Overstretching B-DNA: The Elastic Response of Individual Double-Stranded and Single-Stranded DNA Molecules

TLDR

The study used laser tweezers to stretch individual dsDNA molecules. Under ~65 pN, dsDNA undergoes a reversible, cooperative transition to a 70 % longer form whose stability depends on ionic strength, water activity, and strand cross‑linking, while ssDNA shows a persistence length of 7.5 Å and a stretch modulus of 800 pN, suggesting the overstretched state may influence DNA recombination energetics.

Abstract

Single molecules of double-stranded DNA (dsDNA) were stretched with force-measuring laser tweezers. Under a longitudinal stress of ∼65 piconewtons (pN), dsDNA molecules in aqueous buffer undergo a highly cooperative transition into a stable form with 5.8 angstroms rise per base pair, that is, 70% longer than B-form dsDNA. When the stress was relaxed below 65 pN, the molecules rapidly and reversibly contracted to their normal contour lengths. This transition was affected by changes in the ionic strength of the medium and the water activity or by cross-linking of the two strands of dsDNA. Individual molecules of single-stranded DNA were also stretched giving a persistence length of 7.5 angstroms and a stretch modulus of 800 pN. The overstretched form may play a significant role in the energetics of DNA recombination.