Microbiology, 2021

434 11 • Mechanisms of Microbial Genetics
Figure 11.21
Intercalating agents, such as acridine, introduce atypical spacing between base pairs, resulting in DNA polymerase
introducing either a deletion or an insertion, leading to a potential frameshift mutation.
Radiation
Exposure to either ionizing or nonionizing radiation can each induce mutations in DNA, although by different
mechanisms. Strong ionizing radiation like X-rays and gamma rays can cause single- and double-stranded
breaks in the DNA backbone through the formation of hydroxyl radicals on radiation exposure (Figure 11.22).
Ionizing radiation can also modify bases; for example, the deamination of cytosine to uracil, analogous to the
action of nitrous acid. 4 Ionizing radiation exposure is used to kill microbes to sterilize medical devices and
foods, because of its dramatic nonspecific effect in damaging DNA, proteins, and other cellular components
(see Using Physical Methods to Control Microorganisms).
Nonionizing radiation, like ultraviolet light, is not energetic enough to initiate these types of chemical changes.
However, nonionizing radiation can induce dimer formation between two adjacent pyrimidine bases,
commonly two thymines, within a nucleotide strand. During thymine dimer formation, the two adjacent
thymines become covalently linked and, if left unrepaired, both DNA replication and transcription are stalled
at this point. DNA polymerase may proceed and replicate the dimer incorrectly, potentially leading to
frameshift or point mutations.
Figure 11.22
(a) Ionizing radiation may lead to the formation of single-stranded and double-stranded breaks in the sugar-phosphate
backbone of DNA, as well as to the modification of bases (not shown). (b) Nonionizing radiation like ultraviolet light can lead to the
4 K.R. Tindall et al. “Changes in DNA Base Sequence Induced by Gamma-Ray Mutagenesis of Lambda Phage and Prophage.”
Genetics 118 no. 4 (1988):551–560.
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