PRINCIPLES OF TOXICOLOGY

12.2 GENETIC FUNDAMENTALS AND EVALUATION OF GENETIC CHANGE 249
Figure 12.6 Cytosine modified to uracil by nitrous acid.
basepair substitution mutations or frameshift mutations. These two categories of gene mutations are
induced by different mechanisms and, often, by distinctly different classes of chemical mutagens. Yet
both types of gene mutation are virtually always monitored by measuring some phenotypic change in
the test organism. Microlesions occur at a much lower frequency (10 –5 to 10 –6 ) in comparison to
chromosome aberrations or “macrolesions,” which may be as frequent as 10 –2 to 10 –3 .
As described earlier, the basepair changes induced by point mutations (Figure 12.4) will also alter
RNA codon sequences, which, in turn, change the amino acid sequence of the peptide chain being
Figure 12.7 Alkylating agent (EMS) effects on DNA.