so
that their presence threatens generations unborn as well as the
individuals actually exposed.