Overstretching DNA beyond its B-form contour length

We present here a movie of a typical DNA overstretching experiment as described in the article "Overstretching B-DNA: the Elastic Response of Individual Double Stranded and Single Stranded DNA Molecules" by Steven B. Smith, Yujia Cui, and Carlos Bustamante Science (1996) vol. 271, pp. 795-799 [abstract]

Single molecules of lambda phage dsDNA (contour length 16 microns) were stretched in aqueous buffer with force-measuring laser tweezers. When dsDNA is stretched with a force of 65 pN, it undergoes an overstretching transition over a narrow force range. At 28 microns length (170% B-form length) a molecule has been fully converted to the overstretched form and so the force rises rapidly. This transition is reversible in high salt.

Description of the movie

The movie was produced with a NTSC video camera. Approximately every fifth frame was selected and cropped to reduce file size. Specific item on the screen are explained below.

Frame Num. Action
1-15 Beads (out of focus) move toward left in buffer flow
16 A bead is caught in the laser trap
17-37 The pipette sucks a bead out of the trap
38 A different bead is caught in the trap
39-61 Pipette is moved back and forth to "fish" for DNA
62-75 The presence of DNA is indicated by force pulling trapped bead upstream
76 The flow is stopped
77-95 Overstretching the DNA beyond B-form shows a nearly constant force (~65pN) although the DNA length is changing
96-98 When the length exceeds 170% B-form, the force increases >80 pN
99 The laser trap is turned off
100-109 The DNA tether contracts, first by reverting to B-form and then by entropic elasticity. The tethered bead moves by Brownian motion
110-115 Flow is started and the trapped bead feels a drag force leftward
116-130 Overstretching the DNA in a flow produces a smaller net force on the trapped bead due to hydrodynamic drag
131-134 The molecule reaches 170% B-form length, the force rises
135 Laser trap is shut off
136-142 DNA contracts to B-form
143-170 Overstretching DNA in flow
171 DNA breaks

Click here to see the movie (0.3 Mb MPEG file).

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steve@alice.uoregon.edu