(a) What is the Lk of a 5,250 bp circular duplex DNA molecule with a nick in one
ID: 57610 • Letter: #
Question
(a) What is the Lk of a 5,250 bp circular duplex DNA molecule with a nick in one strand? (b) What is the Lk of the molecule in (a) when the nick is sealed (relaxed)? (c) How would the Lk of the molecule in (b) be affected by the action of a single molecule of E coli topoisomerase I? (d) What is the Lk of the molecule in (b) after eight enzymatic turnovers by a single molecule of DNA gyrase in the presence of ATP? (e) What is the Lk of the molecule in (d) after four enzymatic turnovers by a single molecule of bacterial type I topoisomerase?
Explanation / Answer
Linking number, Lk, represents the number of times each curve winds around the other. The Lk is always an integer. Depending on the orientation of the two curves it is either positive or negative. It is a topological property determining the degree of supercoiling. If both strands are covalently intact, the linking number cannot be changed. The topology theory indicates that the sum of T and W equals the linking number. L = T + W. For instance, the circular DNA of 5,250 bp, the linking number is 5250/10 = 525. If the molecule is relaxed and contains no supercoils, the linking number = twist number, as W is zero.
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