Important_Features_of_Supercoiling - They do this by...

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Important Features of Supercoiling 1. Only closed circular molecules can undergo supercoiling. Well, almost. This is absolutely true for circular prokaryotic genomes and plasmids. In eukaryotes, which have linear chromosomes, supercoiling can occur because the DNA is packaged into large loops which are believed to be anchored to a proteinaceous matrix known as the “chromosome skeleton”. Because the ends of the loops are constrained, limiting the rotational motions of the DNA within each loop, loops behave like circular chromosomes of bacteria. 2. Positive supercoils are given a positive sign by convention and negative supercoils are assigned a negative number. Thus a circular molecule that has no supercoils can through the action of a topoisomerase gain supercoils in either the positive or negative direction i.e. will have either a positive or negative writhe number. 3. Topoisomerases can alter the linking number of a closed circular molecule.
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Unformatted text preview: They do this by changing the number of supercoils in the molecule, which without a change in the twist, would by definition would produce a change in the linking number. How can this happen? Topoisomerases create temporary breaks in the molecule, and once DNA is nicked or broken, the linking number can be altered. There is no inherent directionality to the action of topoisomerases, however. Supercoiling will change in either a positive or negative direction such that the potential energy of the molecule is minimized. Typically, a homogenous population of DNA molecules (i.e. all molecules have the same number of nucleotides and the same amount of supercoiling) will after treatment with a topoisomerase become more heterogeneous, with the formation of subpopulations that have differing degrees of supercoiling. This is illustrated in the gel that I showed in slide 24 of the DNA structure slides....
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