The Analysis of Reverse Mutations

The study of reverse mutations (reversions) can provide useful information about how mutagens alter DNA structure. For example, any mutagen that produces both A-T:G-C and G-C:A-T transitions should be able to reverse its own mutations. However, if the mutagen produces only G-C:A-T transitions, then reversion by the same mutagen is not possible. Hydroxylamine (see Figure 17.20c) exhibits this type of one-way mutagenic activity; it causes G-C:A-T transitions but is incapable of reversing the mutations that it produces; so we know that it does not produce A-T : G-C transitions. Ethylmethanesulfonate (see Figure 17.20a), on the other hand, produces G-C : A-T transitions and reverses its own mutations; so we know that it also produces T-A : C-G transitions.

Analyses of the ability of different mutagens to cause reverse mutations can be sources of insight into the molecular nature of the mutations. We can use reverse mutations to determine whether a mutation results from a base substitution or a frameshift. Base analogs such as 2-aminopurine cause transitions, and intercalating agents such as acridine orange (see Figure 17.22) produce frameshifts. If a chemical reverses mutations produced by 2-aminopurine but not those produced by acridine orange, we can conclude that the chemical causes transitions and not frameshifts. If nitrous acid (which produces both G-C : A-T and A-T : G-C transitions) reverses mutations produced by the chemical but hydroxylamine (which causes only G-C : A-T transitions) does not, we know that, like hydroxylamine, the chemical produces only G-C : A-T transitions. Table 17.4 illustrates the reverse mutations that are theoretically possible among several mutagenic agents. The actual ability of mutagens to produce reversals is more complex than suggested by Table 17.4 and depends on environmental conditions and the organism tested.

0 0

Post a comment