The Elasticity of a Single Supercoiled DNA Molecule

TLDR

The DNA was torsionally constrained. The study examined the elastic behavior of individual λ DNA molecules over‑ and under‑wound by up to 500 turns. Single linear DNA molecules were tethered between a glass surface and a magnetic bead, and a rotating magnetic field applied torque while the stretching force was inferred from bead Brownian fluctuations. A sharp transition from low to high extension occurred at ~0.45 pN for underwound and ~3 pN for overwound DNA, likely reflecting formation of alternative structures relevant to transcription and replication.

Abstract

Single linear DNA molecules were bound at multiple sites at one extremity to a treated glass cover slip and at the other to a magnetic bead. The DNA was therefore torsionally constrained. A magnetic field was used to rotate the beads and thus to coil and pull the DNA. The stretching force was determined by analysis of the Brownian fluctuations of the bead. Here the elastic behavior of individual lambda DNA molecules over- and underwound by up to 500 turns was studied. A sharp transition was discovered from a low to a high extension state at a force of approximately 0.45 piconewtons for underwound molecules and at a force of approximately 3 piconewtons for overwound ones. These transitions, probably reflecting the formation of alternative structures in stretched coiled DNA molecules, might be relevant for DNA transcription and replication.