PRINCIPLES OF TOXICOLOGY

242 MUTAGENESIS AND GENETIC TOXICOLOGY
Figure 12.2 Schematic representation of transcription and translation.
other words, no triplet codon may call for more than a single specific tRNA-amino acid complex
(unambiguous), but several triplets may call for the same tRNA-amino acid (degenerate). This results
from the fact that four nucleotides, which form DNA (DNA is composed of adenine, cytosine, guanine,
and thymine), and the nucleotides forming RNA (RNA is made up of A, C, G, and uracil) may be
combined in triplet form in 64 different ways (4 × 4 × 4 or 4 3 ). The 20 amino acids and three terminal
codes account for less than half of the available codons, leaving well over 30 codons of the possible
64. The biological significance of this degeneracy is that such a characteristic minimizes the influence
of minor mutations (e.g., single basepair deletions or additions) because codons differing only in minor
aspects may still code for the same amino acids. The significance of having an unambiguous code is
clear; the formation of proteins must be perfectly reproducible and exact. Table 12.1 depicts the amino
acids that are coded for by the various triplet codons of DNA, as well as the initiation and termination
signal triplets.
The process of mutagenesis results from an alteration in the DNA sequence. If the alteration is not
too radical, the rearrangement may be transmitted faithfully through the mRNA to protein synthesis,