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Related Experiment Videos

Superhelicity of nucleosomal DNA changes its double-helical repeat.

L E Ulanovsky, E N Trifonov

    Cell Biophysics
    |December 1, 1983
    PubMed
    Summary

    DNA supercoiling in nucleosomes introduces a twist that alters its helical repeat. This change is consistent with the DNA being relatively unconstrained within the nucleosome structure.

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    Area of Science:

    • Molecular Biology
    • Structural Biology
    • Biophysics

    Background:

    • DNA superhelix formation involves bending and twisting deformations.
    • Nucleosomal DNA winding introduces an extra twist angle.
    • The helical repeat of free DNA in solution is approximately 10.55 bp.

    Purpose of the Study:

    • To quantify the change in DNA helical repeat due to nucleosomal superhelicity.
    • To determine if nucleosomal DNA is constrained.
    • To reconcile the helical repeat of DNA in solution versus in nucleosomes.

    Main Methods:

    • Calculation of extra twist angle using the Crick formula.
    • Determination of DNA double-helical repeat changes.
    • Comparison of DNA repeat in solution with estimates from nucleosomal DNA.

    Main Results:

    • An extra twist of -0.5 degrees per base pair was calculated for nucleosomal DNA.
    • This corresponds to a change of -0.15 bp in the DNA double-helical repeat.
    • The observed DNA repeat in nucleosomes (10.38 bp) is explained by superhelicity.

    Conclusions:

    • The superhelicity of nucleosomal DNA perfectly accounts for the observed helical repeat.
    • Nucleosomal DNA appears to be essentially unconstrained.
    • Bending and twisting are key deformations in DNA superhelix formation.

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