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. 2002 Aug 7;99(19):12126–12131. doi: 10.1073/pnas.132378799

Fig 2.

Fig 2.

The experimental configuration. (A) DNA molecules were tagged at their ends with biotin and digoxygenin, allowing attachment to a streptavidin-coated magnetic bead and an antidigoxygenin-coated glass surface. Translation and rotation of a pair of magnets above the DNA molecule changed the extension of the DNA, which was recorded by tracking of the bead's position with an inverted microscope. (B) The behavior of an 11.5-kb duplex DNA molecule under superhelical stress at two applied forces. At F = 0.26 pN, (+) and (−) superhelical turns equivalently reduced DNA extension through the formation of plectonemic supercoils. Increase of the force to 1.9 pN resulted in an asymmetry of the supercoiling vs. extension curve because denatured regions form at the expense of plectonemes in (−) but not (+) supercoiled DNA. For this DNA, 50 turns caused a |σ| of 0.045. (C) Three different DNAs were used in this study: fully duplex 11.5-kb DNA, 8.7-kb duplex DNA with a symmetric mismatch of 12 nucleotides, and 8.7-kb DNA with an asymmetric bulge of 25 unpaired nucleotides.