Single-molecule optical-trapping experiments are now resolving the smallest units of motion in biology, including 1-base-pair steps along DNA. This review initially concentrates on the experimental problems with achieving 1-Å instrumental stability and the technical advances necessary to overcome these issues. Instrumental advances are complemented by insights in optical-trapping geometry and single-molecule motility assay development to accommodate the elasticity of biological molecules. I then discuss general issues in applying this measurement capability in the context of precision measurements along DNA. Such enhanced optical-trapping assays are revealing the fundamental step sizes of increasingly complex enzymes, as well as informative pauses in enzymatic motion. This information in turn is providing mechanistic insight into kinetic pathways that are difficult to probe by traditional assays. I conclude with a brief discussion of emerging techniques and future directions.


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