OVERSTRETCHING B-DNA - THE ELASTIC RESPONSE OF INDIVIDUAL DOUBLE-STRANDED AND SINGLE-STRANDED-DNA MOLECULES

Single molecules of double-stranded DNA (dsDNA) were stretched with fo rce-measuring laser tweezers. Under a longitudinal stress of similar t o 65 piconewtons (pN), dsDNA molecules in aqueous buffer undergo a hig hly 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 contract ed to their normal contour lengths. This transition was affected by ch anges in the ionic strength of the medium and the water activity or by cross-linking of the two strands of dsDNA. Individual molecules of si ngle-stranded DNA were also stretched giving a persistence length of 7 .5 angstroms and a stretch modulus of 800 pN. The overstretched form m ay play a significant role in the energetics of DNA recombination.