CMMB 411 Lecture 3: DNA Topology — Part 3
21 Sep 2018
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A twist is when one single stranded dna molecule passes over another single stranded dna molecule. A supertwist is when a double stranded dna molecule passes another double stranded dna molecule (makes a quad where the two molecules cross) Twist and supertwist linear double stranded dna with free ends. If you don"t break the dna backbone, you can"t get any helical turns. Dna binding proteins bind to linear dna, the area between these proteins is constrained. Linking number (l or lk) is the number of times for one strand dna to cross the other strand when dna lies flat on a plane. L must be an integer (eg. 1, 2, 3, 4) You can only change the linking number by either introducing or breaking twists. If no covalent bond breakage and bond formation in the dna backbone, there is no change in linking number. L = t (twist) + w (writhe) W = +1 = positive supertwist = left-handed supertwist.
