Stretching and breaking duplex DNA by chemical force microscopy.

Abstract

Specific interactions between complementary strands of DNA and other molecules are central to the storage, retrieval and modification of information in biological systems. Although in many cases the basic structures of duplex DNA and the binding energetics have been well characterized, little information is available about the forces in these systems. These forces are of critical importance because they must be overcome, for example, by protein machines during transcription and repair. Recent developments in atomic force microscopy make possible direct measurements of such forces between the individual oligonucleotide strands that form DNA duplexes. We used the chemical force microscopy technique, in which oligonucleotides are covalently linked to the force microscope probe tip and the sample surface, to measure the elongation and binding forces of individual DNA duplexes. The separation forces between complementary oligonucleotide strands were found to be significantly larger than the forces measured between noncomplementary strands, and to be consistent with the unbinding of a single DNA duplex. With increasing applied force, the separation of complementary strands proceeded in a stepwise manner: B-form DNA was stretched, then structurally transformed to a stable form of DNA approximately twice the length of the B form, and finally separated into single-stranded oligonucleotides. These data provide a direct measurement of the forces required to elastically deform and separate double-stranded DNA into single strands. Force microscopy provides a direct and quantitative measurement of the forces and energetics required to stretch and unbind DNA duplexes. Because the measurements can be carried out readily on synthetic oligonucleotides and in the presence of exogenous molecules, this method affords an opportunity for directly assessing the energetics of distorting and unbinding specific DNA sequences and DNA complexes. Such data could provide unique insights into the mechanistic steps following sequence-specific recognition by, for example, DNA repair and transcription factors.