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Radiation Biology
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The subject of radiation biology deals with the effects of ionizing radiations
on living systems. During the passage through living matter, radiation loses
energy by interaction with atoms and molecules of the matter, thereby
causing ionization and excitation. The ultimate effect is the alteration of the
living cells. Radiation biology is a vast subject, and it is beyond the scope
of this book to include the full details of the subject. The following is only
a brief outline of radiation biology, highlighting the mechanism of radiation
damage, radiosensitivity of tissues, different types of effect on living matter,
and risks of cancer and genetic effects from radiation exposure.
The Cell
The cell is the building unit of living matter and consists of two primary
components: the nucleus and the cytoplasm (Fig. 15.1). All metabolic
activities are carried out in the cytoplasm under the guidance of the
nucleus.
The nucleus contains chromosomes, which have a threadlike structure of
two arms connected by a centromere (Fig. 15.2). Chromosomes are formed
of genes, which are the basic units of heredity in the cells of all living species.
Genes are composed of deoxyribonucleic acid (DNA) molecules. The struc-
tural relationship of DNA molecules, genes, and chromosomes is shown
in Figure 15.2. The sequence of genes in the chromosome characterizes a
particular chromosome. Two categories of cells—namely, germ cells (repro-
ductive cells such as oocytes and spermatozoa) and somatic cells (all other
cells)—are based on the number of chromosomes they contain. Whereas
germ cells contain nnumber of individual chromosomes, somatic cells
contain 2nnumber of chromosomes in pairs, where nvaries with species of
the animal. In humans,nis equal to 23; therefore, there are 23 chromosomes
in germ cells and 46 chromosomes in somatic cells.
In the cytoplasm of the cell exist four important organelles—ribosomes,
endoplasmic reticula, mitochondria, and lysosomes—that carry out the