TITLE: "Helix-Coil Transition of Single DNA Molecules"

SPEAKER: Dr. Mark Williams,

TIME: Thursday Mar. 1, 2001 at 4 PM

PLACE: George P. Williams, Jr. Lecture Hall, (Olin 101)

When single DNA molecules are stretched beyond their normal B-form contour length to forces of ~65 pN, they undergo a cooperative overstretching transition, such that very little additional force is required to extend the molecule to 1.7 times its contour length. By using an optical tweezers instrument to measure the transition force as a function of pH and temperature, we have demonstrated that the overstretching transition is a phase transition from the double-stranded helical form of DNA to its single-stranded coil form. We have measured the effect of HIV-1 nucleocapsid protein (NC) on this helix-coil transition. The NC protein is regarded as a nucleic acid chaperone, since it catalyzes the folding of nucleic acids into conformations containing the maximum number of base pairs. Thus, a stable nucleic acid structure that does not represent the lowest energy state of the system is able to overcome an energy barrier in the presence of NC and change its conformation to lower the total energy of the system. When single DNA molecules are stretched through the helix-coil transition in the presence of NC, the stability of the double helical form of DNA and the cooperativity of the transition decrease.