PRINCIPLES OF TOXICOLOGY - Biology East Borneo

12.2 GENETIC FUNDAMENTALS AND EVALUATION OF GENETIC CHANGE 241Figure 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 fractionof these have been confirmed as human carcinogens (see Chapter 13), and no compound has beenshown unequivocally to be mutagenic in humans. However, animal and bacterial tests have demonstrateda mutagenic potential for some occupational and environmental compounds at high exposurelevels, and it is reasonable to consider human exposure to these compounds, particularly in occupationalsituations where contact may be frequent and/or intense. This is not to suggest that very smallexposures common to environmental circumstances are likely to be associated with adverse effects.12.2 GENETIC FUNDAMENTALS AND EVALUATION OF GENETIC CHANGETranscription and TranslationDNA (deoxyribonucleic acid) is the structural and biochemical unit on which heredity and geneticsare based for all species. It is the only cellular macromolecule that is self-replicating, alterable, andtransmissible. 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 polypeptideor protein, which may have a structural, enzymatic, or regulatory function in the organism. Figure 12.2illustrates how the sequence of messages on the DNA molecule is transcribed into the RNA (ribonucleicacid) molecule and ultimately is translated into the polypeptide or protein. The sequence of base pairsin the DNA molecule specifies the appropriate complementary (“mirror image”) sequence that governsthe formation of the messenger RNA (mRNA). Transfer RNAs (tRNA), each of which is specific fora single amino acid, are matched to the appropriate segment of the mRNA. When the amino acids arereleased from the tRNAs and are linked in a continuous string, the polypeptide (or protein) chain isformed.Recognition of the mRNA regions by the tRNA-amino acid complex is accomplished by a systemof triplet, or three-base, codons (in the mRNA) and complementary anticodons (in the tRNA). Thecritical features of this coding system are that it is simultaneously unambiguous and degenerate. In