MCB 250 Lecture Notes - Lecture 6: Nucleic Acid Structure, Linking Number, Writhe
9 Jan 2020
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It just means that there is a coiling super-imposed on the coiling of b form dna: the cell regulates this phenomenon. Linking number: linking number (lk) the total number of times dna strands cross one another, used to describe the topological state of cccdna. Is an invariant property of cccdna (if no covalent bonds have been broken, lk remains constant) Is composed of two parameters, twist (tw) and writhe (wr) Lk = tw + wr: tw = number of helical turns, wr = number of times the helix crosses itself, writhe can exist in two forms, interwound or plectonemic (figure right a, torid or spiral (figure right b) Torsional tension generated by negative supercoils is relieved by unwinding the helix. Type i topoisomerase: example: e. coli topoi and topoiii (note odd numbers, relax dna fewer supercoils, atp independent, pass one strand through the other, change linking number by 1, covalent intermediate: 5"phosphotyrosine linkage to the enzyme.
