Determination of the number of superhelical turns in simian virus 40 DNA by gel electrophoresis

Keller, W.. (1975) Determination of the number of superhelical turns in simian virus 40 DNA by gel electrophoresis. Proceedings of the National Academy of Sciences of the United States of America, 72 (12). pp. 4876-4880.

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Official URL: http://edoc.unibas.ch/dok/A5258014

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Closed-circular, superhelical DNA from simian virus 40 (SV40 DNA I) was treated with an excess of DNA-relaxing enzyme in the presence of increasing amounts of ethidium bromide (EtdBr). After removal of the ethidium, each sample consisted of a group of close-circular DNA molecules differing in their number of superhelical turns (tau) around a mean value of tau in a Gaussian-like distribution. The DNA samples were analyzed by electrophoresis in agarose gels under conditions where the electrophoretic mobility was a function of the number of superhelical turns. Since the distributions around tau of DNA molecules of different samples overlappped, the difference in the mean number of superhelical turns from sample to sample, delta tau, could be determined and used to measure the mean number (tau) for native SV40 DNA I. By this criterion, SV40 DNA I contains a Gaussian-like distribution of molecules differing by integral numbers around a mean value of tau = -24 +/- 2 at 37 degree C [in 0.2 M NaC1, 10 mM Tris-HC1 (pH 7.9), and 0.2 mM EDTA]. The heterogeneity in tau is probably a consequence of thermal fluctuations in the DNA helix at the time when the last phospholiester bond is closed in vivo. When correlated to the buoyant shift of completely relaxed SV40 DNA in a CsC1-propidium diiodide gradient, the number of delta tau = 24 +/- 2 of superhelical versus relaxed DNA implies an unwinding of the DNA helix by 26-28 degree upon intercalation of one molecule of EtdBr.
Faculties and Departments:05 Faculty of Science > Departement Biozentrum > Former Organization Units Biozentrum > Cell Biology (Keller)
UniBasel Contributors:Keller, Walter
Item Type:Article, refereed
Article Subtype:Research Article
Publisher:National Academy of Sciences
Note:Publication type according to Uni Basel Research Database: Journal article
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Last Modified:07 Nov 2017 13:23
Deposited On:22 Mar 2012 13:19

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