b. Ribonucleic Molecule. After replication, there are two identical cells. RNA, which is
the messenger system for DNA, enables the cells to function as specific cells. RNA is a
single-stranded molecule produced by DNA. In a single cell, the DNA molecule partially
splits and produces an RNA molecule. This molecule is given a set of coded
instructions that it takes to the ribosome of the cell. The ribosome of a cell is the
protein-producing part of a cell, which manufactures the necessary nutrients for a cell by
combining certain amino acids that are found within the cell. The RNA molecule
instructs the ribosome how to combine these amino acids. When this has occurred, the
cell will function properly.
c. Disturbing the Functions of Deoxyribonuciel. The above explanation of DNA and RNA
is by no means comprehensive. Cell division is a complex process. Fortunately, DNA
performs its functions without error as long as its structure is not disturbed. Although
other conditions will affect DNA, the one of interest to the x-ray specialist is ionizing
4-23. HOW IONIZING RADIATION AFFECTS DEOXYRIBONUCIEL
Ionization, you will recall, is the removal of an orbital electron. If this electron is
forming the bond between the nitrogen bases, then the ability to replicate exactly is lost.
This means that one of the rungs in a DNA molecule may be incomplete or perhaps
reincorporated in the wrong order.
a. Any modification in the sequence of nitrogen bases will result in a change of
instructions passed from DNA to RNA. When this happens, mutations occur. Mutations
are generally considered to be harmful, but, fortunately, they are often recessive and
are usually overcome by the dominant characteristics of the normal cells.
b. Sickle cell anemia is a mutation not necessarily caused by radiation. It is an
inherited defect in man in which the red blood cells become distorted in the shape of
sickles. They tend to form clumps that block the smaller blood vessels, and their
tendency to burst brings about anemia, usually resulting in early death. This serious
hereditary defect in man is brought about by the substitution of only one nitrogen base
in the DNA molecule--a single change in the sequence of the rungs.
4-24. MECHANISMS OF RADIATION INJURY
Two explanations of radiation damage are known as the target theory and the
indirect damage theory.
a. Target Theory. The target theory is concerned with direct effects. These
effects can be produced at cell level or in the whole animal. In either case, they alter