MCB9b - Topological Linking Number (Lk): Tw ist (Tw) +...

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Topological Linking Number (Lk): Twist (Tw) + Writhe (Wr) Tw = “the number of times Watson strand crosses Crick” helical repeats in B-form DNA are rig ht-handed, considered + direction Wr = “the number of times WC duplex crosses WC duplex” compaction of B-form DNA against itself, also termed supercoiling + supercoils = right handed like B-form DNA; - supercoils = opposite direction Lk = 6 Tw = 6 Wr = 0 Lk o = most favorable Lk = B-form DNA with no supercoils
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Twist and writhe can interconvert, but Lk CANNOT change without cleavage of a phosphodiester bond. DNA in cells is has an Lk less than Lk o like the picture above.
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Fig 4-2 negative supercoil positive supercoil Tw = 6 Wr = 0 Tw = 6 Wr = -1 +1 Lk = 6 no enzyme is required Tw = 6 Wr = -1 Lk = 5 enzyme is required! Lk = 6 must remove + supercoils nucleosome DNA packaging and DNA replication require changes in DNA topology “topoisomers” differ in Lk; Lk is altered by “topoisomerase” enzymes
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Changes Lk in steps of 2. Enzyme breaks one duplex and passes another duplex through the break. Type II topoisomerases use ATP hydrolysis to drive a conformational
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This document was uploaded on 09/12/2011.

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MCB9b - Topological Linking Number (Lk): Tw ist (Tw) +...

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