Although many mutations arise spontaneously, a number of environmental agents, including certain chemicals and radiation, are capable of damaging DNA. Any environmental agent that significantly increases the rate of mutation above the spontaneous rate is called a mutagen.

BASE ANALOGS One class of chemical mutagens consists of base analogs, chemicals with structures similar to those of any of the four standard bases of DNA. DNA polymerases cannot distinguish these analogs from the standard bases, so if base analogs are present during replication, they may be incorporated into newly synthesized DNA molecules. For example, 5-bromouracil (5BU) is an analog of thymine; it has the same structure as thymine except that it has a bromine (Br) atom on the 5-carbon atom instead of a methyl group (Figure 13.16a). Normally, 5-bromouracil pairs with adenine just as thymine does, but it occasionally mispairs with guanine (Figure 13.16b), leading to a transition (T • A → 5BU • A → 5BU • G → C • G), as shown in Figure 13.17. Through mispairing, 5-bromouracil can also be incorporated into a newly synthesized DNA strand opposite guanine. In the next round of replication, 5-bromouracil pairs with adenine, leading to another transition (G • C → G • 5BU → A • 5BU → A • T). In the laboratory, mutations caused by base analogs can be reversed by treatment with the same analog or by treatment with a different analog.

ALKYLATING AGENTS Alkylating agents are chemicals that donate alkyl groups, such as methyl (CH₃) and ethyl (CH₃–CH₂) groups, to nucleotide bases. For example, ethylmethylsulfonate (EMS) adds an ethyl group to guanine, producing O⁶-ethylguanine, which pairs with thymine (Figure 13.18a). Thus, EMS produces C • G → T • A transitions. Ethylmethylsulfonate is also capable of adding an ethyl group to thymine, producing 4-ethylthymine, which then pairs with guanine, leading to a T • A → C • G transition. Because EMS produces both C • G → T • A and T • A → C • G transitions, mutations produced by EMS can be reversed by additional treatment with EMS.

358

DEAMINATION In addition to its spontaneous occurrence (see Figure 13.15), deamination can be induced by some chemicals. For instance, nitrous acid deaminates cytosine, creating uracil, which in the next round of replication pairs with adenine (Figure 13.18b), producing a C • G → T • A transition mutation. Nitrous acid also changes adenine into hypoxanthine, which pairs with cytosine, leading to a T • A → C • G transition. In addition, nitrous acid deaminates guanine, producing xanthine, which pairs with cytosine just as guanine does; however, xanthine can also pair with thymine, leading to a C • G → T • A transition. Nitrous acid produces exclusively transition mutations, and because both C • G → T • A and T • A → C • G transitions are produced, these mutations can be reversed with nitrous acid.

HYDROXYLAMINE Hydroxylamine is a very specific base-modifying mutagen that adds a hydroxyl group to cytosine, converting it into hydroxylaminocytosine (Figure 13.18c). This conversion increases the frequency of a rare tautomer that pairs with adenine instead of guanine and leads to C • G → T • A transitions. Because hydroxylamine acts only on cytosine, it will not generate T • A → C • G transitions; thus, hydroxylamine will not reverse the mutations that it produces.

359

INTERCALATING AGENTS Proflavin, acridine orange, ethidium bromide, and dioxin are intercalating agents (Figure 13.19a), which produce mutations by sandwiching themselves (intercalating) between adjacent bases in DNA, distorting the three-dimensional structure of the helix and causing single-nucleotide insertions and deletions in replication (Figure 13.19b). These insertions and deletions frequently produce frameshift mutations, so the mutagenic effects of intercalating agents are often severe. Because intercalating agents generate both additions and deletions, they can reverse the mutations they produce.