Mutagenesis: Insights from Single-Cell, Real-Time Approaches.

All genetic variation fueling evolution depends on mutations. Although mutations have been extensively studied for almost a century, until a decade ago the investigation of mutations was limited to population-level analysis. This constraint has hampered the exploration of cellular heterogeneity in mutation processes and its evolutionary implications. To overcome these limitations, quantitative visualization methods for studying mutations in the bacterium Escherichia coli at the single-cell level have been developed. These approaches offer the possibility of accessing a major source of mutations, i.e., DNA polymerase errors, and their fate, i.e., repair versus conversion to mutation. In addition, such methods allow for quantitative characterization of the effects of mutations on cell fitness. This article discusses insights into the mutation process derived from these new single-cell mutagenesis approaches.