PRINCIPLES OF TOXICOLOGY

12.2 GENETIC FUNDAMENTALS AND EVALUATION OF GENETIC CHANGE 241
Figure 12.1 Possible consequences of mutagenic event in somatic and germinal cells.
organic compounds currently are in use, a number which increases annually. Only a very small fraction
of these have been confirmed as human carcinogens (see Chapter 13), and no compound has been
shown unequivocally to be mutagenic in humans. However, animal and bacterial tests have demonstrated
a mutagenic potential for some occupational and environmental compounds at high exposure
levels, and it is reasonable to consider human exposure to these compounds, particularly in occupational
situations where contact may be frequent and/or intense. This is not to suggest that very small
exposures common to environmental circumstances are likely to be associated with adverse effects.
12.2 GENETIC FUNDAMENTALS AND EVALUATION OF GENETIC CHANGE
Transcription and Translation
DNA (deoxyribonucleic acid) is the structural and biochemical unit on which heredity and genetics
are based for all species. It is the only cellular macromolecule that is self-replicating, alterable, and
transmissible. Subunits of the DNA molecule are grouped into genes that contain the information,
which is necessary to produce a cellular product. An example of such a cellular product is a polypeptide
or protein, which may have a structural, enzymatic, or regulatory function in the organism. Figure 12.2
illustrates how the sequence of messages on the DNA molecule is transcribed into the RNA (ribonucleic
acid) molecule and ultimately is translated into the polypeptide or protein. The sequence of base pairs
in the DNA molecule specifies the appropriate complementary (“mirror image”) sequence that governs
the formation of the messenger RNA (mRNA). Transfer RNAs (tRNA), each of which is specific for
a single amino acid, are matched to the appropriate segment of the mRNA. When the amino acids are
released from the tRNAs and are linked in a continuous string, the polypeptide (or protein) chain is
formed.
Recognition of the mRNA regions by the tRNA-amino acid complex is accomplished by a system
of triplet, or three-base, codons (in the mRNA) and complementary anticodons (in the tRNA). The
critical features of this coding system are that it is simultaneously unambiguous and degenerate. In